MAIN   LIBRARY  AGRIC.  DEPT. 


Ube  IRural  TTert^oofe  Series 

Edited  by  L.  H.  BAILEY 


SOILS 

THEIR   PROPERTIES   AND   MANAGEMENT 


ftfje  l&ural  EexMSoofc  Series 

Edited  by  L.  H.  BAILEY 

Carleton,  The  Small  Grains. 

B.    M.    Duggar,    Plant     Physiology,    with 

special  reference  to  Plant  Production. 
J.  F.  Duggar,   Southern  Field  Crops. 
Gay,  The  Breeds  of  Live-Stock. 
Gay,    The    Principles    and      Practice     of 

Judging  Live-Stock. 
Goff,    The   Principles   of    Plant    Culture, 

Revised. 
Harper,  Animal  Husbandry  for  Schools. 
Harris    and    Stewart,    The     Principles     of 

Agronomy. 
Hitchcock,  A  Text -book  of  Grasses. 
Jeffery,  Text-Book  of  Land  Drainage. 
Livingston,  Field  Crop  Production. 
Lyon,  Fippin   and    Backman,  Soils  —  Their 

Properties  and  Management. 
Mann,  Beginnings   in  Agriculture. 
Montgomery,  The  Corn  Crops. 
Piper,   Forage  Plants  and  their  Culture. 
Warren,  Elements  of  Agriculture. 
Warren,  Farm  Management. 
Wheeler,  Manures  and  Fertilizers. 
White,  Principles  of  Floriculture. 
Widtsoc,    Principles    of    Irrigation    Prac- 


SOILS 


THEIR    PROPERTIES    AND    MANAGEMENT 


BY 
T.   LYTTLETON   LYON,   Ph.D. 


PROFESSOR    OF    SOIL    TECHNOLOGY,    CORNELL    UNIVEBSITY 


ELMER   0.   FIPPIN,   B.S.A. 

EXTENSION    PROFESSOR    OF    SOIL    TECHNOLOGY 
CORNELL    UNIVERSITY 


HARRY   0.   BUCKMAN,   Ph.D. 

ASSISTANT    PROFESSOR    OF    SOIL    TECHNOLOGY 
CORNELL    UNIVERSITY 


Nefo  gork 

THE   MACMILLAN   COMPANY 

1916 

All  rights  reserved 


36^ 
^ 


si?*: 

COPYRIGHT,    1909   AND    1915, 

By  THE  MACM1LLAN   COMPANY. 


Set  up  and  electrotyped.     Published  September,  1915.     Reprinted 
September,  1915;  January,  August,  1916. 


TSTortoooti  $wss 

J.  S.  Cushing  Co.  —  Berwick  &  Smith  Co. 

Norwood,  Mass.,  U.S.A. 


ACKNOWLEDGMENTS 

The  authors  wish  to  acknowledge  the  originals  of 
the  two  colored  maps  as  occurring  in  Bulletins  60  and 
96  of  the  Bureau  of  Soils,  U.  S.  Department  of  Agricul- 
ture. The  remaining  illustrations  were  drawn  es- 
pecially for  this  text.  Some  few  are  copies  in  part,  or 
in  whole,  from  other  authors.  Acknowledgments  not 
made  in  text  are  due  the  Bureau  of  Soils,  U.  S.  De- 
partment of  Agriculture ;  T.  C.  Chamberlin  ;  E.  W. 
Hilgard;  F.  H.  King  ;  G.  P.  Merrill ;  and  H.  W.  Wiley. 
The  authors  wisli  also  to  express  their  appreciation  to 
Messrs.  A.  B.  Beaumont  and  J.  H.  Bromley  for  their 
careful  work  in  the  preparation  of  the  original  drawings 
and  to  Miss  Lela  G.  Gross  for  aid  on  the  manuscript. 
Thanks  are  also  due  Professor  W.  D.  Bancroft  for  sug- 
gestions as  to  the  theoretical  phases  of  soil  colloids 
and  to  Professor  J.  L.  Stone  for  a  like  favor  regarding 
the  soundness  of  the  chapter  on  fertilizer  practice. 


3402 LO 


TABLE   OF   CONTENTS 

CHAPTER   I       ^ 

PAGES 

Some  General  Considerations  .         .         .         .         .        .  1-12 

Composition  of  the  soil,  1 —  Factors  for  plant  growth, 
2 —  Plant  food  elements,  3  —  Abundance  of  plant  food 
elements,  4 —  Soil-forming  rocks,  5  —  Soil-forming  min- 
erals, 6  —  Relative  abundance  of  minerals,  7 —  Organic 
matter,  8  —  The  soil  and  the  plant,  9. 

V 
CHAPTER   II 

Soil-forming  Processes      * 13-30 

Water,  10  — Wind,  11  — Ice,  12  — Heat  and  cold,  13 

—  Frost,  14 — Plants  and  animals,  15  —  Oxidation  and 
carbonation,  16 — Deoxidation,  17  —  Hydration,  18  — 
Solution,  19  —  A  general  statement  of  weathering,  20  — 
Factors  affecting  weathering,  21  —  Law  of  mineral  and 
rock  decay,  22  —  Special  cases  of  weathering,  23  —  Prac- 
tical relationships  of  weathering,  24. 

CHAPTER   III 
The  Geological  Classification  of  Soils  .         .        .        31-45 

Residual  soils,  25  —  Distribution  of  residual  soils,  26 

—  Cumulose  soils,  27  —  Colluvial  soils,  28  —  Alluvial 
soils,  29  —  Distribution  of  alluvial  soils,  30  —  Marine 
soils,  31  —  Characteristics  of  marine  soils,  32  —  Distri- 
bution of  marine  soils,  33. 

CHAPTER  IV 

Geological  Classification  of  Soils  {Continued)    .        .        46-64 
The  ice  sheet,  34  —  The   American  ice   sheet,  35  — 
Cause  of  the  ice  age,  36  —  The  extension  of  the  ice  sheet, 
37  — The  ice  as  a  soil  builder,  38  —  Glacial  till  soils,  39 


viii  TABLE  OF  CONTENTS 

PAGES 

—  Composition  of  glacial  soils,  40  —  Humus  of  glacial 
soils,  41  —  Glacial  lakes,  42  —  Lacustrine  soils  —  glacial 
lake,  43  —  Lacustrine  soils  —  recent  lake,  44 — iEolian 
soils,  45  —  Loess  soils,  46  —  Distribution  of  loess,  47  — 
Adobe  soils,  48  —  Sand  dunes,  49  —  Volcanic  dust,  50. 

CHAPTER   V 

Climatic  and  Geochemical  Relationships  of  Soils  .  65-82 
Climatic  relationships,  51  —  Geochemical  relationships 
of  residual  and  marine  soils,  52  —  Residual  and  glacial 
soils,  53  —  Effect  of  glaciation  on  agriculture,  54 — Hu- 
mid and  arid  soils,  55  —  Soil  color,  56  —  White  and 
black  soils,  57  —  Red  and  yellow  soils,  58  —  Agricul- 
tural significance  of  color,  59  —  Soil  and  subsoil,  60  — 

N^    Soil  and  subsoil  of  humid  regions,  61  —  Soil  and  subsoil 
of  arid  regions,  62. 

CHAPTER   VI 

The  Soil  Particle 83-107 

Soil  separates  and  mechanical  analysis,  63  —  Princi- 
ples of  mechanical  analysis,  64  —  Mechanical  analysis 
by  water  in  motion.  Schone  elutriator,  65  —  Hilgard's 
churn  elutriator,  66  —  Yoder's  centrifugal  elutriator,  67 

—  Mechanical  analysis  by  water  at  rest  —  Osborne's 
beaker  method,  68  —  Atterberg's  modified  Appiani  silt 
cylinder,  69  —  Centrifugal  soil  analysis,  70  —  Classifica- 
tion of  soil  particles,  71  —  Bureau  of  Soils  classification, 
72  —  Physical  character  of  the  separates,  73 — Mineral- 
ogical  characteristics  of  the  separates,  74  —  The  chem- 
ical constitution  of  soil  particles,  75  —  Value  of  a 
mechanical  analysis,  76  — Soil  class,  77  — Determination 
of  class,  78  —  The  significance  of  texture  and  class,  79. 

CHAPTER   VII 
Some  Physical  Properties  of  the  Soil   ....     108-125 
Arrangement  of  soil  particles,  80  — The  absolute  spe- 
cific gravity  of  the  soil,  81  —  Apparent  specific  gravity, 


TABLE  OF  CONTENTS 


IX 


82  —  Actual  weight  of  a  soil,  83  —  Pore  space  in  soil,  84 
—  The  number  of  soil  particles,  85  —  Surface  exposed 
by  soil  particles,  86  —  The  effective  mean  diameter  of 
soil  particles,  87. 


CHAPTER  VIII 
The  Organic  Matter  of  the  Soil  ..... 
The  source  and  distribution  of  organic  matter,  88  — 
Composition  of  plants,  89  —  Decay  of  organic  matter  in 
soils,  90  —  Composition  of  soil  humus,  91  —  The  work  of 
Oswald  Schreiner,  92  —  Toxic  material  in  the  soil,  93  — 
End  products  of  humus  decay,  94  —  Carbonized  mate- 
rials of  soil,  95  —  The  estimation  of  soil  organic  matter, 
96  —  The  estimation  of  soil  humus,  97  —  The  organic 
content  of  representative  soils,  98  —  The  humus  content 
of  soil,  99  —  The  influence  of  the  original  material  on 
the  resultant  humus,  100  —  Effects  of  organic  matter  on 
soil,  101  —  Maintenance  of  soil  organic  matter,  102. 


126-152 


CHAPTER  IX 
The  Colloidal  Matter  of  Soils  ..... 
The  colloidal  state,  103  —  The  properties  of  colloids, 
104  —  Colloidal  phases,  105  — Flocculation,  106  — Com- 
mon soil  colloids  and  their  generation,  107  —  Prepara- 
tion of  colloids,  108  —  Colloids  and  soil  properties,  109 
—  Factors  affecting  colloids,  110  —  Estimation  of  col- 
loidal content,  111. 


153-169 


CHAPTER   X 

Soil  Structure 170-197 

Plasticity,  112  — The  cause  of  plasticity,  113  —  The 
importance  of  plasticity,  114  —  Cohesion,  115  —  Methods 
of  determining  cohesion,  116  —  Factors  affecting  cohe- 
sion, 117  —  Moisture  limits  for  successful  tillage,  118  — 
Control  of  cohesion  and  plasticity,  119  —  Soil  tilth,  120 
—  Granulation,  121  —  Forces  facilitating  granulation, 
122  —  Wetting  and  drying,  123  — Freezing  and  thawing, 


X  TABLE   OF  CONTENTS 

PAGES 

124  —  Addition  of  organic  matter,  125  —  Action  of  plant 
roots  and  animals,  126  —  Addition  of  lime,  127  —  Til- 
lage, 128  —  The  action  of  the  plow,  129  —  Resume^  130. 

CHAPTER  XI 
The  Forms  or  Soil  Water  and  their  Movement  .  .  198-242 
Methods  of  expressing  soil  moisture,  131  —  Kinds  of 
water  in  the  soil,  132  —  Hygroscopic  water,  133  —  Effects 
of  texture  and  humus  on  hygroscopicity,  134  —  Nature 
of  the  film,  135  —  Effect  of  humidity  and  temperature 
on  hygroscopic  water,  136  —  Determination  of  hygro- 
scopicity, 137  —  Heat  of  condensation,  138  —  Capillary 
water,  139  —  Surface  tension  and  the  force  developed 
thereby,  140  —  The  form  of  water  surfaces  between  soil 
particles,  141 — Factors  affecting  capillary  water,  142  — 
Surface  tension  and  the  amount  of  capillary  water,  143 

—  Texture  and  the  amount  of  capillary  water,  144  — 
Effect  of  structure  on  the  amount  of  capillary  moisture, 
145  —  Organic  matter  and  the  amount  of  capillary  mois- 
ture, 146  —  Determination  of  capillary  water,  147  — The 
moisture  equivalent  of  soils,  148  —  The  maximum  re- 
tentive power  of  soils,  149  —  Capillary  movement,  150 

—  Factors  affecting  rate  and  height  of  capillary  move- 
ment, 151  —  Effect  of  thickness  of  film  on  capillary 
movement,  152  —  Surface  tension  and  capillary  move- 
ment, 153  —  Effect  of  texture  on  capillary  movement, 
154  —  Texture  and  capillary  pull  of  soils,  155  —  Effect 
of  structure  on  capillary  movement,  156  —  Gravitational 
water,  157  —  Pressure  and  the  movement  of  gravity 
water,  158  —  Effect  of  temperature  on  the  flow  of  grav- 
ity water,  159  —  Effect  of  texture  and  structure  on  the 
flow  of  gravity  water,  160 — Determination  of  the  quan- 
tity of  free  water  that  a  soil  will  hold,  161  —  The  calcu- 
lation of  the  free  water  of  a  soil,  162  —  Value  of  studying 
flow  and  composition  of  gravitational  water,  163  —  The 
study  of  gravity  water  by  means  of  tile  drains,  164  — 
The  lysimeter  method  of  studying  gravitational  water, 
165  —  Thermal  movement  of  water,  166. 


TABLE  OF  CONTENTS  xi 

CHAPTER   XII 

paqss 

The  Water  of  the  Soil  in  its  Relation  to  Plants  .  243-263 
Relations  of  water  to  the  plant,  167  —  The  water 
requirement  of  plants,  168  —  Factors  affecting  trans- 
piration, 169  —  Effect  of  crop  and  climate  on  transpira- 
tion, 170  —  Effect  of  soil  moisture  on  transpiration,  171 
—  Influence  of  fertility  on  transpiration,  172  —  Effect  of 
texture  on  transpiration,  173  —  Actual  amounts  of  water 
necessary  to  mature  a  crop,  174  —  Role  of  capillarity  in 
the  supplying  of  the  plant  with  water,  175  —  Influence 
of  water  on  the  plant,  176  —  Availability  of  the  water  in 
the  soil,  177  —  Unavailable  soil  water,  178  —  The  wilting 
coefficient  of  plants,  179  —  Determination  of  the  wilting 
point,  180  —  Calculation  of  the  wilting  point,  181  —  Re- 
lation of  texture  to  the  wilting  point,  182  —  Available 
and  superavailable  water,  183  —  Optimum  moisture  for 
plant  growth,  184. 

CHAPTER   XIII 

The  Control  of  Soil  Moisture 264-288 

Run-off  losses,  185  —  Percolation  losses,  186  —  Methods 
of  checking  loss  by  run-off  and  leaching,  187  —  Evapo- 
ration losses,  188  —  Methods  of  checking  evaporation 
losses,  189  —  Mulches  for  moisture  control,  190  —  Kinds 
of  mulches,  191  —  The  functions  of  a  mulch,  192  —  The 
soil  mulch  versus  the  dust  mulch,  193  —  Formation  of  a 
mulch,  194  —  Depth  of  a  mulch,  195  —  Resume"  of  mulch 
control,  196  —  Water  saved  by  a  mulch,  197  —  Effect  of 
mulehes  other  than  on  moisture,  198  —  General  useful- 
ness of  a  mulch,  199  —  Other  practices  affecting  evapo- 
ration losses,  200  —  Fall  and  early  spring  plowing,  201 — 
Rolling,  202  —  Shelters,  203  —  Level  cultivation,  204  — 
Plants,  205  —  Summary  of  moisture  control,  206. 

CHAPTER   XIV 

Soil  Heat 289-326 

Relation  of  heat  to  germination  and  growth,  207  — 
Chemical   and   physical    changes  due   to  heat,   208  — 


Xll 


TABLE  OF  CONTENTS 


Sources  of  soil  heat,  209  —  Factors  affecting  soil  tem- 
perature, 210  — Specific  heat,  211  —  Variations  of  spe- 
cific heat,  212 — Specific  heat  based  on  volume  of  soil, 
213  — Effect  of  water  on  specific  heat,  214  — Absorptive 
power  of  soils  for  heat,  215  —  Effect  of  color  on  absorp- 
tion of  heat,  216 — Effects  of  texture  and  structure  on 
heat  absorption,  217  —  Radiation  of  heat  by  soil,  218  — 
Conductivity  and  convection  of  heat  in  soils,  219  — 
Measurement  of  conductivity,  220  —  Effect  of  texture 
on  conductivity  of  heat,  221  —  Effects  of  humus  and 
structure  on  conductivity,  222  —  Influence  of  moisture 
on  heat  conductivity  in  soil,  223  —  Effect  of  evaporation 
of  water  on  soil  temperature,  224  —  Effect  of  organic 
decay  on  soil  temperature,  225  —  Relation  of  slope  to 
soil  temperature,  220  —  Heat  supply  and  its  effects,  227 
—  Control  of  soil  temperature,  228. 


CHAPTER   XV 


\J 


Availability   of   Plant   Nutrients   as   Determined   by 
Chemical  Analysis  .         .         .         . 

Solubility  of  the  soil  in  various  solvents,  229  —  Com- 
plete solution  of  the  soil,  230  —  Partial  solution  with 
strong  acids,  231  —  Significance  of  a  strong  hydrochloric 
acid  analysis,  232  —  Relation  of  texture  to  solubility, 
233  —  Nature  of  the  subsoil,  234  —  Calcium  carbonate, 
235  —  Deficiency  of  ingredients  and  manurial  needs,  236 
—  Partial  solution  with  weak  acids,  237  —  Advantages 
in  the  use  of  dilute  acids,  238  —  The  one-per-cent  citric 
acid  method,  239  —  Usefulness  of  the  citric  acid  method, 
240  —  Dilute  mineral  acids,  241  —  Extraction  with  an 
aqueous  solution  of  carbon  dioxide,  242  —  Extraction 
with  pure  water,  243  —  Influence  of  absorption,  244  — 
Other  factors  influencing  extraction,  245  —  The  soil  solu- 
tion in  situ,  246  —  Devices  for  obtaining  a  soil  solution, 
247  —  Composition  and- concentration  of  the  soil  solu- 
tion, 248  —  Variability  in  composition  and  concentration 
of  the  soil  solution,  249  —  Discussion  of  the  theories 
regarding  soil  solutions,  250. 


327-348 


TABLE   OF  CONTENTS  xiii 


CHAPTER    XVI 


PAGES 

The  Absorptive  Properties  of  Soils  ....  349-374 
Substitution  of  bases,  251  — Time  required  for  absorp- 
tion, 252  —  Insolubility  of  certain  absorbed  substances, 
253  —  Influence  of  size  of  particle,  254  —  Causes  of  ab- 
sorption, 255  —  Zeolites,  256  —  Chabazite,  257  —  Pres- 
ence of  zeolites  questioned,  258  —  Absorption  of  phos- 
phoric acid,  251)  —  Formation  of  insoluble  phosphates, 
260  —  Absorption,  261  —  Absorption  by  colloids,  262  — 
Absorptive  properties  of  colloidal  matter,  263  —  Selective 
absorption,  264 — Absorptive  power  of  colloidal  silicates, 
265  —  Absorption  by  colloids  versus  absorption  by  zeo- 
lites, 266  —  Absorption  by  organic  matter,  267  —  Ab- 
sorption of  water  vapor  and  of  gases  by  soils,  268  — 
Absorption  of  ammonia,  269  —  Absorption  of  carbon 
dioxide,  270  —  Absorption  of  nitrogen  and  oxygen,  271 

—  Relation  of  temperature  to  gas  absorption,  272  — 
Relation  of  absorptive  capacity  to  productiveness,  273 

—  Absorption  as  related  to  drainage,  274  —  Substances 
usually  carried  in  drainage  water,  275  —  Drainage  rec- 
ords at  Rothamsted,  276  —  Drainage  records  at  Brom- 
berg,  277  —  Losses  of  nitrogen  and  calcium,  278  — 
Composition  of  surface  water,  279. 

CHAPTER   XVII 

Acid  or  Sour  Soils 376-390 

Nature  of  soil  acidity,  280  —  Positive  acidity,  281  — 
Negative  acidity,  282  —  Production  of  sour  soils,  283  — 
Removal  of  bases  by  drainage  as  a  cause  for  acidity,  284 

—  Removal  of  bases  by  plants,  285  —  Effect  of  green 
manures  on  acidity,  286  —  Effect  of  fertilizers  on  soil 
acidity,  287  —  Acidity  in  relation  to  climate  and  to  for- 
mation of  soil,  288  —  Weeds  that  nourish  on  sour  soils, 
289  —  Crops  adapted  to  sour  soils,  290  —  Crops  that  are 
injured  by  acid  soils,  291  —  Qualitative  tests  for  acidity, 
292  —  Litmus  paper  test,  293  —  Ammonia  test,  294  — 
Zinc  sulfide  test,  295  —  Litmus  paper  and  potassium 
nitrate,  296  —  Acid  test  for  carbonates,  297  —  Plants  as 


XIV 


TABLE   OF  CONTENTS 


indicators  of  acidity,  298  —  Quantitative  determinations 
of  acidity,  299  —  Potassium  nitrate  method,  300  —  Lime- 
water  method,  301  —  Resume,  302. 


CHAPTER   XVIII 

Alkali  Soils        ......... 

Composition  of  alkali  salts,  303  —  White  and  black 
•  alkali,  304  —  Effect  of  alkali  on  crops,  305  —  Effect  on 
different  plants,  306  —  Other  conditions  that  influence 
the  action  of  alkali,  307  —  Accumulation  of  alkali,  308  — 
Irrigation  and  alkali,  309  —  Handling  of  alkali  lands,  310 
—  Eradication  of  alkali,  311 — Leaching  with  under- 
drainage,  312  —  Correction  with  gypsum,  313  —  Scraping, 
314  —  Flushing,  315  —  Control  of  alkali,  316  —  Cropping 
with  tolerant  plants,  317  —  Alkali  spots,  318. 


391-403 


CHAPTER   XIX 

Absorption  of  Nutritive  Salts  by  Agricultural  Plants 

How  plants  absorb  nutrients,  319  —  Relation  between 

root-hairs  and  soil  particles,  320  —  Liebig  and  Sachs  on 

solvent  action  of  plant-roots,  321  — -  Czapek's  experiment, 

322  —  Secretion  of  an  oxidizing  enzyme  by  plant-roots, 

323  —  Importance  of  carbon  dioxide  as  a  solvent,  324  — 
Insufficiency  of  carbon  dioxide,  325 —  The  present  status 
of  the  question,  326  —  Possible  root  action  on  colloidal 
complexes,  327  —  Why  crops  vary  in  their  absorptive 
powers,  328  —  Extent  of  absorbing  systems,  329  —  Ab- 
sorptive activity,  330  —  The  absorptive  power  of  cereals, 
331 — The  feeding  of  grass  crops,  332 — Leguminous 
crops,  333  —  Root  crops,  331  —  Vegetables,  335  —  Fruits, 
336  —  Mineral  substances  absorbed  by  plants,  337  —  Re- 
lation of  plant  growth  to  concentration  of  nutrient  solu- 
tion, 338  —  Quantities  of  plant  food  materials  removed 
by  crops,  339  —  Quantities  of  plant  food  materials  con- 
tained in  soils,  340  —  Possible  exhaustion  of  mineral 
nutrients,  341. 


404-420 


TABLE  OF  CONTENTS  XV 

v 
CHAPTER   XX 

PAGES 

Organisms  in  the  Soil 421-442 

Macrooryanisms. 

Rodents,  342— Worms,  343  — Insects,  344— Large 
fungi,  345  —  Plant  root,  346. 

Microorganisms. 

Plant  microorganisms,  347 — Plant  microorganisms 
injurious  to  higher  plants,  348  —  Plant  microorganisms 
not  injurious  to  higher  plants,  349 — Bacteria,  350  — 
Distribution  of  bacteria,  351  —  Numbers  of  bacteria,  352 
—  Numbers  as  influenced  by  season,  353  —  Conditions 
affecting  growth,  351  —  Oxygen,  355  —  Moisture,  356  — 
Temperature,  357  —  Organic  matter,  358  —  Soil  acidity, 
359  —  Functions  of  soil  bacteria,  300  —  Decomposition 
of  mineral  matter,  361 — Influence  of  certain  bacteria 
and  molds  on  the  solubility  of  phosphates,  362  —  Decom- 
position of  non-nitrogenous  organic  matter,  363  —  De- 
composition of  nitrogenous  organic  matter,  364. 


CHAPTER    XXI 

The  Nitrogen  Cycle 443-474 

Decay  and  putrefaction,  365  —  Ammonification,  366  — 
Bacteria  and  substances  concerned  in  ammonification, 
367  —  Nitrification,  3(58 — Effect  of  organic  matter  on 
nitrification,  369  —  Effect  of  soil  aeration  on  nitrification, 
370 — Effect  of  sod  on  nitrification,  371  —  Depths  at 
which  nitrification  takes  place,  372  —  Loss  of  nitrates 
from  the  soil,  373  —  Nitrate  reduction,  374  —  Nitrate 
assimilating  organisms,  375 — Denitrification,  376  —  Ni- 
trogen fixation  through  symbiosis  with  higher  plants, 
377  —  Relation  of  bacteria  to  nodules  on  roots,  378  — 
Transfer  of  nitrogen  to  the  plant,  379  —  Soil  inoculation 
for  legumes,  380  —  Nitrogen  fixation  without  symbiosis 
with  higher  plants,  381  — Nitrogen-fixing  organisms,  382 
—  Mixed  cultures  of  nitrogen-fixing  organisms,  383  — 
Nitrogen  fixation  and  denitrification  antagonistic,  384. 


XVI 


TABLE  OF  CONTENTS 


Treatment  of  Soils  with  Volatile  Antiseptics  and  Heat. 

Effects  of  carbon  bisulfide  and  heat  on  properties  of 
soils,  385  —  Hypotheses  to  account  for  effects  of  carbon 
bisulfide  and  of  heat,  386  —  Koch's  theory,  387  —  Hiltner 
and  Stormer's  theory,  388  —  Russell  and  Hutchinson's 
theory,  389  —  Greig-Smith's  theory,  390. 

CHAPTER   XXII 
The  Soil  Air 

Factors  that  Determine  Volume. 

Texture,  391  —  Structure,  392  —  Organic  matter,  393 
—  Moisture  content,  394. 
Composition  of  Soil  Air. 

Analyses  of  soil  air,  395  —  Sources  of  carbon  dioxide 
in  soil  air,  396  —  Production  of  carbon  dioxide  as  affect- 
ing composition,  397. 
Functions  of  Soil  Air. 

Oxygen,  398  — Carbon  dioxide,  399. 
Movement  of  Soil  Air. 

Diffusion  of  gases,  400  —  Movement  of  water,  401  — 
Changes  in  atmospheric  pressure,  402  —  Changes  of  tem- 
perature in  atmosphere  or  in  soil,  403  —  Suction  produced 
by  wind,  404. 

Methods  for  Modifying  the  Volume  and  Movement  of 
Soil  Air. 

Tillage,  405  —  Manures,  406  —  Underdrainage,  407  — 
Irrigation,  408  —  Cropping,  409. 


• 


475-488 


CHAPTER   XXIII 
Commercial  Fertilizers .     489-533 

Early  ideas  of  the  function  of  manures,  410  —  Devel- 
opment of  the  idea  of  the  nutrient  function  of  manures, 
411  —  Classes  of  manures,  412  —  Commercial  fertilizers, 
413  —  Fertilizer  constituents,  414. 
Fertilizers  Used  for  their  Nitrogen. 

Forms  in  which  nitrogen  exists  in  soils,  415  —  Forms 
in  which  nitrogen  is  absorbed  by  plants,  416 — Use  of 
nitrates  by  plants,  417  — Ammonia  as  a  plant  food,  418 


TABLE  OF  CONTENTS  xvil 

PAGES 

—  Utilization  of  humus  compounds  by  plants,  419  — 
Sodium  nitrate,  420  —  Ammonium  sulfate,  421  —  Ferti- 
lizers containing  atmospheric  nitrogen,  422  —  Cyanamid, 
423  —  Composition  of  cyanamid,  424  —  Changes  of  cal- 
cium cyanamid  in  the  soil,  425  —  The  use  of  cyanamid, 
426  —  Calcium  nitrate,  427  —  Organic  nitrogen  in  ferti- 
lizers, 428  —  Availability  of  organic  nitrogenous  ferti- 
lizers, 429. 

Fertilizers  Used  for  their  Phosphorus. 

Bone  phosphate,  430 — Mineral  phosphates,  431  — 
Superphosphate  fertilizers,  432  —  Reverted  phosphoric 
acid,  433  —  Relative  availability  of  phosphate  fertilizers, 
434  —  Changes  that  occur  when  superphosphate  is  added 
to  soils,  435  —  Other  factors  influencing  the  availability 
of  tricalcium  phosphate,  436  —  Effect  of  plants  on  the 
availability  of  tricalcium  phosphate,  437  —  Effect  of 
basicity  on  tricalcium  phosphate,  438  —  Influence  of  fer- 
menting organic  matter,  439  —  Influence  of  other  salts, 
440. 
Fertilizers  Used  for  their  Potassium. 

Stassfurt   salts,   441  —  Wood   ashes,   442  —  Insoluble 
potassium  fertilizers,  443. 
Sulfur  and  Sulfates  as  Fertilizers. 

The  use  of  free  sulfur,  444  —  Sulfur  as  sulfate,  445. 
Catalytic  Fertilizers. 

Nature  of  catalytic  action,  446  —  Catalytic  action  of 
soils,  447  —  Substances  used  as  catalytic  fertilizers,  448 

—  Manganese,  449  —  Physiological  role  of  manganese, 
450  —  Action  of  manganese  as  a  fertilizer,  451 — Forms 
of  manganese  and  response  of  soils,  452. 


CHAPTER    XXIV 

Soil  Amendments 534-545 

Salts  of  calcium,  453  — Effect  on  tilth  and  bacterial 
action,  454 — Liberation  of  plant  food  materials,  455  — 
Influence  of  lime  on  the  formation  of  nitrates  in  soil,  456 
—  Effect  on  toxic  substances  and  plant  diseases,  457  — 


xviii  TABLE  OF  CONTENTS 

PAGES 

The  lime-magnesia  ratio,  458—  Forms  of  calcium,  459  — 
Caustic  limes,  460  —  Carbonate  of  lime,  461  — Relative 
effectiveness  of  caustic  lime  and  carbonate,  462  —  Sul- 
fate of  calcium,  463  — Common  salt,  464  — Muck,  465. 

CHAPTER   XXV 
Fertilizer  Practice 546-576 

Effects  of  nitrogen  on  plant  growth,  466  —  Effects  of 
phosphorus  on  plant  growth,  467  —  Effects  of  potassium 
on  plant  growth,  468  —  Law  of  the  minimum,  469  —  Fer- 
tilizer brands,  470  —  Fertilizer  inspection  and  control, 
471 —Trade  values  of  fertilizers,  472  — The  buying  of 
mixed  goods,  473  —  Home-mixing  fertilizers,  474  —  Fer- 
tilizers not  to  be  mixed,  475 — How  to  mix  fertilizers, 
476  —  Factors  affecting  the  efficiency  of  fertilizers,  477  — 
Method  and  time  of  applying  fertilizers,  478  —  Fertiliz- 
ing crops,  479  —  Systems  of  fertilization,  480. 

CHAPTER   XXVI 

Farm  Manures 577-618 

General  character  and  function  of  farm  manures,  481 
—  Variable  composition  of  manures,  482  —  Litter,  483  — 
Class  of  animal,  484  —  Individuality,  condition,  and  age 
of  animal,  485  —  Food  of  animal,  486 —  Handling  of  the 
manure,  487  —  Composition  and  character  of  farm  ma- 
nures, 488  —  Actual  plant-food  in  liquid  and  solid  excre- 
ment, 489  —  Production  of  manure,  490  —  Heiden's 
formula,  491  —  Poultry  manure,  492  —  Commercial  and 
agricultural  evaluation  of  manures,  493  —  The  fermenta- 
tion of  manure,  494— Aerobic  action,  495  —  Anaerobic 
action,  496  —  Fermentation  in  general,  497  —  Gases  from 
manures,  498 — Change  of  bulk  and  composition  of  rot- 
ting manure,  499 — Fire-fanging  of  manure,  500  — 
Waste  of  farm  manures,  501  —  Losses  due  to  digestion, 
502  —  Losses  due  to  leaching  and  fermentation,  503  — 
Increased  value  of  protected  manure,  504 — The  money 
waste  of  manure,  505  —  Handling  of  manures,  506  — 
Care  of  manure  in  the  stalls,  507  —  Hauling  directly  to 


TABLE  OF  CONTENTS  xix 

PAGES 

the  field,  508  —  Cement  pit,  509  —  Covered  barnyard, 
510  —  Piles  outside,  511  — Distribution  of  manure  in  the 
field,  512  —  Reinforcement  of  manure,  513 — Benefits 
from  reinforcing,  514 — Lime  and  manure,  515  —  Com- 
posting, 516  —  Manure  and  muck,  517  —  Effects  of  ma- 
nure on  the  soil,  518  —  Residual  effect  of  manure,  519  — 
Place  of  manure  in  the  rotation,  520  —  Resume^  521. 

CHAPTER   XXVII 

Green  Manures 619-626 

Effects  of  green-manuring,  522  —  Quantities  of  plant 
constituents  added  by  green-manuring,  52:}  —  Decay  of 
green  manure,  624  —  Crops  suitable  for  green  manures, 
525  —  When  to  use  green  manures,  526  —  When  to  turn 
under  green  crops,  527  —  How  to  turn  under  green  mate- 
rial, 528  —  Green  manures  and  lime,  529  —  Green  manure 
and  the  rotation,  530. 

CHAPTER   XXVIII 

Land  Drainage 627-662 

Extent  of  drainage  needed  in  humid  regions,  531  — 
History  of  drainage,  532  —  Effects  of  land  drainage  on 
the  soil,  533  —  Methods  of  drainage,  534  —  Construction 
of  small  open  ditches,  535  —  Construction  of  large  open 
ditches,  536  —  Construction  of  early  types  of  under- 
drains,  537  —  Stone  drains,  538  — Tile  drains,  539  — 
Quality  of  tile,  540  —  Shapes  of  tile,  541  — Protection  of 
joints,  542  —  Entrance  of  roots  into  tile,  543  —  Protection 
of  joints  on  curves,  544  —  Foundation  for  tile,  545  — 
Arrangement  of  drainage  systems,  546 — 'Grade  of  tile 
drains,  547  —  Depth  of  drains,  548 — Distance  between 
drains,  549 — Construction  of  drainage  trenches  for  tile, 
650  —  Laying  tile,  551  —  Size  of  tile,  552  —  Amount  of 
water  to  be  removed  from  land,  553  —  Carrying  capacity 
of  a  tile-drain  system,  554  —  Cost  of  drainage,  555  — 
Storm  channels,  550  —  Silt  basins,  557  —  Surface  intakes, 
558  —  Outlets,  559  —  Muck  and  peat  soil,  560  —  Drain- 


XX 


TABLE  OF  CONTENTS 


age  of  irrigated  and  alkali  lands,  561—  Vertical  drainage, 
562  —  Drainage  by  means  of  explosives,  563  —  Re\sum£, 
564. 

CHAPTER   XXIX 

Tillage  

Objects  of  tillage,  565  —  Implements  of  tillage,  566  — 
Effects  on  the  soil,  567  —  Classes  of  tillage  implements, 
568  —  Plows,  569  —  Pulverizing  action  of  the  plow,  570 
—  Types  of  plows,  571  —  Shapes  of  moldboard  plows, 
572  —  Position  of  the  furrow  slice,  673  —  Depth  and 
width  of  furrow,  574  —  Plow  sole,  575  —  Hillside  plow, 
576  —  Covering  rubbish,  677  —  Subsoil  plow,  578  — 
Cultivators,  579  —  Cultivators  proper,  580  —  Leveler 
and  harrow  types  of  cultivator,  581  —  Seed  cultivators, 
582  —  Packers  and  crushers,  583  —  Rollers,  584  —  Clod 
crushers,  585  — Efficient  tillage,  586. 


663-681 


CHAPTER   XXX 

Irrigation  and  Dry  Farming 682-717 

Relation  of  irrigation  to  rainfall,  587  —  Extent  of  irri- 
gated land,  588 — History  of  irrigation,  589  —  Develop- 
ment of  irrigation  practice  in  the  United  States,  590  — 
Irrigation  in  humid  regions,  591  —  The  Reclamation 
Service,  592  —  Legal,  economic,  and  social  effects  of 
irrigation,  593  —  Divisions  of  irrigation,  594  —  Source  of 
water  for  irrigation,  595  —  Canals,  596  —  Preparation  of 
land  for  irrigation,  597  —  Methods  of  applying  water, 
598  —  Overhead  sprays,  599  —  Subirrigation,  600  — 
Methods  most  used  in  arid  regions,  601  —  Flooding,  602 

—  Furrows,  603  —  Size  and  form  of  furrows,  604  —  Unit 
of  measurement,  605  —  Amount  of  water  to  apply,  606 

—  Time  to  apply  water,  607  —  Conservation  of  moisture 
after  irrigation,  608  —  Sewage  irrigation,  609. 

Dry  Farming. 

Practices  in  dry  farming,  610  —  Storage  of  water  in 
the  soil,  611  —  Conservation  of  moisture,  612  —  Alternate 
cropping,    613  —  Drought-resistant    crops,    014  — Soils 


TABLE  OF  CONTENTS  xxi 

PAGES 

associated  with  dry  farming,  615  —  Extent  of  dry  farm- 
ing, 616. 

CHAPTER   XXXI 

The  Soil  Survey 718-740 

The  classification  of  soils  by  survey,  617  —  Factors 
employed  in  classification,  618  —  Texture,  the  soil  class, 
619 — Special  properties,  the  soil  series,. 620 — Source  of 
material,  the  soil  group,  621  —  Agency  of  formation,  the 
soil  province,  622  —  Climate,  623 —  The  practical  classi- 
fication of  soils  in  the  United  States,  624  —  The  soil  type 
and  soil  series,  625 — The  equipment  for  survey  work, 
626  —  Procedure  in  the  field,  627  —  Collection  of  soil 
samples,  628  —  The  accuracy  and  detail  of  the  soil  sur- 
vey, 629  —  The  soil  survey  report,  630  —  The  soil  map, 

631  —  The  extent  of  soil  surveys  in  the  United  States, 

632  —  Surveys   by  state  institutions,  633  —  Surveys  in 
other  countries,  634  —  Use  of  the  soil  survey,  635. 


SOILS:     THEIR    PROPERTIES 
AND   MANAGEMENT 

CHAPTER  I 

SOME  GENERAL  CONSIDERATIONS 

The  broken  and  weathered  fragments  of  rock  that 
cover  in  a  thin  layer  the  solid  part  of  the  earth  and  that 
furnish  the  foothold  and,  in  part,  the  sustenance  for  plant 
life,  are  termed  soil.  Soil  comes  from  rock  and  returns 
to  rock.  It  is  merely  a  transitory  stage  in  the  change 
from  one  form  of  rock  to  another.  It  is  never  still.  From 
the  time  when  the  particle  leaves  the  disintegrating  rock 
until  it  is  again  cemented  in  the  skeleton  of  the  earth, 
it  is  subjected  to  almost  constant  movement  and  to  the 
action  of  numerous  forces  that  change  it  chemically  and 
physically.  It  is  the  movement,  the  strain  and  the 
stress,  the  hard  treatment  at  the  hands  of  disintegrating 
agencies,  that  make  the  soil  useful  to  plant  life. 

It  was  only  the  simpler  forms  of  plants,  however,  that 
first  throve  on  the  pulverized  rock.  Tribe  after  tribe  of 
plants  has  invaded  the  soil.  Each  has  wrested  from  it 
the  mineral  matter  necessary  for  its  growth  and  develop- 
ment. Each  has,  in  the  end,  left  not  only  the  mineral 
matter  that  it  obtained  from  the  disintegrated  rock,  but 
also  the  carbon  and  the  oxygen  that  had  been  won  from  the 


2/  :  -SOJLS;  .PROPERTIES  AND  MANAGEMENT 

air  in  the  struggle  for  life.  Primitive  plants  have  been 
followed  by  more  highly  organized  ones  as  the  incursions 
have  gone  on,  and  always  to  the  profit  of  the  soil,  until 
the  soil  has  accumulated  a  great  store  of  organic  matter 
and  a  teeming  population  of  microscopic  life. 

This  debris  of  rock  and  plant  residue  that  has  accumu- 
lated through  the  centuries  of  struggle  is  the  arable  soil 
from  which  man  obtains  his  bread.  The  study  of  this 
soil  is  a  history  of  strife  and  struggle,  and  as  the  light  of 
investigation  is  turned  on  it,  new  contestants,  new  opera- 
tions, new  results,  and  new  principles  are  brought  to  view 
and  the  story  must  be  retold. 

1.  Composition  of  the  soil.  —  Broadly  speaking,  the 
soil  is  composed  of  two  general  classes  of  materials,  rock 
and  organic  matter.  The  former  usually  makes  up  the 
bulk  of  the  soil,  while  the  latter  occurs  under  normal 
conditions  in  relatively  small  amounts.  In  spite  of  this 
low  proportion,  however,  its  presence  is  of  vital  impor- 
tance to  productivity.  The  soil  has  also  three  general 
phases  —  the  physical,  the  chemical,  and  the  biological. 
In  the  physical  phase,  the  size  and  shape  of  particle,  the 
movement  of  air  and  water,  and  other  physical  proper- 
ties are  dealt  with ;  in  the  chemical  phase,  the  composi- 
tion of  the  particle,  of  the  organic  matter,  and  of  the  soil 
solution  is  of  dominant  importance ;  in  the  biological  phase, 
the  soil  is  seen  to  be  not  an  inert  material,  but  teeming 
with  life  —  minute  forms  of  life,  to  be  sure,  but  of  great 
importance  in  the  manufacture  of  food  for  plants.  Under 
these  three  general  phases,  then,  the  changes  going  on  in 
a  soil  may  be  studied,  and  they  are  found  to  be  directed 
primarily  toward  the  production  and  maintenance  of  con- 
ditions favorable  for  plant  growth.  The  soil  is  not  a 
simple  medium  to  study,  but  is  extremely  complicated 


SOME   GENERAL    CONSIDERATIONS  3 

for  two  reasons :  first,  because  of  the  complicated  nature 
of  its  two  general  constituents ;  and  secondly,  because  of 
the  action  and  interaction  of  these  constituents  with  each 
other. 

2.  Factors  for  plant  growth.  — The  growth  and  devel- 
opment of  a  plant  are  largely  the  result  of  two  sets  of 
factors,  the  internal  and  the  external.  The  former  de- 
pends on  the  nature  of  the  plant  itself,  the  latter  on  its 
environment.  The  external  factors  of  plant  growth  under 
normal  conditions  may  be  classified  as  follows :  (1)  me- 
chanical support,  (2)  air,  (3)  heat,  (4)  light,  (5)  water,  and 
(6)  food.  With  the  exception  of  light,  the  soil  supplies, 
either  wholly  or  in  part,  all  the  conditions  named.  As 
a  mere  mass  of  ground-up  rock  with  which  are  mixed 
varying  quantities  of  decayed  organic  matter,  the  soil 
acts  as  a  medium  for  root  development  and  thereby  pro- 
vides a  foothold  for  the  plant.  Air,  heat,  and  water  are 
supplied  as  a  consequence  of  the  inherent  physical  condi- 
tion of  a  soil.  The  circulation  of  water  serves  to  bring 
food  into  solution  for  absorption  by  the  rootlets.  Thus 
the  two  prime  functions  of  the  soil  are  realized  —  the 
supplying  of  plant-food  and  of  a  foothold  for  plant  life. 

3.  Plant-food  elements.1  —  While  the  physical  condition 
of  the  soil  has  tremendous  influence  on  plant  growth,  the 
food  elements  must  first  be  considered,  since  their  avail- 
ability is  so  closely  related  to  the  factors  that  function 
in  soil  formation.  Ten  elements  are  usually  considered 
as  absolutely  necessary  for  plant  growth.  They  may  be 
classified  as  follows  :  — 

1  For  a  complete  discussion  of  the  plant-food  elements  as  re- 
lated to  the  plant,  see  Russell,  E.  J.  Soil  Conditions  and  Plant 
Growth,  Chapter  II,  pp.  30-46.  New  York  City.  2d  edition, 
1915. 


4  SOILS:    PBOPERTIES  AND  MANAGEMENT 

Elements  obtained  from  Elements  coming  directly  from l 

air  or  water  the  soil  itself 

Carbon  Nitrogen  Magnesium 

Oxygen  Phosphorus  Iron 

Hydrogen  Potassium  '  Sulfur 

Nitrogen  Calcium 

Carbon  is  obtained  very  largely  by  the  plant  directly 
from  the  air  as  carbon  dioxide  (C02) ;  while  oxygen 
comes  directly  from  the  atmosphere  or  from  water,  which 
is  also  the  source  of  at  least  a  part  of  the  hydrogen  utilized 
in  vegetative  growth.  The  other  elements,  except  in 
the  case  of  leguminous  crops,  are  taken  wholly  from  the 
soil  solution  itself. 

While  all  these  elements  found  in  the  soil  must  be 
available  in  order  that  plants  may  grow  normally,  only 
a  very  few  ever  become  limiting  factors.  The  three 
elements  most  likely  to  be  lacking  in  a  soil  from  a  food 
standpoint  are  nitrogen,  phosphorus,  and  potassium. 
They  may  be  designated  as  the  primary  elements  for 
plant  growth.  The  other  elements  are  usually  present 
in  amounts  many  times  greater  than  will  ever  be  needed 
by  crops.  Calcium,  while  necessary  in  large  quantities 
in  a  soil,  is  largely  an  amendment,  and  very  seldom  may 
limit  plant  growth  because  of  being  in  too  minute  quantity 
to  supply  the  food  needs  of  a  crop.  The  liming  of  a  soil 
is  for  other  purposes  than  the  supplying  of  calcium  for 
plant  nutrition.  Sulfur  is  supposed,  in  certain  soils, 
to  limit  plant  growth  because  of  its  insufficiency,  but 
ordinarily  it  is  never  found  in  a  minimum  quantity. 

Nitrogen  exists  in  the  soil  largely  as  a  portion  of  the 

1  Sodium,  silicon,  and  aluminium  are  found  in  plants,  but  are 
not  essential  to  proper  growth. 


SOME  GENERAL   CONSIDERATIONS  5 

partially  or  wholly  decayed  organic  matter  present  therein. 
It  is  utilized  by  the  plant  ordinarily  in  the  form  of  nitrate. 
The  atmosphere,  composed  of  four-fifths  nitrogen  by 
volume,  has  been  the  original  source  of  this  element; 
and  through  natural  processes  which  are  continually 
at  work  the  nitrogen  has  been  transferred  to  the  soil. 
The  encouragement  of  this  natural  fixation,  thus  draw- 
ing upon  the  great  body  of  gas  surrounding  the  earth, 
has  become  of  great  practical  importance  in  agricultural 
operations. 

Phosphorus  has  its  origin  in  the  mineral  apatite  and 
exists  in  most  soils  largely  as  a  tricalcium  phosphate 
(Ca3(P04)2).  In  case  of  a  lack  of  lime  or  of  the  presence 
of  considerable  quantities  of  humus,  phosphorus  may  be 
present  as  ferric  or  aluminium  phosphates  or  as  organic 
phosphoric  acid.  Phosphorus  is  probably  taken  up  by 
the  plant  as  the  mono- or  di-calcic  phosphate  (CaH^PO^ 
or  Ca2H2(P04)2). 

The  potassium  of  the  soil  exists  largely  in  feldspar 
(K20 .  A12C>3 .  6  Si02),  in  mica,  or  in  hydrated  aluminium 
silicates,  which,  while  rather  insoluble,  supply  potash 
to  the  soil  solution  in  the  bicarbonate,  chloride,  nitrate, 
or  sulfate  forms.  It  is  from  such  compounds  that  the 
plant  draws  upon  the  soil  for  this  element. 

4.  Abundance  of  plant-food  elements.  —  Having  con- 
sidered the  plant-food  elements,  especially  those  of  primary 
importance,  it  is  of  interest  to  note  their  distribution  in 
the  earth's  crust.  Clarke 1  estimates  the  composition 
of  the  lithosphere,  which  makes  up  93  per  cent  of  the 
known  terrestrial  matter,  as  follows :  — 


1  Clarke,  F.  W.    Data  of  Geochemistry.     U.  S.  Geol.  Survey, 
Bui.  491,  p.  33.     1911. 


SOILS:    PROPERTIES  AND  MANAGEMENT 


Oxygen 
Silicon 
Aluminium 
Iron       .     . 
Calcium     . 
Magnesium 


47.17 
28.00 
7.84 
4.44 
3.42 
2.27 


Sodium 

Potassium 

Hydrogen 

Carbon 

Sulfur 

Phosphorus 


2.43 
2.49 
.23 
.19 
.11 
.11 
i.*7 


The  briefest  scrutiny  of  this  table  reveals  the  fact  that 
the  lighter  elements  are  the  more  abundant  in  the  earth's 
crust.  The  first  four  elements  make  up  eighty-seven  per 
cent,  while  the  primary  elements  of  plant  growth  either 
are  lacking  or  are  present  only  in  very  small  quantities. 

5.  Soil-forming  rocks.  —  As  has  been  stated,  ordinary 
soil  is  made  up  largely  of  inorganic  matter  which  is  derived 
from  ground-up  rock  material.  Therefore,  in  any  study 
of  soil  origin  or  formation,  however  cursory,  the  atten- 
tion must  be  directed  toward  geological  conditions,  not 
because  of  their  mere  geological  interest  but  because  of 
their  ultimate  bearing  on  soil  fertility  and  crop  growth. 
In  the  soil  we  expect  to  find,  and  do  find,  fragments  of 
the  commonest  rocks,  because  those  most  exposed  and 
those  present  to  the  largest  extent  at  the  earth's  surface 
must  be  the  ones  to  break  down  into  soil.  Therefore 
the  commonest  soil-forming  rocks  are  the  rocks  that 
are  met  so  commonly  in  the  field.  They  may  be  classified 
broadly  under  three  heads  —  igneous,  sedimentary,  and 
metamorphic.     Some  of  the  common  types  are  as  follows : 


Igneous 

Sedimentary 

Metamorphic 

Granite 

Limestone 

Schist 

Syenite 

Sandstone 

Gneiss 

Diorite 

Shale 

Marble 

Diabase 

Dolomite 

Slate 

Gabbro 

Quartzite 

Peridotite 

SOME  GENERAL   CONSIDERATIONS  7 

The  igneous  rocks  furnish  material  for  the  formation 
of  the  types  constituting  the  other  groups.  They  may 
be  divided  in  a  general  way  into  two  classes  —  one  con- 
taining a  high  percentage  of  silica  and  some  free  quartz, 
the  other  having  a  medium  or  low  silica  content  and  no 
quartz.  The  former  is  designated  as  acid,  and  the  latter 
as  basic  since  it  contains  a  high  percentage  of  the  alkalies 
and  the  alkaline  earth  minerals.  Granite  and  gabbro 
are  excellent  examples,  respectively,  of  these  general 
groups  of  rock. 

The  sedimentary  rocks,  formed  from  material  derived 
from  the  igneous  rocks,  have  been  deposited  usually 
under  fresh-  or  salt-water  conditions.  The  development 
of  pressure  has  in  many  cases  been  instrumental  in  the 
consolidation  of  this  material.  The  limestone  and  the 
dolomite  deposited  by  precipitation  may  be  expected 
to  be  comparatively  soluble  rocks.  Shale  is  merely  a 
more  or  less  hardened  clay,  while  sandstone  varies  ac- 
cording to  the  cementing  material  which  serves  to  hold 
its  sand  grains  together.  This  cement  may  be  iron 
(Fe203),  calcium  carbonate  (CaCOs),  or  silica  (SiQa). 

The  action  of  heat,  usually  with  pressure,  on  either 
igneous  or  sedimentary  rocks,  results  in  the  third  group, 
the  metamorphic.  Thus,  granite  on  metamorphosis 
may  form  either  a  gneiss  or  a  schist ;  limestone  or  dolomite 
may  form  marble ;  shale  may  form  slate :  and  sandstone 
may  form  quartzite. 

On  examination,  ordinary  rock  is  found  to  be  composed 
of  one  or  more  minerals.  In  other  words,  it  is  a  mineral 
aggregate.  The  mineral,  in  turn,  is  a  natural  compound 
of  approximately  a  constant  chemical  composition,  usually 
displaying  a  crystalline  form  and  other  well-defined 
physical    properties.     In    order    to    illustrate    the    com- 


8  SOILS:    PROPERTIES  AND   MANAGEMENT 

plication  that  may  arise,  the  mineral  composition  of  some 
common  rocks  is  given  below :  — 

Granite  —  Quartz,  orthoclase,  and  plagioclase  with  mica 
and  hornblende. 

Syenite  —  Orthoclase  and  mica  with  hornblende  and 
augite. 

Basalt  —  Plagioclase  and  hornblende  or  augite  with 
apatite,  pyrite,  and  mica. 

Peridotite  —  Olivine  with  augite,  pyrite,  mica,  and  horn- 
blende. 

Limestone  —  Calcium  or  magnesium  carbonate  with  traces 
of  silica  and  iron. 

Sandstone  —  Sand  cemented  with  iron,  silica,  or  calcium 
carbonate. 

This  complex  character  of  rocks  has  an  important 
bearing  on  the  question  of  soil  formation,  since  the  presence 
or  absence  of  certain  minerals  may  have  considerable  in- 
fluence on  the  physical  or  chemical  characteristics  of  the 
resultant  soil.  It  is  the  minerals,  therefore,  rather  than 
the  rocks  themselves,  that  must  be  looked  to  in  a  study 
of  the  great  mass  of  inorganic  matter,  some  active  and 
some  inactive,  which  makes  up  the  bulk  of  ordinary 
soils.  The  question  of  the  composition  of  a  soil  thus 
becomes  more  intensely  geologic  as  we  proceed. 

6.  Soil-forming  minerals.  —  A  great  many  minerals 
have  been  discovered,  studied,  and  classified,  but  only  a 
comparatively  few  occur  in  any  abundance  in  the  normal 
soil.  Nevertheless,  it  may  be  said  that  practically  all 
soils  contain  all  the  common  rock-forming  minerals. 
This  is  to  be  expected,  as  fragments  of  practically  all  the 
common   rocks  go  to  make  up  an  ordinary  soil.     The 


SOME  GENERAL    CONSIDERATIONS 


9 


following  list  will  give  some  idea  of  the  minerals  normally 
present  in  soils  :  — 

COMMON    SOIL-FORMING    MINERALS 

1.  Quartz  .     .     Si02 

2.  Orthoclase     K20  .  A1203 .  6  Si02 

3.  Plagioclase    Na20  .  A1203 .  6  Si02,  CaO  .  A1203 .  2  Si02 

or  combinations 

4.  Hornblende   Chiefly  Ca(MgFe)3SiA2  with 

Na2Al2Si4Oi2  and  (MgFe)2 .  (AlFe)2 . 
Si20i2 

5.  Augite  .     .     Chiefly      CaMgSi206      with 

(AlFe)2Si206 

6.  Muscovite     2  H20  .  K20  .  3  A1203 .  6  Si02 


(MgFe) 


7.  Biotite  .     . 

8.  Olivine      . 

9.  Serpentine 

10.  Epidote     . 

11.  Apatite 

12.  Zircon  .     . 

13.  Chlorite     . 

14.  Calcite 

15.  Dolomite  . 

16.  Gypsum 

17.  Talc      .     . 

18.  Hematite  . 

19.  Siderite 

20.  Limonite   . 

21.  Kaolinite  . 

22.  Zeolites     . 


(Si04); 


6SiOs 


(HK),  (MgFe),  (AlFe), 

2  (MgFe)O .  Si02 

3  MgO  .  2  Si02 .  2  H20 

H20.4Ca0.3(AlFe)203 

3  Ca3P208  +  (CaFl2)      or      (CaCl2)     or 

combinations 
Zr02 .  Si02 

H40(FeMg)23Ali4Sii3O90 
CaC03 

CaC03 .  MgC03 
CaS04 .  2  H20 
H20.3Mg0.4Si02 
Fe203 
FeC03 

2  Fe203 .  3  H20 
2  H20  .  A1203 .  2  Si02 
Complex  hydrated  aluminium  silicates  of 

Ca,  K,  and  Na  as  Philolite  (CaK2N2) 

Al2Sii0O24 .  5  H20 


10         SOILS:    PROPERTIES  AND   MANAGEMENT 

There  are  certain  of  these  minerals  that  merit  especial 
attention  because  of  particular  attributes  which  they  may 
impart  to  a  soil.  Quartz,  for  example,  is  very  common 
in  all  soils,  making  up  usually  from  85  to  99  per  cent  of 
their  composition.  It  is  a  makeweight  material,  how- 
ever, as  it  is  used  to  a  very  slight  extent  by  most  plants ; 
but  it  adds  a  stability  to  the  soil  that  perhaps  the  soil 
would  not  otherwise  have,  and  this  function  is  of  con- 
siderable significance.  Of  greater  importance  from  the 
plant-food  standpoint  are  the  feldspars,  of  which  orthoclase 
is  probably  primary  because  it  is  the  source  and  store- 
house of  the  soil  potash.  Acted  upon  by  physical  and 
chemical  agencies,  it  slowly  supplies  the  soil  solution  with 
potassium,  which  in  turn  nourishes  the  plant.  The  micas 
also  may  furnish  considerable  potash  for  crop  growth. 
The  plagioclase,  instead  of  being  rich  in  potassium,  as 
the  formula  indicates,  contain  the  more  basic  elements, 
calcium  and  magnesium,  as  also  do  the  pyroxenes  and 
amphiboles  represented  by  augite  and  hornblende.  Olivine 
and  serpentine,  also  silicates,  are  particularly  rich  in 
magnesium.  Practically  all  the  phosphorus  in  the  soil, 
either  organic  or  inorganic,  has  had  its  origin  in  the 
mineral  apatite ;  yet  this  mineral  is  present  in  rocks  and 
soil  usually  in  very  small  quantities,  making  up  not  more 
than  0.6  per  cent  of  the  bulk  of  igneous  rocks.  More- 
over it  is  a  rather  insoluble  material.  This  fact,  together 
with  the  small  quantities  occurring  in  soil-forming  rocks, 
may  account  for  the  need  of  phosphorus  in  many  otherwise 
fertile  soils. 

Calcium,  so  important  as  a  basic  material  in  soil,  may 
be  supplied  to  a  certain  extent  by  other  minerals  besides 
those  already  named  —  calcite,  dolomite,  and  gypsum 
being  perhaps  the  most  important,  especially  the  calcium 


SOME  GENERAL   CONSIDERATIONS  11 

carbonate  in  either  the  crystalline  or  the  amor- 
phous forms.  Plenty  of  calcium  in  a  soil  tends  not  only 
to  better  physical  conditions,  but  also  to  improve  chemical 
reactions  and  biological  activity.  The  iron  of  the  soil 
minerals  is  of  importance  in  its  color  relationships,  for 
when  oxidized  to  the  hematite  form  a  bright  red  may  be 
imparted,  while  a  yellow  may  result  if  limonite  is  pro- 
duced. Color  has  great  significance  in  a  general  estimate 
of  soil  productivity  and  is  always  an  important  factor  in 
soil  identification  and  survey.  The  tendency  of  most 
iron  compounds  in  the  soil  is  toward  the  hematite  or  the 
limonite  form  when  subjected  to  oxidation  and  hydration. 

Kaolinite  is  a  product  of  rock  decomposition  and  is 
considered  to  be  of  considerable  importance  in  most  clays 
or  clay  loams.  It  is  almost  always  impure  and  in  this 
form  is  designated  as  kaolin.  Kaolin  and  the  soil  zeolites, 
which  are  hydrated  aluminium  silicates  carrying  chiefly 
calcium,  sodium,  and  potassium,  are  really  the  end  prod- 
ucts of  rock  decay  and  therefore  are  secondary  minerals. 
Consequently  they  must  always  be  considered  in  any 
study  of  soil  formation  or  of  soil  utilization,  particularly 
as  they  may  serve  to  enrich  the  soil  solution  in  plant-food 
held  by  them  in  physical  and  chemical  combinations. 

7.  Relative  abundance  of  minerals.  —  D'Orbigny  1  pre- 
sents the  following  table  as  a  result  of  his  calculation  on 
the  distribution  of  certain  minerals  in  the  earth's  crust :  — 
Percentage  Percentage 

Feldspars 48     Carbonates       ....     1 

Quartz 35     Hornblende,  augite,  etc.     1 

Mica 8     All  other  minerals     .     .     2 

Talc 5 

1  Hall,  A.  D.     The  Soil,  p.  16.     New  York  City.     1907. 


12  SOILS:    PROPERTIES  AND  MANAGEMENT 

This  agrees  in  general  with  the  distribution  of  these 
minerals  in  the  earth's  surface  and  accounts  for  their 
universal  presence  in  all  soils. 

8.  Organic  matter.  —  The  minerals  as  listed  account 
for  all  the  elements  of  plant-food  obtained  from  the 
soil  except  nitrogen,  which,  as  already  indicated,  is  found 
very  largely  locked  up  in  proteid  and  other  nitrogenous 
material.  The  incorporation  of  organic  matter  in  any 
soil,  either  by  natural  or  by  artificial  means,  besides 
tending  to  better  its  physical  condition  also  enriches 
it  in  its  total,  or  gross,  nitrogen  content.  Though  this  or- 
ganic matter  is  so  necessary  in  a  fertile  soil,  its  addition 
and  thorough  incorporation  occurs  late  in  the  process  of 
soil  formation.  Through  the  agency  of  bacteria  and 
other  organisms  the  organic  compounds  are  slowly  sim- 
plified, new  compounds  are  split  off,  and  nitrogen  is  in- 
troduced into  the  soil  solution  mainly  as  nitrate,  which 
is  one  of  the  principal  forms  in  which  it  may  be  used  by 
plants  growing  on  the  soil. 

9.  The  soil  and  the  plant.  —  Observed  from  the  agri- 
cultural standpoint,  then,  the  soil  becomes  purely  a 
medium  for  crop  production.  Its  composition,  both 
mineral  and  organic,  is  of  vital  importance  in  the  further- 
ance of  such  a  use.  All  the  physical,  chemical,  and 
biological  agencies  become  directed  toward  this  end. 
The  study  of  the  soil  and  a  better  understanding  of  its 
function  will  allow  the  great  class  of  landowners  not 
only  to  increase  their  crops,  and  consequently  their 
profits,  but  at  the  same  time  to  maintain  as  far  as  possible 
the  fertility  of  our  greatest  national  resource.  A  rational 
study  of  the  soil  should  ultimately  lead  to  a  study  of 
conservation  in  its  bearing  both  to  present  prosperity 
and  to  the  welfare  of  posterity. 


CHAPTER  II 

> 

SOIL-FORMING  PROCESSES 

After  the  first  proper  estimate  of  the  relations  between 
the  crop  and  the  soil,  the  next  step  is  toward  the  mode  of 
soil  formation  and  the  agencies  concerned.  As  might  be 
expected,  this  is  a  complicated  problem  from  the  fact 
that  most  rocks  are  so  heterogeneous  in  their  composition. 
The  question  becomes  still  further  involved  because  of 
the  many  factors  that  are  continually  functioning  in  rock 
decay.  This  process  of  the  breaking  down  of  rock  masses 
and  their  gradual  evolution  into  soil  is  called  weathering.1 
Rock  weathering  may  be  defined  specifically  as  the  changes 
that  rock  masses  undergo  due  to  the  physical  and  chemical 
activities  of  atmospheric  agents.  Everything  on  the 
earth's  surface  is  seeking  a  more  stable  condition,  and 
therefore,  from  a  geological  standpoint,  is  continually 
changing.  If  a  soil  represents  a  more  stable  condition 
than  the  exposed  rock,  the  rock  slowly  evolves  toward 
the  soil.  Again,  if  a  soil  presents  constituents  not  wholly 
stable,  that  soil  will  change  by  an  elimination  or  an 
alteration  of  these  components.  The  soil,  then,  is  a 
geologic  unit.  It  is  a  transition  product  from  one  con- 
dition to  another. 

This  weathering,  which  brings  about  such  changes  and 
is  such  a  factor  in  the  modifications  of  our  topography,  is 

1  For  a  complete  discussion  of  weathering,  see  Merrill,  G.  P. 
Rocks,  Rock  Weathering,  and  Soils.     New  York.     1906. 

13 


14         SOILS:    PROPERTIES  AND  MANAGEMENT 

very  superficial  and  affects  the  earth  to  but  relatively 
shallow  depths  However,  from  the  fact  that  it  provides 
a  medium  for  crop  growth  and  at  the  same  time  is  largely 
instrumental  in  maintaining  the  fertility  of  this  medium, 
its  agencies  and  processes  become  of  great  significance. 

The  forces  of  weathering,  while  very  diverse  not  only 
as  to  action  but  also  as  to  product,  permit  of  an  outline 
so  clear  that  the  true  relationships  at  once  become  ap- 
parent. This  classification  may  be  made  under  two 
heads,  mechanical  and  chemical,  as  follows :  — 

Forces  of  weathering 

I.   Mechanical  changes,  or  disintegration 

A.  Erosion  and  denudation 

Water,  wind,  ice 

B.  Temperature 

Heat  and  cold,  and  frost 

C.  Plants  and  animals 

II.   Chemical  changes,  or  decomposition 

A.  Oxidation  and  carbonation 

B.  Deoxidation 

C.  Hydration 

D.  Solution 

10.  Water.  —  The  three  great  agencies  of  erosion  and 
denudation  are  water,  wind,  and  ice.  They  are  instru- 
mental not  only  in  the  breaking  up  of  rocks,  but  also  in 
transporting  the  resultant  materials.  Water  is  especially 
of  importance,  as  its  denuding  effects  are  very  rapid  when 
viewed  over  geological  periods.  It  is  estimated  that 
the  United  States  is  being  planed  down  at  the  rate  of 
one  inch  in  seven  hundred  and  sixty  years.  This  is  rapid 
enough  to  dig  the  Panama  Canal  in  seventy-three  days. 


SOIL-FORMING  PROCESSES  15 

The  water,  in  order  to  be  a  successful  cutting  agent,  must 
be  laden  with  sediment,  so  that  its  carrying  power  largely 
determines  its  power  of  erosion.  In  other  words,  it  must 
be  armed. 

From  the  time  when  the  raindrops  beat  down  on  a 
surface  until  they  have  been  gathered  into  rivulets  and 
streams  and  finally  discharged  into  the  ocean,  they  are 
engaged  in  moving  the  detrital  matter  already  produced. 
The  Mississippi  River  is  working  fast  enough  at  the 
present  time  to  reduce  the  continent  of  North  America 
to  sea  level  in  four  million  years.  The  Appalachian 
Mountains,  born  in  Paleozoic  times,  have  lost  vastly 
more  material  than  now  remains  for  us  to  view.  Our 
river  and  lake  soils  are  due  to  the  cutting  and  carrying 
power  of  the  streams.  The  deltas,  and  the  marine  soils 
of  the  Atlantic  and  Gulf  coasts,  afford  other  examples 
of  such  effects.  The  continual  pounding  and  grinding 
of  waves  are  no  mean  factor  in  rock  disintegration.  The 
rounding  of  the  sands  is  a  mute  evidence  of  this  great 
force. 

11.  Wind.  —  The  wind  as  a  soil-forming  agent  has, 
like  water,  two  phases  of  action  —  erosive  and  transpor- 
tive.  Sweeping  over  the  land  in  dry  weather,  it  has  the 
power  of  picking  up  innumerable  fine  particles  which 
may  abrade  rocks  very  noticeably  over  a  term  of  years. 
The  fluting  of  exposed  rocks,  especially  in  arid  regions, 
the  undermining  of  cliffs,  and  the  polishing  of  stones 
to  a  smoothness  equal  to  that  of  glass,  are  frequent  occur- 
rences. The  roughening  of  windowpanes  in  houses  near 
the  seashore  during  severe  storms,  and  the  illegibility 
of  old  tombstones,  are  of  common  record.  Great  areas 
of  soil  have  been  deposited  by  winds,  especially  in  the 
United  States.     The  loess  of  the  Mississippi  Valley  and 


16         SOILS:    PROPERTIES  AND  MANAGEMENT 

the  adobe  of  the  Southwest  owe  their  origin,  at  least 
partially,  to  the  carrying  power  of  wind  at  a  time  when 
aridity  existed  over  all  this  area. 

12.  Ice.  —  When  in  large  bodies,  as  in  glaciers,  ice 
exerts  a  tremendous  grinding  power.  Glacial  ice,  by  its 
mobility  and  motility,  adapts  itself  to  all  topography, 
and  as  it  moves  slowly  forward  it  grinds  and  scours  and 
abrades  even  the  hardest  rocks.  The  great  masses  of 
pebbles  and  rocks  which  are  picked  up  and  imbedded 
by  glaciers,  especially  in  their  lower  surfaces,  increase 
their  cutting  power  many  fold.  The  effect  of  glaciers 
is  of  particular  interest  because  of  the  fact  that  all  of 
the  northern  part  of  the  United  States  was  covered  at 
one  time  with  a  great  ice  sheet,  and  our  northern  soils 
are  due  either  directly  or  indirectly  to  the  advances  and 
retreats  of  this  ice  sheet.  Formed  in  northern  latitudes 
due  to  a  change  in  climatic  conditions,  the  ice  sheet 
slowly  covered  many  thousands  of  square  miles  of  terri- 
tory, and  as  the  ice  was  usually  several  thousand  feet 
thick,  hills,  and  often  mountains,  were  overridden.  Their 
tremendous  weight  made  the  grinding  action  almost 
irresistible.  In  the  retreat,  or  melting  back,  of  the  ice, 
a  mantle  of  this  ground-up  and  well-mixed  material  was 
deposited  as  soil;  while  the  streams  flowing  from  its 
front,  or  into  glacial  lakes,  were  furnished  with  heavy 
sediments  for  distribution  in  other  regions. 

13.  Heat  and  cold.  —  The  changes  in  temperature  of 
the  air,  and  the  soils  and  rocks,  tend  vastly  to  augment 
the  effect  of  the  denuding  agents.  Constant  expansion 
and  contraction  is  productive  of  weakness  and  ultimate 
physical  breakdown.  Heat  is  conducted  slowly  through 
rocks,  thus  leading  to  differential  heating  and  unequal 
expansion  or  contraction.     Rocks,  as  already  noted,  are 


SOTL-FOBMING  PROCESSES  17 

usually  mineral  aggregates,  and  these  minerals  vary 
in  their  coefficients  of  expansion.  With  every  change 
of  temperature,  differential  stresses  are  set  up  which 
ultimately  must  produce  a  considerable  effect.  When 
the  separate  minerals  expand  they  expand  differently, 
and  when  they  contract  they  never  again  assume  quite 
their  former  relationships  to  one  another.  Thus  crevices, 
cracks,  and  rifts  are  created  in  rocks,  especially  those  of 
heterogeneous  mineral  composition.  The  expansion  coef- 
ficient of  granite  is  .0000048  of  an  inch  to  a  foot  for  every 
degree  Fahrenheit,  while  that  of  marble  is  about  .0000056 
of  an  inch.  This  seems  to  be  very  slight,  but  it  must  be 
remembered  that  under  natural  conditions  large  surface 
areas  of  rock  are  concerned.  A  sheet  of  granite  100  feet 
long  will  expand  one-half  an  inch  with  a  change  of  75° 
Fahrenheit,  which  is  not  an  uncommon  variation  of 
temperature  in  arid  regions  or  high  altitudes.  This 
leads  to  chipping,  flaking,  and  exfoliation.  The  rock 
fragments  may  range  from  microscopic  sizes  to  large 
blocks,  which  are  often  split  off  witH  great  violence. 

14.  Frost.  —  Great  as  is  the  action  of  a  simple  change 
of  temperature,  its  effects  become  many  fold  more  ap- 
parent when  water  is  present.  We  then  have  the  action 
of  frost.  The  cracks  and  crevices  made  by  heat  and  cold 
will  in  a  humid  region  become  filled  with  water.  This 
moisture,  on  freezing,  exerts  a  very  great  force.  The 
expansive  power  of  water  passing  from  the  liquid  to  the 
solid  state  is  equal  to  about  150  tons  to  a  square  foot, 
which  is  equivalent  to  the  weight  of  a  column  of  rock 
about  a  third  of  a  mile  in  height.  Moreover,  most  rocks 
contain  a  certain  amount  of  water  in  themselves.  This 
water  is  recognized  in  excavation  operations  as  quarry 
water.     The  passage  of  the  quarry  water  to  a  solid  state 


18  SOILS:    PROPERTIES  AND  MANAGEMENT 

must  result  most  disastrously  to  the  physical  condition 
of  the  rock.  This  action  of  frost  is  by  no  means  com- 
plete when  the  rock  is  fined  mechanically  to  a  soil,  but  is 
continued  on  the  soil  itself.  Such  further  fining  is  of 
the  greatest  importance  in  bettering  the  physical  condi- 
tion of  the  soil,  and  is  usually  designated  as  a  wetting  and 
drying  and  freezing  and  thawing  process.  It  is  to  such 
forces,  more  than  to  any  other  action,  that  the  farmer 
owes  the  good  tilth  of  his  soil. 

15.  Plants  and  animals.  —  Plants  and  animals  unite 
their  forces  with  those  already  mentioned  to  bring  about 
further  physical  change.  Unlike  the  modifications  due  to 
erosion,  denudation,  and  temperature,  these  agencies  affect 
the  soil  to  a  greater  extent  than  they  affect  the  parent  rock. 
In  other  words,  they  begin  their  work  after  the  minerals 
have  been  reduced,  at  least  partially,  to  the  form  of  a 
soil.  Simple  plants,  as  mosses  and  lichens,  will  develop 
readily  on  rock  ledges  and  coarse  rock  fragments.  They 
send  their  rootlets  into  the  crevices  and  exert  a  prying 
and  loosening  effect.  They  also  catch  dust,  provide 
humus,  and  gradually  accumulate  a  soil  in  which  higher 
and  still  higher  species  of  plants  may  grow.  Their 
chemical  effects,  especially  regarding  solution  and  oxida- 
tion, aid  in  this  disintegration.  The  distribution  of 
organic  matter  through  the  soil  by  the  extension  and 
death  of  plant  roots  is  of  no  mean  importance  in  soil 
fertility.  Bacteria  also  may  be  a  factor  in  rock  decay, 
not  only  through  their  action  on  the  humus  material 
but  also  through  a  direct  attack  on  the  rocks  themselves. 
Their  influence,  however,  is  probably  mostly  chemical. 

Animals  also  effect  the  fining  of  rock  fragments  and 
soils,  from  their  burrowing  and  mixing  tendencies.  Such 
rodents  as  gophers  and  squirrels  open  up  the  soil,  thus 


SOIL-FORMING   PROCESSES  19 

providing  better  circulation  of  air  and  water.  This 
brings  about  a  deeper  and  more  effective  action  of  the 
other  physical  agencies  of  weathering.  Earthworms 
produce  similar  effects.  Their  holes  provide  channels 
for  ready  drainage,  and  large  quantities  of  soil  are  brought 
to  the  surface  yearly  by  them.  Darwin  estimates  that 
this  amounts  to  as  much  as  one  or  two  inches  in  a 
decade. 

16.  Oxidation  and  carbonation.  —  The  physical  and 
chemical  forces  do  not  act  alone,  but,  as  a  general  thing, 
combine  in  their  effects.  Thus  one  set  of  factors  aids 
and  accelerates  the  other.  Scarcely  has  the  disintegration 
of  a  rock  begun,  then,  before  its  decomposition  is  also 
apparent.  Of  the  chemical  forces  oxidation  is  usually, 
especially  near  the  surface  of  the  earth,  the  first  to  be 
noticed.  It  is  perceptibly  manifested  in  rocks  carrying 
iron,  and  consists  in  such  a  change  that  the  added  oxygen 
may  be  accommodated.  Sulfides  readily  succumb  and 
become  oxides,  while  these  same  oxides  are  prone  to  take 
up  oxygen  to  their  fullest  extent.  This  oxidation  is  dis- 
closed by  a  discoloration  of  the  rock,  which  is  first  streaked 
and  stained  with  iron  oxide  but  at  last  changed  to  a 
uniform  ochre.  The  change  may  be  exemplified  by  the 
following  reactions :  — 

2FeS2  +  702  +  4H20  =  2FeO  +  4  H2S04 
4  FeO  +  02  =  2  Fe203  (red) 

While  not  all  the  minerals  contain  iron,  enough  of  them 
do  to  impart  a  fatal  weakness  to  most  rocks.  The  ferrous 
oxide  (FeO),  being  soluble,  is  washed  out  and  the  rock  is 
creviced  and  crumbled.  A  way  is  now  open  for  more 
energetic  physical  and  chemical  decay. 

With  the  oxidizing  action  there  is  also  the  influence  of 


20         SOILS:    PROPERTIES  AND  MANAGEMENT 

carbon  dioxide  (C02),  which  is  universally  a  constituent 
of  air  and  is  a  product  of  the  decaying  vegetable  matter 
present  in  most  soils.  This  means  that  the  water  cir- 
culating among  rock  fragments,  especially  those  of  a  soil, 
is  heavily  charged  with  this  compound.  The  carbon a- 
tion  may  be  illustrated  as  follows :  — 

2  FeS2  +  7  02  +  4  H20  +  2  C02  =  2  FeC03  +  4  H2S04  or 
2  NaOH  +  C02  =  Na2C03  +  H20 

17.  Deoxidation.  —  Deoxidation  is  an  opposite  re- 
action to  oxidation,  being  a  loss  of  oxygen  either  to  the 
air  or  to  some  other  compound.  With  hematite  it  might 
take  place  as  follows  :  — 

2  Fe203  -  02  =  4  FeO 

Under  normal  conditions,  however,  it  is. not  a  very  im- 
portant factor,  since  most  rock  fragments  and  soil  are 
fairly  well  aerated,  at  least  too  well  aerated  to  allow  this 
reverse  process  to  occur.  In  poorly  drained  soil  or  in 
soil  very  rich  in  humus  and  carrying  organic  acids  it 
may  occur,  and  is  usually  manifested  by  the  development 
of  blue  and  gray  colors,  indicating  that  a  reduction  has 
taken  place.  The  bleaching  of  sands,  sandstones,  and 
clays  may  be  due  partially  to  this,  and  also  to  a  removal 
of  the  ferriferous  salts  in  solution.  Some  subsoils  dis- 
play this  phenomenon.  The  average  farmer,  however, 
need  not  concern  himself  with  the  injuries  that  may  re- 
sult from  deoxidation. 

18.  Hydration.  —  Hydration  usually  accompanies  oxi- 
dation, but  when  occurring  at  great  depths  it  may  be 
practically  the  only  change  the  minerals  have  imdergone. 
Minerals,  especially  feldspars,  become  clouded  and  lose 
their  luster  on  this  assumption  of  chemically  combined 


SOIL-FORMING   PROCESSES  21 

water.  There  is  also  a  considerable  increase  in  bulk,  this 
being  often  as  much  as  88  per  cent  during  the  transition 
of  a  rock  to  a  soil.  Hydrated  minerals,  while  apparently 
sound,  quickly  succumb  when  exposed  to  forces  of  weather- 
ing which  are  more  superficial  in  their  effects.  Car- 
bonation  and  oxidation  usually  take  place  as  correla- 
tive actions  with  hydration.  A  simple  example  of 
hydration  is  shown  in  the  change  of  hematite  to  limo- 
nite,  which  occurs  in  practically  every  case  when  iron 
is  allowed  to  oxidize  from  pyrite  or  a  simpler  oxide  to 
the  higher  forms  :  — 

2  Fe203  (red)  +  3  H20  =  2  Fe203 .  3  H20  (yellow) 

19.  Solution.  —  As  it  is  now  quite  evident  that  weather- 
ing, especially  the  chemical  manifestations,  is  largely  a 
simplification  of  compounds,  and  that  water  is  almost 
universally  present,  some  solution  must  occur.  These 
simple  materials  are  particularly  prone  to  enter  solution 
because  of  the  presence  of  carbon  dioxide,  which,  by 
acidifying  the  soil  water,  intensifies  its  solvent  action  to 
a  considerable  extent  and  consequently  increases  its  power 
as  a  weathering  agent.  The  atmosphere  contains  amounts 
of  this  gas  ranging  from  3.87  to  4.48  parts  in  10,000, 
while  considerable  amounts  are  brought  down  on  the 
rocks  and  the  soil  in  snow  and  rain.  The  carbon  dioxide 
evolved  directly  into  the  soil  water  from  decaying  organic 
matter  also  aids  in  keeping  the  soil  charged  with  this  gas. 
This  means,  then,  that  solution  is  largely  a  process  of 
carbonation,  especially  after  the  soluble  constituents  have 
been  thrown  out  into  the  soil  solution.  It  is  evident 
that  oxidation,  carbonation,  hydration,  and  solution  act 
in  unison  to  bring  about  the  chemical  decay  of  the  rock 
and  the  soil.     This  combined  action  may  be  represented 


22         SOILS:    PROPERTIES   AND  MANAGEMENT 

by  showing  the  various  stages  that  orthoclase  may  undergo 
in  producing  a  residual  clay :  — 

KAlSisOg  +  HOH  =  HAlSi308  +  KOH 

2  KOH  +  C02  =  K2C03  +  H20 
HAlSi308  -  2  Si02  =  HAlSi04  (kaolinite) 

The  silica  in  this  case  may  become  quartz  or  colloidal 
silica,  or,  what  is  more  probable,  may  unite  with  certain 
elements  to  produce  complex  hydrated  silicates. 

20.  A  general  statement  of  weathering.  —  The  question 
of  rock  weathering  is  complicated  because  no  one  action 
can  be  considered  alone.  All  forces  are  acting  together, 
tending  to  produce  a  great  complex  of  reaction  and  inter- 
action. No  amount  of  explanation  or  speculation  can 
ever  fully  clarify  the  question  as  to  the  formation  of  a 
soil  from  a  parent  rock.  Nevertheless,  knowing  in  general 
the  separate  forces  and  reactions  produced,  we  may  formu- 
late the  phenomenon  in  a  general  and  superficial  way. 
The  change  that  a  rock  undergoes  in  the  formation  of  a 
residual  clay  is  first  a  physical  breaking-down  accom- 
panied by  chemical  changes,  which  consist  in  the  hydra- 
tion of  the  feldspars,  the  oxidation  of  the  iron,  and  the 
solution  and  carbonation  of  the  soluble  bases. 

21.  Factors  affecting  weathering.  —  It  is  readily  to  be 
seen  that  the  activity  of  the  various  agencies  of  weather- 
ing will  be  modified  by  certain  factors  which  determine 
not  only  the  kind  of  rock  decay  but  also  its  rate.  Of 
these,  climate  is  probably  of  the  greatest  importance. 
The  difference  in  the  weathering  in  an  arid  region  as 
compared  to  that  in  a  humid  region  will  illustrate  this 
point.  Under  arid  conditions  the  physical  forces  will 
dominate  and  the  resulting  soil  will  be  coarse.  Freezing 
and  thawing,  heat  and  cold,  the  action  of  the  wind,  and 


SOIL-FORMING  PROCESSES  23 

the  effect  of  animals,  will  be  almost  the  sole  agents. 
In  humid  regions,  however,  the  forces  are  more  varied 
and  practically  the  full  quota  will  be  at  work.  Chemical 
decay  will  accompany  the  disintegration,  and  the  resultant 
product  will  be  finer  and  more  minutely  divided.  The 
separate  minerals  will  show  also  the  change  of  color  and 
loss  of  luster  due  to  the  decomposition  of  some  of  their 
essential  elements.  The  same  rocks,  then,  will  behave 
differently  under  different  climatic  conditions.  A  granite, 
for  instance,  is  a  very  insoluble  rock  as  compared  with  a 
limestone,  and  in  a  humid  region  where  chemical  agencies 
are  dominant  it  would  be  markedly  more  resistant.  If, 
however,  these  two  rocks  are  placed  under  arid  conditions 
where  the  physical  forces  are  potent,  particularly  as  re- 
gards change  of  temperature,  the  comparison  is  different. 
The  limestone,  being  homogeneous,  is  not  affected  by 
atmospheric  changes;  but  the  stresses  set  up  in  granite 
due  to  differential  contraction  and  expansion  must  ulti- 
mately reduce  it  to  fragments. 

As  weathering  is  confined  to  the  very  surface  of  the 
earth,  the  exposure  or  position  of  a  rock  will  determine 
the  kind  and  the  rate  of  decay.  If  the  rock  is  very  deep 
below  the  surface,  only  hydration  may  occur ;  while  if 
it  exists  as  an  exposed  ledge,  the  full  force  of  the  weather- 
ing agents  will  be  sustained.  If  the  debris  of  the  decayed 
rocks  is  not  removed,  this  serves  as  a  blanket  for  the 
protection  of  the  rocks  below.  The  transportive  powers 
of  weathering  are  important  in  maintaining  a  clean  sur- 
face for  action. 

The  texture  of  the  rock  is  also  a  factor.  Other  things 
being  equal,  a  coarsely  crystalline  rock  will  disintegrate 
and  decompose  more  rapidly  than  one  of  finer  grain. 
The  coarser  the  grain,  the  larger  the  amount  of  interstitial 


24         SOILS:    PROPERTIES  AND  MANAGEMENT 

space  and  the  greater  the  encouragement  to  physical 
agencies.  As  physical  changes  open  the  way  for  chemical 
decay,  coarse  texture  will  ultimately  encourage  decomposi- 
tion as  well  as  disintegration. 

Lastly,  the  disruptive  forces  of  the  rock  will  be  in- 
fluenced by  the  chemical  composition  of  the  minerals 
and  the  mineral  composition  of  the  rock.  A  rock  made 
up  of  minerals  that  offer  but  little  resistance  to  decay 
will  naturally  reduce  readily  and  quickly  to  a  soil. 
Rocks  that  very  largely  bear  minerals  which  are  re- 
fractory in  their  nature,  however,  may  never  decompose 
far  enough  or  rapidly  enough  to  give  a  soil  of  any  agri- 
cultural significance.  The  next  step,  then,  in  the  study 
of  soil  formation  is  a  consideration  of  the  relative  resistance 
of  the  minerals  and  the  rocks. 

22.  The  law  of  mineral  and  rock  decay.  —  Considerable 
work  has  been  done  on  the  comparative  solubility  of 
minerals  both  in  pure  and  carbonated  water,  but  in  most 
cases  it  has  proved  somewhat  inconclusive.  Neverthe- 
less we  are  able,  by  consulting  the  work  of  Miiller,1  Clark,2 
Daubree,3  and  others,  to  arrange  some  of  the  commoner 
minerals  in  the  order  of  their  solubility,  the  most  resistant 
minerals  heading  the  list :  — 

1.  Quartz  6.  Epidote  11.  Apatite 

2.  Muscovite  7.  Serpentine  12.  Olivine 

3.  Biotite  8.  Talc  13.  Calcite 

4.  Orthoclase  9.  Hornblende 

5.  Plagioclase        10.  Augite 

1  Miiller,  R.  Solution  of  Rocks  in  Carbonated  Water.  Jahrb. 
k-k  Geol.  Reichsanstalt,  Vol.  XXVII,  p.  25.     1877. 

2  Clark,  F.  W.  Data  of  Geochemistry.  U.  S.  Geol.  Survey, 
Bui.  330,  p.  401.     1908. 

3  Daubree,  A.  Solubility  of  Orthoclase.  Etudes  de  Geol. 
Experimentale,  pp.  27  and  252.     Paris,  1847. 


r 

SOIL-FORMjrfG  PROCESSES  25 

The  next  step  is  to  deduct  some  general  law  which  can 
be  shown  to  govern  the  resistance  of  these  minerals. 
Such  a  statement  would  aid  considerably  in  the  making  of 
general  deductions  regarding  weathering.  The  siliceous 
content  of  some  of  these  minerals,  taken  in  the  order  as 
above,  throws  considerable  light  on  this  phase :  — 

Per  cent  of  Si02  Per  cent  of  Si02 

Quartz 100      Hornblende  ....       45 

Orthoclase    ....      65      Olivine 41 

Plagioclase   ....      55      Calcite trace 

Another  case  might  be  cited  in  a  comparison  of  the 
chemical  composition  of  anorthite,  hornblende,  and 
olivine :  — 

Anorthite      .  CaAl2Si208 

Hornblende  .  Ca(MgFe)2(Si03)  with  {^ggSj 

Olivine     .     .  (MgFe)2Si04 

It  is  to  be  noted  that  immediately  as  the  resistance  of  a 
mineral  declines,  its  content  of  silica  decreases  and  the 
percentage  of  the  basic  constituents  increases.  Silica 
and  aluminium,  then,  mark  resistance  to  decay;  while 
calcium,  magnesium,  sodium,  potassium,  and  iron  func- 
tion in  increasing  susceptibility  to  decay.  The  law  of 
mineral  resistance  may  be  formulated  as :  "  The  more 
basic  a  rock  becomes,  the  more  rapid  is  its  decomposi- 
tion ;   and  the  more  acid,  the  less  marked  is  its  decay."  * 

This  general  law  certainly  should  apply  to  rocks  that 
are  made  up  of  the  minerals  listed  above.  One  example 
will  show  this  clearly.  The  igneous  rocks,  as  already 
stated,  may  be  divided  into  two  groups,  acid  and  basic. 

1  Buckman,  H.  O.  The  Formation  of  Residual  Clay.  Trans. 
Amer.  Ceramic  Soc,  Vol.  XIII,  p.  362.     February,  1911. 


26 


SOILS:    PROPERTIES  AND  MANAGEMENT 


This  acidity  and  basicity  is  determined  by  the  presence 
of  silica  and  the  alkalies,  respectively,  as  carried  by 
certain  essential  minerals.  Suppose  we  name  some 
representative  igneous  rocks  in  the  order  of  their  acidity, 
and  list  some  of  the  minerals  carried  by  them :  — 

1.  Granite     .     .     Quartz,  orthoclase,  and  mica 

2.  Diabase    .     .     Plagioclase,  mica,  hornblende,  or  augite 

3.  Peridotite      .     Principally  olivine 

It  is  to  be  seen  that  the  minerals  contained  by  granite 
are  more  resistant  than  those  carried  by  either  the  diabase 
or  peridotite,  while  the  olivine  of  the  last  group  is  near 
the  foot  of  the  list  when  the  minerals  are  arranged  in  the 
order  of  their  resistance. 

The  following  data x  bear  out  the  argument  presented 
above  as  to  the  relative  resistance  of  rocks :  — 

Proportional  Amounts  of  Fresh  Rocks  Soluble  in  Boil- 
ing Hydrochloric  Acid  and  Sodium  Carbonate  Solutions 


Si02  .  . 
Al203Fe203 
CaO  .  . 
MgO .  . 
K20  .  •. 
Na20       . 


Phonolite 

Diabase 

21.64 

10.85 

12.60 

15.65 

1.07 

3.09 

.40 

2.20 

.28 

1.21 

5.45 

.50 

41.44 

33.50 

Granite 

9.49 

8.36 

.60 

.71 

1.68 

1.23 

22.07 


It  is  evident,  then,  that  the  law  of  mineral  resistance 
applies  to  rocks  as  well  as  to  the  separate  minerals,  al- 
though its  application  thereto  is  much  more  complex 
and  difficult  to  interpret. 


1  Merrill,  G.  P.     Rocks,  Rock  Weathering,  and  Soils,  p.  366, 
New  York.     1906. 


SOIL-FORMING  PROCESSES 


27 


23.  Special  cases  of  weathering.  —  The  weathering  of 
granite  and  .limestone  under  different  climatic  conditions 
has  already  been  compared.  The  changes  that  take 
place  in  these  rocks  as  they  are  evolved  to  residual  clays 
may  now  be  considered.  The  following  analyses  serve 
to  show  on  what  elements  the  losses  are  likely  to  be  most 
serious  during  the  process:  — 

Fresh  Granite  and  its  Resultant  Clay1 


Si02  . 
A1203 . 
Fe203 . 
CaO  . 
MgO. 
K20  . 
Na20. 
P205  . 
Ignition 


Rock 


60.69 

16.89 

9.06 

4.44 

1.06 

4.25 

2.82 

.25 

.62 


Clay 


45.31 

26.55 

12.18 

00.00 

.40 

1.10 

.22 

.47 

13.75 


Percentage 
Lost 


52.45 
00.00 
14.35 
100.00 
74.70 
83.52 
95.03 
00.00 
Gain 


Virginia  Limestone  and  its  Residual  Clay2 


Si02 

A1203 

Fe203 

CaO 

MgO 

K20 

Na20 

p2o5 
co2 

H20 


Rock 


7.41 
1.91 

.98 

28.29 

18.17 

1.08 

.09 

.03 
41.57 

.57 


Clay 


57.57 
20.44 

7.93 
.51 

1.21 

4.91 
.23 
.10 
.38 

6.69 


Percentage 
Lost 


27.30 
00.00 
24.89 
99.83 
99.38 
57.49 
76.04 
68.78 
99.15 
Gain 


1  Merrill,  G.  P.     Bui.  Geol.  Soc.  Araer.,  Vol.  8,  p.  160.     1879. 
2Diller,  J.  S.     U.  S.  Geol.  Survey,  Bui.  150,  p.  385.     1898. 


28 


SOILS:    PROPERTIES  AND  MANAGEMENT 


Soils  have  resulted  in  both  cases  from  the  decay  of 
these  rocks.  In  the  case  of  the  granite  the  resulting  soil 
was  a  deep  red  clay,  with  quartz  grains  present.  The 
soil  from  the  limestone  was  a  plastic  clay,  high  in  silica 
and  aluminium.  Leaching  has  probably  gone  on  to  a 
very  great  extent  in  both  soils.  It  is  noticeable  also 
that  the  basic  constituents  have  suffered  the  greatest 
losses,  especially  calcium,  magnesium,  sodium,  and 
potassium.  The  carbonate  has  almost  wholly  disap- 
peared from  the  limestone  clay,  showing  that  a  limestone 
soil  may  not  necessarily  be  rich  in  lime.  As  a  matter  of 
fact,  the  chances  are  that  if  it  is  residual  it  will  be  lacking 
in  that  compound.  When  shown  diagrammatically  (See 
Figs.  1  and  2),  the  changes  that  the  parent  rocks  have 
undergone  chemically  in  forming  a  clay  will  become 
apparent. 


iS 

», 

\ 

Fe*C 

N 

/. 

fa 

\ 

\ 

Afr 

/ 

\ 

1gO 

\ 

L 

\ 

H 

o 

\ 

\ 

I 

/ 

\ 

\ 

/ 

a 

\ 

J 

(ajFresf)  roc/( 

\ 

( 

\> 

1* 

(b)G 

lay 

s 

Fig.   1.  —  Diagrammatic  representation  of  the  chemical  composition  of 
fresh  granite  and  its  residual  clay.     See  analyses  above. 


SOIL-FORMING  PROCESSES 


29 


As  shown  by  the  diagrams,  the  soil  from  the  granite  does 
not  differ  greatly  from  the  original  rock,  except  in  loss 
of  bases,  assumption  of  water,  and  increase  of  organic 


Fig.  2.  —  Diagram  showing  the  chemical  composition  of  Virginia  lime- 
stone and  its  residual  clay.     See  analyses  above. 


matter.  The  residual  clay  from  the  limestone  presents 
greater  differences,  due  to  the  almost  entire  disappear- 
ance of  calcium  carbonate.  The  diagrams  for  the  two 
clays  resemble  each  other  fairly  closely  in  spite  of  their 
widely  differing  sources.  Because  weathering  causes 
the  persistence  and  accumulation  of  silica,  aluminium, 
and  iron,  and  a  loss  of  the  basic  materials,  all  soils  as 
they  weather  tend  to  approach  a  similar  composition. 
Yet,  owing  to  a  difference  in  the  adjustment  of  the  forces 
at  work  and  in  the  time  element,  no  two  soils  will  ever 
be  exactly  alike.  Soils  will  differ,  then,  from  the  original 
rock  and  from  one  another  according  to  the  intensity 


80         SOILS:    PROPERTIES  AND  MANAGEMENT 

and  character  of  the  weathering  and  the  constitution  of 
the  parent  minerals. 

24.  Practical  relationships  of  weathering.  —  Weather- 
ing processes  result  in  a  general  simplification  of  com- 
pounds. Their  action  first  affects  the  rock,  with  the 
result  that  a  soil  is  produced ;  but  they  still  remain  ac- 
tive in  the  soil  after  it  is  in  a  condition  to  support  plants. 
The  physical  agencies  especially  tend  to  loosen  and  fine 
the  soil,  contributing  largely  to  its  tilth.  The  farmer 
encourages  such  influences  by  plowing  his  land  and  by 
other  operations.  Were  it  not  for  such  weathering 
action,  the  soil  would  become  physically  unable  to  afford 
foothold  for  plants.  The  continued  chemical  changes 
resulting  in  solution  and  carbonation  provide  a  soil  water 
rich  in  plant-food  nutriment.  Weathering,  then,  by  a 
slow  process  over  geologic  periods  has  provided  us  with 
soil,  and  by  the  same  slow  process  is  maintaining  the 
fertility  of  this  creation.  The  encouragement  and  control 
of  such  an  agency  is  of  no  small  importance  in  agricultural 
practice. 


CHAPTER  III 


THE  GEOLOGICAL  CLASSIFICATION  OF  SOILS 


Weathering  must  be  considered  as  affecting  soils  both 
in  situ  and  in  motion.  This  gives  two  general  classes  of 
materials  —  those  that  have  not  been  shifted  far  from 
their  place  of  origin,  and  those  in  the  formation  of  which 
the  transporting  agencies  have  been  instrumental.  These 
two  general  groups,  designated  as  sedentary  and  trans- 
ported,1 are  subject  to  considerable  subdivision,  as 
follows :  — 

Residual 


Sedentary 


Cumulose 


Transported 


Gravity  —  Colluvial 
[  Alluvial 

Water  I  Marine 

I  Lacustrine 

Ice  —  Glacial 

Wind  —  iEolian 


25.  Residual  soils.  —  This  group  of  soils  covers  wide 
areas  of  our  arable  regions  and  comes  from  many  kinds  of 
rocks.  Residual  soils  are  old  soils,  the  oldest  with  which 
we    have   to    deal    in    agricultural    operations.     Since    a 


1  See  Trowbridge,  A.  C.     A  Classification  of  Common  Sedi- 
ments.   Jour,  of  GeoL,  Vol.  22,  No.  4,  pp.  420-436.     1911. 

31 


32 


SOILS:    PROPERTIES  AND  MANAGEMENT 


residual  soil  is  formed  in  situ,  the  rocks  that  underlie  it, 
if  sound,  show  the  character  and  composition  of  the  rocks 
from  which  the  soil  was  actually  a  product.  In  such 
soils  the  changes  that  a  rock  undergoes  in  forming  a 
residual  clay  are  to  be  studied  to  the  best  advantage. 
An  examination  of.  the  various  grades  of  material  that  are 
found  overlying  the  country  rock  (Fig.  3)  in  an  area  where 


'-:'■    :/r\  '^~- : '«!<',- ■*      '"  "■        '  :    *..*•""-  ^ 


;?*•■*: 


Fig.  3.  —  The  gradual  transition  of  country  rock  into  residual  soil  by 
weathering  in  situ. 

this  residual  mantle  exists,  reveals  more  or  less  accurately 
the  gradations  from  rock  to  soil.  Residual  soils,  besides 
being  old  soils,  are  usually  nonsjjc&tified  and  present  a 
heterogeneous  mass  of  material.  Since  they  have  been 
subjected  to  leaching  over  vast  periods,  a  very  large 
amount  of  the  soluble  materials  have  been  washed  out, 
tending  to  leave  high  percentages  of  the  persistent  ma- 
terials, such  as  silica,  l'rnn^n^  aluminium  An  analysis 
of  an  Arkansas  limestone,  its  residual  clay  and  the  cal- 
culated percentage  loss  of  the  various  constituents  pres- 
ent in  the  fresh  rock,  illustrates  this  point :  — 


GEOLOGICAL   CLASSIFICATION  OF  SOILS  33 

Arkansas  Limestone  and  its  Residual  Clay  l 


Si02 

A1203 

Fe20.- 

MnO 

CaQL 

MgO 

K20 

Na20 

C02 


Fresh  Rock 

Clay 

4.13 

33.69 

4.19 

30.30 

2.35 

1.99 

4.33 

14.98 

44.79 

3.91 

.30 

.26 

.35 

.96 

.16 

.61 

34.10 

.00 

Percentage 
Lost 


.00 
11.35 

.  89.56 
57.59 
98.93 
89.38 
66.36 
53.26 

100.00 


The  vast  age  of  such  soils  tends  to  bring  about  great 
oxidation,  so  that  most  of  the  iron  has  changed  to  hematite 
and  limonite.  Since  almost  all  soils  contain  considerable 
iron,  the  prevailing  colors  of  residual  soils  are  reds  and 
yellows,  depending  on  the  degree  of  oxidation  and  hydra- 
tion. Grays  and  browns  may  exist,  however,  where  iron 
has  been  lacking  or  oxidation  has  been  feeble.  In  texture 
such  soils  usually  present  very  fine  conditions.  Having 
been  attacked  by  both  the  physical  and  the  chemical 
agencies,  the  particles  have  been  reduced  to  a  very  fine 
state  of  division.  Over  residual  areas  the  heavier  soils 
predominate,  as  silts,  silt  loams,  clays,  and  clay  loams. 
Very  often  sand  or  chert  may  l>e  present,  having  been  a 
constituent  of  the  original  rock  mass. 

An  examination  of  the  particles  of  a  residual  soil  usually 
shows  them  to  be  in  an  advanced  stage  of  decay.  The 
feldspars  have  lost  their  luster  and  have  become  opaque. 
The  iron  has  become  oxidized,  and  the  soluble  bases  have 


1  Penrose,  R.  A.  F. 
Vol.  I,  p.  179.     1890. 

D 


Ann.    Rept.  Geol.    Survey    Arkansas, 


34         SOILS:    PROPERTIES  AND  MANAGEMENT 

either  disappeared  or  changed  their  combinations  to  more 
stable  forms.  The  tendency  of  all  soils  is  toward  a  con- 
dition of  equilibrium,  and  consequently  they  approach, 
but  never  reach,  a  common  composition.  This  does  not 
apply  to  their  productivity,  because  many  other  factors 
besides  chemical  composition  go  to  determine  cropping 
power.  Residual  limestone  soils,  therefore,  become  poorer 
and  poorer  in  lime  as  their  age  increases.  The  organic 
matter  of  residual  soils  largely  depends,  in  amount  and 
condition,  on  climatic  factors.  If  rainfall  and  tempera- 
ture, for  instance,  are  favorable  for  the  rapid  and  con- 
tinued development  of  a  natural  vegetation,  the  soil 
will  be  rich  in  humus,  so  rich  at  times  as  to  mask  to  a 
certain  extent  the  red  color  so  characteristic  of  such  soils. 
If  plants  do  not  grow  well  on  this  soil,  however,  it  will  be 
low  in  organic  matter  and  probably  in  poor  physical  con- 
dition, so  vital  is  humus  to  a  proper  foothold  for  plant 
life.  Two  residual  soils  coming  from  the  same  kind  of 
rocks  may  vary  rather  widely  in  their  general  character- 
istics, especially  as  to  crop  productivity. 

26.  Distribution  of  residual  soils.  —  Residual  soils  are  of 
wide  distribution  in  the  United  States,1  particularly  in  the 
eastern  and  central  parts.  A  glance  at  the  soil  map  of  this 
country  (See  Fig.  4)  shows  four  great  provinces  —  the  Pied- 
mont Plateau,  the  Appalachian  Mountains  and  Plateaus, 
the  Limestone  Valleys  and  Uplands,  and  the  Great  Plains 
Region.  The  age  of  these  soils  varies  in  the  order  named, 
showing  that  while  they  are  very  old  as  compared  with 
other  soils  yet  to  be  discussed,  there  may  be  vast  periods 
of  geologic  time  between  their  beginnings.     As  a  matter 

1  For  a  full  discussion  of  the  origin  and  characteristics  of  the 
soils  of  the  United  States,  see  Marbot,  C.  F.,  and  others.  U.  S. 
D.  A.,  Bur.  Soils,  Bui.  96.     1913. 


V, 


ils,  Bui.  96,  1913. 


GEOLOGICAL   CLASSIFICATION   OF  SOILS  35 

of  fact,  there  is  probably  a  greater  difference  in  age  be-, 
tween  the  soils  of  the  Piedmont  Plateau  and  those  of 
the  Great  Plains  Region  than  has  elapsed  since  the  latter 
were  formed.  The  soils  of  the  Piedmont  Plateau  have 
been  formed  mostly  from  gneiss  and  schist.  In  fact, 
the  Piedmont  Plateau  is  the  remnant  of  the  old  continent, 
Appalachia,  which  was  in  existence  in  early  Cambrian 
times.  The  rocks  of  the  Appalachian  Mountains  and 
Plateaus  are  limestone,  sandstone,  and  shales.  The  Great 
Plains  Region  presents  limestone,  sandstone,  and  shale 
of  the  Cretaceous,  Permian,  and  Carboniferous  ages,  be- 
sides much  unconsolidated  material.  The  soils  of  these 
provinces,  extending  as  they  do  over  great  areas,  vary 
within  wide  limits  due  to  rock  formation,  climatic  con- 
ditions, and  age;  yet  certain  common  characteristics, 
as  already  pointed  out,  are  exhibited  by  all. 

27.  Cumulose  soils.  —  This  type  of  soil  is  of  a  very 
different  character  from  the  one  just  under  discussion, 
being  made  up  largely  of  organic  matter  with  the  mineral 
constituents  of  secondary  importance.  At  relatively 
recent  periods  shallow  lakes,  ponds,  and  basins  were 
formed,  partly  by  stream  action,  partly  by  marsh  con- 
ditions along  sea  or  lake  coasts,  or,  what  is  commoner  in 
the  northern  part  of  the  United  States,  by  glaciation. 
Any  basin  that  contains  water  throughout  the  year  serves 
as  a  place  for  the  formation  of  cumulose  soil.  The  highly 
favorable  moisture  relations  along  the  banks  and  shores 
of  such  standing  water  encourage  the  growth  of  many 
plants  such  as  algse,  moss,  reeds,  flags,  grass,  and  the  like. 
These  plants  thrive,  die,  and  fall  down  only  to  be  covered 
by  the  water  in  wThich  they  were  growing.  The  water 
shuts  out  the  air  to  a  large  extent,  prohibits  rapid  oxida- 
tion, and  thus  acts  as  a  preservative  for  the  rapidly  collect- 


36         SOILS:    PROPERTIES  AND  MANAGEMENT 

ing  organic  matter.  Year  after  year  this  process  goes 
on,  and  year  after  year  the  bed  of  cumulose  material 
becomes  deeper  and  deeper.  Large  shrubs,  and  even 
forests,  often  grow  on  such  land.  Time  and  the  lack  of 
water  are  the  factors  that  may  limit  the  depth  of  such 
beds.  Accumulations  of  this  nature  are  found  dotted 
over  the  entire  country.  Their  size  may  vary  from  a 
few  acres  to  several  thousand.  Along  streams  the  old 
abandoned  beds  offer  a  common  opportunity  for  the 
beginning  of  such  accumulations.  Along  large  bodies 
of  water,  marshes,  either  salt  or  fresh,  may  allow  the 
process  to  go  on.  Shallow  basins  scraped  and  gouged 
out  by  advancing  glaciers  are  frequently  occupied  by 
such  material.  In  the  last-named  case  the  beds  are 
more  or  less  independent  of  topography,  and  may  be 
found  on  hillsides,  or  even  on  hilltops,  as  well  as  in  the 
lower,  lands. 

Cumulose  materials  may  be  grouped  under  two  heads, 
peat  and  muck.  The  only  difference  is  in  their  stage  of 
decay.  In  peat  the  stem  and  leaf  structure  of  the  original 
plants  can  still  be  detected,  and  identification  is  quite 
possible.  In  muck,  however,  the  putrefaction  and  decay 
have  gone  so  far  that  the  plant  tissue  has  lost  its 
identity  as  such  and  is  merged  into  that  complicated 
and  indefinite  material  called  humus.  The  composition 
of  peat  and  muck  may  be  much  altered  by  the  wash- 
ing-in  of  mineral  matter  from  above.  In  some  cases 
the  beds  may  be  from  80  to  85  per  cent  organic,  while 
in  other  cases,  due  to  this  foreign  material,  the  percent- 
age may  drop  to  as  low  as  15,  giving  a  black  or  swamp 
marsh  mud. 

The  following  analyses  illustrate  the  composition  of 
Representative  cumulose  soils  :  — 


GEOLOGICAL   CLASSIFICATION  OF  SOILS 


37 


Mineral  matter 
Organic  matter 
Nitrogen 
P2O5  .... 
K20  .  .  .  . 
Moisture     .     . 


1 

2 

31.60 

24.79 

68.40 

67.63 

2.63 

2.03 

.20 

.19 

.17 

.15 

— 

7.58 

80.40 
15.77 

.15 

.65 

3.83 


1.  Muck  —  Pickel,  G.  M.  Muck  :  •  Composition  and  Utili- 
zation.    Fla.  Exp.  Sta.,  Bui.  13.     1891. 

2.  Muck  —  Rept.  Can.  Exp.  Farms,  1910.  Rept.  of  chem- 
ist, p.  160. 

3.  Marsh  mud  —  Rept.  Can.  Exp.  Farms,  1910.  Rept.  of 
chemist,  p.  137. 

Muck  soils,  while  usually  not  of  large  extent,  become  of 
extreme  value  when  drained,  especially  if  they  are  near 
a  good  market.  They  are  of  particular  value  in  trucking 
operations,  being  adapted  to  such  crops  as  onions,  celery, 
lettuce,  and  the  like.  Usually  they  must  not  only  be 
provided  with  drainage,  but  also  be  treated  with  fertili- 
zers carrying  phosphorus  and,  especially,  potash.  It  is 
also  a  good  practice  to  start  vigorous  decay  by  the  appli- 
cation of  barnyard  manure,  as  the  nitrogen  carried  by 
muck  soils  is  usually  not  very  readily  available  to  plants. 
In  many  cases  muck  and  peat  may  be  underlaid  at  vary- 
ing depths  by  marl,  which  is  a  soft,  impure  calcium 
carbonate.  Before  and  at  the  beginning  of  the  organic 
accumulation  these  basins  were  inhabited  by  Mollusca, 
which  at  death  deposited  their  shells  on  the  bed  of  the 
inclosure.  These  shells  are  now  found  in  a  more  or  less 
fragmentary  condition,  usually  mixed  with  sand  and 
clay  and  covered  to  a  varying  depth  with  peat  or  muck. 
Such  material,  because  of  its  richness  in  lime,  is  valuable 


38 


SOILS:    PROPERTIES   AND  MANAGEMENT 


as  a  soil  amendment,  and  often  where  it  is  found  pure 
enough  in  quality  and  in  sufficiently  large  quantities  it 
is  handled  commercially.  When  it  contains  large  amounts 
of  phosphorus,  as  it  does  in  some  cases,  it  may  be  used 
as  a  fertilizer.  The  following  analyses  *  show  the  general 
character  of  this  soil :  — 


Si02       .  . 
A1203 .  Fe203 

CaO       .  . 

MgO      .  . 

K20       .  . 

Na20     .  . 

P205      .  . 

C02       .  . 
Ignition 


25.28 

5.65 

3.02 

3.30 

37.52 

48.51 

.12 

1.96 

.22 

.23 

.25 

.30 

.40 

Trace 

29.02 

39.80 

4.17 

.25 

28.  Colluvial  soils.  —  This  class  of  soil  is  not  of  great 
importance,  first  because  of  its  small  area  and  its  inac- 
cessibility, and  secondly  because  it  is  usually  a  coarse, 
loose  soil,  rather  unfavorable  for  plant  growth.  It  is 
formed,  as  its  name  indicates,  in  regions  of  precipitous 
topography,  and  is  made  up  of  fragments  of  rocks  de- 
tached from  the  heights  above  and  carried  down  the 
slopes  by  gravity.  Talus  slopes,  cliff  debris,  and  other 
heterogeneous  rock  detritus  are  examples  of  colluvial 
soil.  Avalanches  are  made  up  largely  of  such  material. 
As  the  physical  forces  of  weathering  are  most  active  in 
the  formation  of  these  soils,  the  amount  of  solution  and 
oxidation  is  small.     The  upper  part  of  the  accumulation 


1  Kerr,  W.  C.     Geology  of   North   Carolina,  Vol.   I,   p.   195. 
1875. 


GEOLOGICAL   CLASSIFICATION   OF  SOILS 


39 


exhibits  this  isolated  physical  action  to  the  greatest  extent, 
the  particles  being  angular,  coarse,  and  comparatively 
fresh;  farther  down  the  slope  the  material  may  merge 
by  degrees  into  ordinary  soil.  Such  soils  are  usually 
shallow  and  stony,  and  approach  the  original  rock  in 
color  unless  large  amounts  of  organic  matter  have  ac- 
cumulated (Fig.  5). 


Fig.  5. — Diagram  showing  the  formation  of  a  colluvial  soil,  (a),  bed 
rock  ;  (6),  dismantled  cliff ;  (d),  coarse  unproductive  talus ;  (p),  soil 
capable  of  bearing  plants. 


29.  Alluvial  soils.  —  In  considering  the  importance  of 
water  as  a  weathering  agent,  it  was  found  that  it  had 
both    cutting    and    transporting    powers.     The    alluvial 


40         SOILS:    PROPERTIES  AND  MANAGEMENT 

soil  is  a  direct  result  of  both  these  activities.  The  carry- 
ing power  of  water  varies  directly  as  the  sixth  power  of 
its  velocity ;  so  that  a  doubling  of  the  velocity  increases 
the  transportive  ability  sixty-four  times.  It  is  estimated  1 
that  water  flowing  at  the  rate  of  three  inches  a  second 
will  carry  only  fine  clay,  but  if  this  rate  is  increased  to 
twenty-four  inches  a  second,  pebbles  the  size  of  an  egg 
will  be  moved  along  the  stream  bed.  Any  checking  of 
the  velocity  of  a  stream  will  cause  it  to  deposit  the  ma- 
terial carried  in  suspension,  the  larger  particles  first  and 
the  finest  when  the  current  becomes  very  sluggish.  This 
brings  about  one  of  the  important  characteristics  of  an 
alluvial  soil,  its  stratification.  Wherever  material  is 
being  laid  down  by  water  this  phenomenon  is  exhibited, 
due  to  the  rapid  changes  in  velocity.  As  a  stream  ap- 
proaches nearer  and  nearer  to  its  outlet,  its  bed  becomes 
less  and  less  inclined  and  the  current  more  and  more 
sluggish.  This  tends  toward  an  aggrading  of  the  stream 
bottom  from  the  deposited  material.  Such  a  condition 
naturally  increases  the  probability  of  overflow  in  high 
water.  Overflow  at  a  time  when  the  stream  is  carrying 
its  maximum  of  sediment  causes  the  deposition  of  a  thin 
layer  of  soil  over  the  areas  covered  by  the  water.  This 
soil  is  stratified  according  to  the  conditions  under  which 
it  was  laid  down,  the  finer  particles  usually  being  carried 
farther  and  often  deposited  in  slack  water  or  lagoons. 
Also,  a  stream  on  a  gently  inclined  bed  may  begin  to 
swing  from  side  to  side  in  long,  gentle  curves,  due  to  the 
deposition  of  alluvial  material  on  the  inside  of  the  curve 
and  the  cutting  by  the  current  on  the  opposite  bank. 
This  results  in  oxbows,  lagoons,  and  similar  inclosures, 

!Geikie,A.   Text  Book  of  Geology,  p.  380.   New  York.    1893. 


GEOLOGICAL   CLASSIFICATION  OF  SOILS  41 

% 

ideal  for  the  deposition  of  alluvial  matter.  Deltas  are 
another  good  example  of  alluvial  deposits,  whether  oc- 
curring in  ocean,  gulf,  or  lake.  Due  to  a  change  in  grade, 
a  stream  may  cut  down  through  its  already  well-formed 
alluvial  deposit,  leaving  terraces  on  one  or  both  sides. 
Often  two,  or  even  three,  terraces  may  be  detected  along  a 
valley,  marking  a  time  when  the  stream  bed  was  at 
these  elevations.  On  the  lower  slopes  of  hills  border- 
ing valleys,  the  colluvial  deposits  may  touch  or  even 
mingle  with  the  alluvial,  and  furnish  a  stream  with  some 
of  its  detritus. 

Alluvial  soils,  then,  are  found  as  narrow  ribbons  along 
streams.  They  are  always  young  soils,  and  are  still  in 
process  of  formation.  Since  in  most  cases  they  are  de- 
posited by  slowly  moving  water,  the  texture  of  such  soils 
is  fine,  the  soils  being  mostly  clays,  silts,  and  fine  sandy 
loams.  Found  in  low  lands,  alluvial  soils  need  drainage 
to  a  large  extent.  Because  of  the  favorable  moisture  con- 
ditions these  soils  usually  have  a  very  large  amount  of 
organic  material,  as  vegetation  grows  readily  under  such 
conditions.  Considerable  humus  is  also  washed  into 
alluvial  materials  at  the  time  of  their  deposition.  The 
soil  is  usually  deep,  and,  because  of  the  high  organic  con- 
tent, universally  of  good  physical  condition,  although 
very  heavy  stiff  clays  may  be  found  in  certain  cases. 
The  character  of  the  soils  and  the  rocks  from  which  the 
detritus  has  been  obtained  exerts  considerable  influence 
on  its  character.  For  example,  a  red  soil  will  often 
give  rise  to  a  reddish  alluvial  soil,  while  a  soil  or  a  rock 
poor  in  lime  will  certainly  not  be  parent  to  a  soil  very 
much  richer  in  that  constituent. 

30.  Distribution  of  alluvial  soils.  —  The  distribution 
of  alluvial  soils  in  the  United  States  is  not  wide,  although 


42 


SOILS:    PROPERTIES  AND  MANAGEMENT 


these  soils  exist  along  almost  every  stream  east  of  the 
Great  Plains  Region.  Their  best  and  widest  develop- 
ment is  found,  as  the  map  indicates  (See  Fig.  4)  along 
the  lower  Mississippi  river,  where  they  may  often  show 
a  lateral  extension  of  one  hundred  miles.  Extensions  of 
this  band  are  noted  along  the  Missouri,  Ohio,  and  Upper 
Mississippi  rivers.  All  streams  flowing  east  exhibit 
areas  of  such  soils,  these  areas  varying  with  the  size  and 
velocity  of  the  stream. 

The  soils  of  the  alluvial  province  may  be  divided  under 
two  heads  because  of  topographic  differences  —  (1)  the 
first  bottoms,  or  present  flood  plains ;  and  (2)  the  terraces, 
or  old  flood  plains.  These  soils  differ  in  their  elevation, 
drainage,  and  age,  but  their  general  characteristics  are 
similar ;  the  surface  features  in  both  cases  vary  from  a  flat 
to  a  gently  rolling  topography  (Fig.  6).  Erosion,  especially 
in  the  terraces,  may  have  obliterated  some  of  the  out- 


Fig.  6.  —  Cross-section  of  typical  alluvial  soils.  (a),  bed  rock; 
(r),  stream;  (6),  present  flood  plain,  a  recent  alluvial  soil;  (c),  flood 
plain  terrace ;   (d),  very  old  stream  terrace,  an  old  alluvial  soil. 


standing  features.  Alluvial  soils,  being  very  rich,  are 
particularly  adapted  to  trucking  crops,  although  in  most 
cases  they  are  utilized  for  more  extensive  farming.  When 
well  drained  and  protected  from  overflow,  they  are  the 
richest  and  most  valuable  of  soils. 


GEOLOGICAL    CLASSIFICATION   OF  SOILS  43 

31.  Marine  soils.  —  The  sediments  which  are  con- 
tinually being  carried  away  by  rivers  are  eventually 
deposited  in  the  sea,  the  coarser  fragments  near  the 
shore,  the  finer  particles  at  considerable  distances.  This 
layer  of  material,  varying  in  thickness,  consists  of  stratified 
gravels,  sands,  and  clays,  and  is  of  a  rather  recent  age 
compared  with  the  residual  soils.  It  has  not  become 
consolidated  as  yet,  because  of  insufficient  pressure  and 
time.  When  such  material  becomes  raised  above  the 
sea,  due  to  a  change  in  land  elevation,  it  is  classified  as  a 
marine  soil.  It  has  been  worn  and  triturated  by  a  num- 
ber of  agencies.  First,  the  forces  necessary  to  throw  it 
into  stream  suspension  were  active,  and  next  it  was 
swept  into  the  ocean  to  be  deposited  and  stratified, 
possibly  after  being  pounded  and  eroded  by  the  waves 
for  years.  At  last  came  the  emergence  above  the  sea 
and  the  action  of  the  forces  of  weathering  in  situ.  The 
latter  effects  are  not  of  great  moment,  since  with  our 
most  important  marine  soils  they  have  been  at  work 
for  but  a  comparatively  short  time,  speaking  geologically. 

32.  Characteristics  of  marine  soils.  —  Marine  soils, 
while  much  younger  than  residual  soils,  are  usually  more 
worn  and  ordinarily  show  a  less  amount  of  the  important 
food  elements.  This  is  because  of  their  almost  con- 
tinuous contact  with  water  from  the  time  when  they  are 
swept  into  the  streams  until  they  rise  above  the  sea  level 
as  a  soil.  They  are  generally  characterized  as  sandy 
soils,  because  the  forces  to  which  they  have  been  sub- 
jected have  worn  out  and  dissolved  most  of  the  minerals 
except  quartz.  This  gives  them  a  coarse  texture  and 
fits  them  particularly  for  trucking  soils.  Sands,  sandy 
loams,  and  loams  predominate  usually  in  such  soil  prov- 
inces,   although    clays    and    silts    may   occasionally   be 


44         SOILS:    PROPERTIES  AND  MANAGEMENT 

found.  These  soils  are  usually  low  in  humus,  and  con- 
sequently must  be  handled  with  reference  to  the  possi- 
bilities of  increasing  their  organic  content.  Lack  of 
humus  makes  the  predominating  color  of  the  soil  light, 
ranging  from  light  gray  to  brown  and  dark  brown.  The 
character  of  these  soils  is  governed  to  some  extent  by  the 
origin  of  the  sediments;  different  rocks,  particularly  if 
weathered  under  different  climatic  conditions,  may  give 
rise  to  widely  different  marine  soils.  The  climatic  con- 
ditions to  which  marine  materials  are  subjected  after 
being  raised  above  the  sea  may  also  be  a  considerable 
diversifying  agency. 

33.  Distribution  of  marine  soils.  —  Marine  soils  are 
found  very  widely  distributed  in  the  eastern  United  States, 
and  make  up  one  of  our  most  important  soil  provinces. 
Beginning  at  Long  Island  at  the  north  (See  Fig.  4), 
they  extend  southward  along  the  coast  in  a  band  ranging 
from  one  hundred  to  two  hundred  miles  in  width.  The 
western  edge  of  the  Atlantic  coastal  plain  is  marked  by 
the  great  "  Fall  "  Line,  or  the  edge  of  the  old  continent 
Appalachia.  It  is  from  this  area  that  most  of  the  sedi- 
ments of  the  Atlantic  marine  soils  were  derived.  Proceed- 
ing southward,  we  find  that  Florida  is  practically  all  of 
marine  origin,  together  with  a  great  area  of  the  Gulf 
coast  extending  westward  to  central  Texas  and  having 
an  average  width  of  two  hundred  fifty  miles.  This 
gulf  marine  soil  is  considerably  younger  than  that  of 
the  Atlantic  coast.  It  is  cut  into  two  parts  by  the  allu- 
vial soils  of  the  Mississippi,  and  is  covered  by  a  narrow 
band  of  alluvial  soil  on  the  eastern  bank  of  that  stream. 
The  sediments  of  the  Gulf  coastal  plain  were  derived 
from  the  erosion  and  denudation  of  the  old  lands  to  the 
northward. 


GEOLOGICAL   CLASSIFICATION  OF  SOILS  45 

The  soils  of  the  Atlantic  and  Gulf  coastal  provinces, 
formed  as  vast  outwash  plains,  are  very  diversified,  due 
to  source  of  material,  age,  and  climatic  conditions.  They 
comprise  a  sufficient  range  in  texture  and  climate  to 
support  a  highly  varied  agriculture.  There  are  great 
tracts  of  general  farming  land,  besides  wide  areas  of 
special-purpose  soils  adapted  to  highly  specialized  in- 
dustries. The  latter  soils  require  refined  and  intensive 
methods  of  cultivation.  Predominantly  sandy,  these 
soils  are  easy  to  cultivate,  and .  they  are  well  drained 
except  in  the  lower  coastal  plain  belt.  Good  aeration  is 
usually  found  because  of  their  open  structural  condition. 
Severe  leaching  occurs  in  times  of  heavy  rainfall,  for  the 
same  reason.  When  sufficiently  supplied  with  organic 
matter,  carefully  fertilized,  and  cultivated  properly, 
these  soils  support  a  great  variety  of  crops,  such  as  cotton, 
corn,  oats,  forage  crops,  and  peanuts,  besides  vegetables 
and  fruits  of  many  varieties. 


CHAPTER  IV 

GEOLOGICAL   CLASSIFICATION  OF  SOILS 
(CONTINUED) 

Ice  in  the  form  of  glaciers  has  been,  as  already  stated, 
a  very  great  factor  in  soil  formation,  especially  in  the 
north  temperate  zones  of  North  America,  Europe,  and 
Asia.  Not  only  was  the  old  mantle  of  material  swept 
from  the  land  by  the  advance  of  the  ice,  but  a  new  soil 
was  laid  down  as  drift  material.  This  drift  was  some- 
times merely  ground-up  rock,  sometimes  rock  flour 
mixed  with  the  original  residual  soil,  and  sometimes 
glacial  material  wholly  reworked  and  considerably  strati- 
fied by  water.  Besides  this,  the  streams  of  water  that 
issued  from  under  the  glaciers  were  instrumental  in 
many  cases  in  distributing  sediments  a  considerable 
number  of  miles  southward  of  the  ice  front.  Glacial 
lakes,  also,  when  in  existence  for  sufficiently  long  periods, 
furnished  basins  for  the  distribution  and  deposition  of 
materials  derived  from  the  erosive  and  grinding  power 
of  the  ice.  The  ice  also  furnished  a  large  amount  of 
very  fine  detritus,  which  was  susceptible  to  wind  move- 
ment. This  material,  reworked  and  deposited  as  bars 
in  stream  beds,  was  carried  many  miles  by  the  prevailing 
westerly  winds  during  a  period  of  aridity  following  the 
glacial  epoch.  It  now  exists  over  wide  areas,  especially 
in  the  Middle  West  of  the  United  States  and  in  northern 
China,  in  which  places  it  reaches  its  best  development. 

46 


GEOLOGICAL   CLASSIFICATION   OF  SOILS  47 

It  is  the  important  soil  of  these  regions.  Glaciation  was 
instrumental,  then,  either  directly  or  indirectly,  in  the 
formation  of  three  general  classes  of  materials  —  glacial 
drift  soils,  glacial  lake  soils,  and  a  certain  class  of  iEolian 
materials  designated  as  loess  and  adobe. 

34.  The  ice  sheet.1 — If  in  any  region,  but  more  likely 
one  of  some  elevation,  the  temperature  and  the  snowfall 
stand  in  such  relationship  that  the  heat  of  summer  does 
not  offset  the  winter  accumulation  of  snow,  great  snow 
fields  form.  As  this  condition  persists  year  after  year, 
and  the  snow  becomes  deeper  and  more  widely  spread, 
the  temperature  is  reduced  to  such  an  extent  as  to  in- 
crease the  proportion  of  the  precipitation  which  persists 
through  the  summer's  heat.  The  pressure  of  the  over- 
lying snow,  and  the  water  from  the  melting  surface,  bring 
about  a  change  of  the  snow  into  ice.  Often  a  recrystalli- 
zation  appears  to  occur  without  a  melting  and  refreezing. 
As  the  depth  of  ice  increases,  the  phenomenon  of  move- 
ment is  inaugurated  as  the  thickness  of  the  ice  at  the 
center  develops  strong  lateral  pressure.  Ice,  when  under 
great  stress,  exhibits  a  plasticity  which  it  does  not  or- 
dinarily possess.  As  it  moves  slowly  forward  under 
this  tremendous  pressure,  and  with  a  thickness  of  develop- 
ment almost  incredible,  it  conforms  itself  to  every  un- 
evenness  of  the  surface  it  may  be  invading.  It  rises  over 
hills,  or  shapes  itself  to  valleys  and  even  small  depressions, 
with  surprising  ease.  The  rate  of  advance  or  retreat  of 
a  glacier  is  determined  by  the  rate  at  which  its  edges 
are  wasting  or  melting  away.  If  the  melting  is  slow,  the 
ice  front  advances ;    if  it  just  balances  the  advance,  the 

1  For  a  complete  discussion  of  glaciers  and  glaciation,  see 
Salisbury,  R.  D.  The  Glacial  Geology  of  New  Jersey.  GeoL 
Survey  of  New  Jersey,  Vol.  5.     1902. 


48 


SOILS:    PROPERTIES  AND  MANAGEMENT 


ice  front  is  at  a  standstill ;  but  if  the  wasting  is  rapid, 
and  the  advance  of  the  glacier  is  not  fast  enough  to  re- 
place this  waste,  the  ice  is  said  to  be  retreating.  A  great 
ice  sheet  may  exhibit  all  three  of  these  conditions  many 
times  in  its  history. 


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Fig.  7.  —  Map  of  North  America,  showing  the  area  covered  by  the 
great  ice  sheets,  the  three  centers  of  accumulation,  and  the  approxi- 
mate southward  extension. 


35.   The  American  ice  sheet.  —  The  northern  part  of 
the  American  continent  was  at  one  time  covered  by  a 


GEOLOGICAL   CLASSIFICATION  OF  SOILS  49 

great  sheet  of  ice  possessing  all  the  properties  described 
above.  Accumulation  seems  to  have  occurred  in  three 
well-defined  centers,  from  which,  over  long  geologic 
periods,  the  ice  slowly  moved  southward,  encroaching 
upon  and  covering  thousands  of  square  miles.  The 
ice  cap  of  Greenland  is  a  very  good  example  of  the  con- 
ditions then  existing  in  the  northern  part  of  the  United 
States.  The  area  covered  by  glaciers  in  North  America 
at  the  time  of  the  greatest  extension  of  the  ice  is  estimated 
as  4,000,000  square  miles.  The  thickness  of  the  sheet 
was  probably  very  great,  ranging  from  a  few  feet  at  the 
margin  to  probably  a  mile  or  more  toward  the  centers; 
at  least  it  was  thick  enough  to  override  some  of  the 
highest  mountains  of  the  New  England  ranges.  Local 
glaciation  also  occurred  on  the  hill  and  mountain  tops, 
which  tended  to  increase  the  apparent  thickness  of  the 
ice  mantle. 

36.  Cause  of  the  ice  age.  —  The  ice  age  was  not  one 
unbroken  invasion  and  retreat  of  the  ice  cap,  but  was, 
as  is  conceded  by  all  authorities  on  glaciation,  really 
divided  into  epochs.  Five  great  invasions  appear  to 
affect  at  least  the  central  part  of  the  United  States, 
possibly  without  bringing  about  a  disappearance  of  the 
ice  across  the  Canadian  border.  These  interglacial 
periods  are  shown  by  forest  beds,  accumulations  of 
organic  matter,  and  evidences  of  erosion  between  the 
drift  deposited  by  the  successive  ice  sheets.  Some  of 
the  interglacial  periods  evidently  were  times  of  warm, 
and  even  semitropical,  climate.  Just  exactly  what  was 
the  cause  of  the  ice  age  is  still  under  dispute.  The  most 
probable  theory,  both  as  to  its  occurrence  and  as  to  its 
disappearance,  is  that  a  change  in  the  carbon  dioxide 
content  of  the  atmosphere  took  place.     It  is  believed 


50         SOILS:    PROPERTIES  AND  MANAGEMENT 

that  doubling  the  amount  now  present  would  bring  about 
tropical  climate  in  the  temperate  zones,  while  halving 
it  would  cause  frigid  conditions  and  a  probable  return 
of  the  great  ice  fields. 

37.  The  extension  of  the  ice  sheet.  —  While  the 
advancing  ice  in  general  exhibited  well-defined  viscosity, 
certain  parts  were  more  or  less  rigid.  This  was  especially 
true  of  the  parts  near  the  edges  of  the  sheet.  These 
parts  had  become  filled  in  their  advance,  particularly 
near  the  bottom,  with  earthy  and  stony  material,  which 
aided  the  erosive  processes  to  a  very  great  degree.  The 
eroding  and  denuding  power  of  the  glaciers  is  shown 
everywhere  by  the  gouged-out  valleys  and  by  the  scratches, 
or  striae,  on  exposed  rocks.  As  this  sheet  of  ice  slowly 
advanced  a  few  inches  or  a  few  feet  a  day,  the  mantle  of 
residual  soil  was  carried  away  or  mixed  with  the  rock  flour 
constantly  formed  by  the  moving  ice.  The  original  soil 
was  really, an  instrument  for  more  effective  ice  action. 
The  scouring  effect  is  observed  now  to  the  best  advantage 
in  valleys  which  lay  longitudinally  to  the  ice  movement, 
as  did  the  valleys  of  the  Finger  Lakes  of  central  New 
York.  Valleys  lying  at  right  angles  to  the  ice  were  very 
often  partially  or  wholly  filled  with  debris,  and  the  en- 
tire topography  was  altered.  Rivers  flowing  under  the 
ice  often  left  large  amounts  of  materials  designated  now 
as  eskers  and  kames.  The  mixing,  grinding,  trans- 
porting, and  stratification  that  went  on  emphasizes 
again  the  great  influence  of  glaciation  on  general  topog- 
raphy and  soils. 

The  greatest  southward  extension  of  the  ice  in  the  United 
States  is  marked  by  a  great  terminal  moraine  (Fig.  8). 
It  is  supposed  that  the  margin  of  the  sheet  was  stationary 
at  this  point  for  a  sufficiently  long  period  to  allow  this 


GEOLOGICAL   CLASSIFICATION   OF  SOILS 


51 


52         SOILS:    PROPERTIES  AND  MANAGEMENT 

narrow  band  of  material  to  collect  by  the  continual 
melting  of  the  ice  and  a  consequent  dumping  of  its  load 
of  debris.  This  moraine  is  by  no  means  continuous, 
and  for  miles  across  the  continent  no  trace  of  it  can  be 
found.  It  extends,  roughly,  eastward  from  the  Canadian 
border  in  Washington  to  the  upper  sources  of  the  Missouri 
River,  then  down  that  river  to  St.  Louis,  up  the  Ohio 
River,  northeastward  until  the  southwest  border  of 
New  York  is  reached,  and  then  southeast  to  New  York 
City  and  Long  Island.  Many  other  moraines  are  found 
to  the  northward,  marking  points  where  the  ice  became 
stationary  for  a  time  during  its  retreat. 

38.  The  ice  as  a  soil  builder.  —  It  was  during  these 
retreats  that  the  ice  acted  as  a  soil-forming  agent.  Ma- 
terial gathered  and  ground  by  the  ice  as  it  pushed  to  the 
southward  was  finely  pulverized  and  it  is  only  natural 
to  suppose  that  this  debris  was  deposited  as  the  ice  slowly 
retreated  by  the  melting  back  of  its  margins.  The 
material  laid  down  as  a  great  mantle  over  the  glaciated 
areas  is  called  drift.  Some  of  this  has  been  reworked 
and  stratified  by  water,  but  a  very  large  proportion  has 
remained  untouched  since  it  was  laid  down  by  the  melt- 
ing ice.  It  presents  in  most  cases  —  except  at  the  very 
surface,  where  weathering  may  have  occurred  or  organic 
matter  accumulated  —  exactly  the  same  condition  as 
when  deposited.  This  mass  of  un stratified  material  is 
heterogeneous,  both  as  to  size  of  the  particles  that  make 
it  up  and  as  to  its  rock  composition.  It  may  be  coarse 
and  bowldery,  especially  in  mountains  or  where  there 
are  gneisses  or  schists,  or  it  may  be  very  fine  where  the 
rocks  are  soft.  Bowlder  clay  is  a  term  sometimes  used 
in  describing  the  matrix  of  this  glacial  deposit.  In  some 
cases  foliation  occurs,  and  often  coarse  and  fine  layers 


GEOLOGICAL    CLASSIFICATION   OF  SOILS  53 

of  till  may  alternate.  This  great  mantle  of  material, 
varying  in  thickness  and  constitution  according  to  the 
underlying  rocks  and  the  strength  of  glaciation,  gives  a 
great  soil  province  in  northern  United  States  which 
may  be  designated  as  glacial  till  soil. 

39.  Glacial  till  soils.  —  The  glacial  till  soils  may  be 
characterized  physically  as  heavy  or  relatively  heavy 
soils.  The  tremendous  force  of  the  grinding  has  pro- 
duced fine  particles,  and  as  a  consequence  clay  loams 
and  silt  loams  predominate.  Such  soils  usually  have  a 
subsoil  which  is  finer  than  the  surface  material  and  may 
be  so  impervious  to  water  as  to  produce  bad  drainage 
conditions.  The  individual  particles  of  a  glacial  soil 
are  found  to  be  unweathered  to  a  great  degree  unless  the 
soil  is  mixed  with  some  of  the  old  mantle  of  residual 
material  which  once  overspread  all  our  glaciated  areas. 
The  particles  are  jagged  and  unrounded ;  the  feldspars 
retain  all  of  their  luster,  and  the  iron  stains  so  common 
in  residual  soils  are  almost  absent.  As  the  glacial  soils 
are  young  soils,  their  colors  are  seldom  made  up  of  reds 
and  yellows,  but  grays  and  browns  prevail.  Red  may 
occur,  however,  where  red  sandstones  have  been  glaciated 
or  where  red  residual  soil  has  become  incorporated  in 
the  till.  Where  considerable  organic  matter  has  accu- 
mulated the  soil  is  usually  very  black.  The  subsoils 
in  the  glaciated  areas  usually  present  colors  ranging 
from  light  grays  to  light  browns.  Blue  or  mottled  clay 
or  clay  loam  is  often  found,  due  to  a  lack  of  aeration  in 
the  soil ;  to  the  soil  expert  such  a  condition  near  the  sur- 
face indicates  a  need  of  drainage. 

40.  Composition  of  glacial  soils.  —  The  chemical  com- 
position of  glacial  soil  approaches  more  nearly  than  that 
of  any  other  soil  the  composition  of  the  original  rock. 


54  SOILS:    PROPERTIES   AND   MANAGEMENT 

This  close  resemblance  to  the  parent  rock  is  not  sur- 
prising, since  glacial  soil  is  ground-up  rock  material  of 
recent  formation  on  which  the  weathering  agencies  have 
as  yet  had  little  time  for  action.  Therefore  the  amounts 
of  the  important  constituents  in  such  soil  are  governed 
largely  by  the  composition  of  the  original  rock.  The 
lime  content  is  due  to  such  a  relationship,  and  the  agri- 
cultural value  of  the  soil  is  greatly  influenced  thereby, 
since  large  amounts  of  calcium  are  of  great  importance 
to  soil  fertility.  The  hill  soils  of  central  New  York 
(Volusia  series)  come  from  shales  poor  in  lime,  and  the 
soil  owes  its  properties  very  largely  to  this  lack,  which 
is  traceable  to  the  parent  rock.  On  the  other  hand,  cer- 
tain glacial  soils  of  the  Mississippi  Valley  (Miami  series) 
formed  from  sandstones  and  limestone,  contain  plenty 
of  lime  due  to  the  nature  of  their  rock  origin.  Glacial 
soils  from  limestone  always  contain  plenty  of  lime,  a 
condition  that  is  far  from  true  with  residual  soils. 

41.  Humus  of  glacial  soils.  —  The  humus  content  of 
glacial  soils  depends  to  a  large  extent  on  the  climatic 
conditions  under  which  the  soil  has  existed  since  its 
formation.  If  environmental  factors  have  been  such 
as  to  encourage  the  accumulation  of  organic  matter, 
these  soils  will  exhibit  the  deep  black  color  that  arises 
from  the  presence  of  such  material.  If,  however,  con- 
ditions do  not  encourage  the  natural  growth  of  a  heavy 
vegetation,  the  amount  of  organic  matter  in  such  virgin 
soil  will  be  low.  Lime  may  be  a  very  great  factor  in 
such  soils,  not  only  in  the  encouragement  of  plant  growth, 
but  also  in  the  proper  decay  of  the  plant  tissue  after 
it  has  become  incorporated  with  the  soil. 

Glacial  till  soils  are  found  distributed  over  all  the  area 
north  of  the  great  terminal  moraine,  and  stretch,  roughly, 


GEOLOGICAL    CLASSIFICATION   OF  SOILS  55 

from  New  England  to  the  Pacific  coast  (see  Fig.  4). 
They  comprise  a  great  variety  of  soils,  not  only  as  to  their 
physical  character,  but  also  as  to  fertility.  They  are 
adapted  to  many  crops,  but  general  farming  is  practiced 
on  them  to  the  greatest  degree.  This  means  extensive, 
rather  than  intensive,  operations.  In  some  localities 
dairying  has  been  developed  to  a  large  extent,  and  has 
proved  to  be  not  only  a  means  of  obtaining  paying  re- 
turns from  such  soils,  but  at  the  same  time  a  method  of 
keeping  up  their  fertility. 

42.  Glacial  lakes.  —  Such  great  masses  of  ice  could 
not  advance  and  retreat,  again  and  again,  on  such  an 
extensive  scale,  without  causing  the  formation  of  great 
torrents  of  water.  It  is  more  than  probable  that  at  all 
times  great  streams  gushed  from  the  ice  front,  laden 
with  much  sediment.  Often  these  streams  were  under 
pressure,  which  when  released  caused  an  immediate 
deposition  of  material.  As  long  as  the  ice  front  stood 
south  of  the  east  and  west  divide,  this  water  found  ready 
egress  and  flowed  rapidly  away  to  deposit  its  load  as 
gravelly  outwash,  river  terraces,  valley  trains,  and  allu- 
vial fans.  These  formed  alluvial  soils  of  varied  character, 
depending  on  the  size  of  the  materials  carried.  There 
came  a  time,  however,  in  the  retreat  of  the  ice,  when  the 
front  stood  north  of  the  divide  and  only  a  small  pro- 
portion of  the  water  found  itself  free  to  flow  over  the  divide 
and  away  to  the  southward.  The  remaining  ftvater  was 
ponded  between  the  ice  front  and  the  old  divide.  Thus 
glacial  lakes  were  produced,  of  large  or  small  extent, 
according  to  the  position  of  the  ice.  The  location  of 
such  lakes  is  shown  on  the  soil  map  of  the  United  States. 
The  ponded  water  remained  in  this  condition  for  many 
years,   subject,   of   course,   to   changes   concordant  with 


56         SOILS:    PROPERTIES  AND  MANAGEMENT 

the  oscillation  of  the  ice  front.  With  the  ice  melting 
rapidly  on  the  hilltops,  these  lakes  were  constantly  fed 
by  torrents  from  above  which  were  laden  with  sediment 
derived  not  only  from  under  the  ice,  but  also  from  the 
unconsolidated  till  sheet  over  which  it  flowed.  As  a 
consequence,  there  were  in  the  glacial  lakes  deposits  rang- 
ing from  coarse  delta  materials  near  the  shore  to  fine  silts 
and  clay  in  the  deeper  and  stiller  water.  Such  materials 
now  cover  large  areas  (see  Fig.  4),  not  only  in  New  York 
State  and  along  the  Great  Lakes,  but  also  in  the  Red 
River  Valley  and  in  the  northerly  inclined  valleys  of  the 
Rocky  Mountains  and  the  Cascade  and  Sierra  Nevadas. 
They  make  up  by  far  the  most  important  lacustrine 
soils. 

43.  Lacustrine  soils  —  glacial  lake.  —  Glacial  lake  soils 
probably  present  as  wide  a  variation  in  physical  char- 
acteristics as  any  of  our  great  soil  provinces.  Being 
deposited  by  water,  they  have  been  subject  to  much 
sorting  and  stratification,  and  range  from  coarse  gravels 
on  the  one  hand  to  fine  clays  on  the  other.  They  are 
generally  found  as  the  lowland  soils  in  any  region,  al- 
though they  may  occur  well  up  on  the  hillsides  if  the 
shores  of  the  old  lakes  encroached  thus  far.  The  color 
of  such  soils  varies  from  gray  to  black,  according  to 
the  degree  of  organic  matter  present.  The  humus  con- 
tent of  such  soils,  as  with  the  glacial  till,  varies  with 
climate,  and  may  be  high,  low,  or  medium  according 
to  conditions.  The  thickness  of  glacial  lake  deposits  is 
variable,  ranging  from  a  few  feet  to  many  feet.  In 
chemical  composition  they  closely  approximate  the  soil 
from  which  they  are  derived.  This  is  particularly  true 
as  regards  the  presence  of  lime.  The  Dunkirk  soil 
of    southern    New  York,    a  wash    from '  the    lime-poor 


GEOLOGICAL   CLASSIFICATION  OF  SOILS  57 

Volusia  series  of  the  highlands,  is  low  in  lime;  while 
the  same  soil  just  south  of  Lake  Ontario,  obtaining  its 
wash  from  a  limestone  till  (Ontario  series),  is  rich  in 
lime.  As  may  be  inferred  from  the  above  comparison, 
the  glacial  lake  soils  of  the  United  States  are  variable 
in  their  fertility. 

The  distribution  of  the  glacial  lake  deposits,  as  seen 
from  the  soil  map  of  the  United  States,  is  fairly  wide. 
Such  soils  are  found  in  areas  large  enough  to  be  of  great 
agricultural  influence,  extending  from  New  England 
westward  along  the  Great  Lakes  until  their  greatest 
expanse  is  reached  in  the  Red  River  Valley.  These  de- 
posits make  up  some  of  the  most  important  soils  of  the 
northern  states.  They  are  valuable  not  only  for  ex- 
tensive cropping  with  grain  and  hay,  but  also  for  fruit 
and  trucking  crops.  The  Ice  Age  was  certainly  not 
in  vain  as  far  as  the  production  of  fertile  soils  is  con- 
cerned. 

44.  Lacustrine  soils  —  recent  lake.  —  There  is  yet 
another  lacustrine  soil  to  be  considered  besides  the  one 
just  discussed  —  recent  lake  soil.  While  the  glacial  lake 
deposits  were  formed  many  thousands  of  years  ago,  the 
lake  soils  of  the  second  group  are  in  process  of  construc- 
tion. It  is  a  well-known  fact  in  physical  geography 
that  lakes  are  only  enlarged  stream  beds,  and  are  doomed 
ultimately  to  be  filled  by  river  sediments.  Such  soils 
have  been  reclaimed  to  a  certain  extent,  but  their  acreage 
is  not  large  enough  to  give  them  the  importance  of 
the  glacial  lake  soils.  The  lake  soil  is  usually  of  a  fine 
character,  rich  in  humus,  of  good  tilth.  If  properly 
drained,  it  is  almost  invariably  highly  productive,  and 
is  adapted  to  a  variety  of  crops  depending  on  climatic 
conditions. 


58         SOILS:    PROPERTIES  AND   MANAGEMENT 

45.  jEolian  soils.  —  During  glaciation  much  fine  ma- 
terial was  carried  miles  below  the  front  of  the  glaciers 
by  streams  that  found  their  sources  therein.  This  fine 
sediment  was  deposited  over  wide  areas  by  the  over- 
loaded rivers.  The  accumulations  occurred  below  the 
ice  front  at  all  points,  but  seem  to  have  reached  their 
greatest  development  in  what  is  now  the  Missouri  Valley. 
There,  too,  the  sediment  seemed  finest,  and,  coming 
mainly  from  glaciated  limestones,  was  very  rich  in  cal- 
cium. It  is  generally  agreed  by  glacialists  that  a  period 
of  aridity,  at  least  as  far  as  this  particular  region  is  con- 
cerned, immediately  followed  the  retreat  of  the  ice. 
The  low  rainfall  of  this  period  was  accompanied  by 
strong  westerly  winds.  These  winds,  active  perhaps 
through  centuries,  were  instrumental  in  the  picking-up 
and  distributing  of  this  fine  material  over  wide  areas 
of  the  Mississippi,  Ohio,  and  Missouri  valleys.  One 
strong  argument  for  this/Eolian  origin  is  that  the  soil  is 
found  in  its  deepest  and  most  characteristic  development 
along  the  eastern  banks  of  the  large  streams.  Especially 
noticeable  is  the  extension  down  the  eastern  side  of  the 
Mississippi  River  almost  to  the  Gulf  of  Mexico.  This 
wind-blown  material,  called  loess,  is  found  over  wide 
areas  in  the  United  States,  in  most  cases  covering  the 
original  till  mantle.  It  covers  eastern  Nebraska  and 
Kansas,  southern  and  central  Iowa  and  Illinois,  northern 
Missouri,  and  parts  of  Ohio  and  Indiana,  besides  a  wide 
band,  as  already  noted,  extending  southward  along  the 
eastern  border  of  the  Mississippi  River.  Due  to  its 
mode  of  origin,  its  depth  is  always  greatest  near  the 
streams  and  gradually  becomes  less  farther  inland.  In 
places,  notably  along  the  Missouri  and  Mississippi  rivers, 
its   accumulation  has  given  rise  to  great  bluffs  which 


GEOLOGICAL   CLASSIFICATION  OF  SOILS  59 

bestow  a  characteristic  topography  to  that  region.  The 
loess  soil  is  found  also  covering  the  great  areas  of  China 
and  Siberia,  and  thus  it  is  one  of  the  important  soils  of 
the  world.  Another  soil,  made  up,  at  least  partially,  of 
wind-blown  material  and  found  in  Arizona  and  New 
Mexico,  is  called  adobe.  Volcanic  soils  of  the  western 
United  States  and  elsewhere  are  to  some  extent  of  wind 
origin.  Sand  dunes  are  of  iEolian  origin,  but  these  sink 
into  insignificance  as  to  agricultural  value  when  com- 
pared with  the  soils  named  above,  especially  loess. 

46.  Loess  soils.  —  Loess  is  usually  a  fine  calcareous 
silt  or  clay,  of  a  yellowish  or  yellowish  buff  color.  While 
it  may  be  readily  pulverized  when  subjected  to  cultiva- 
tion, it  possesses  remarkable  tenacity  in  resisting  ordinary 
weathering.  The  vertical  walls  and  escarpments  formed 
by  this  soil  show  one  of  its  striking  physical  character- 
istics. In  China  1  caves  that  house  thousands  of  persons 
are  dug  in  the  defiles  and  canons  existing  in  this  deposit. 
Another  feature  of  loess  is  the  presence  of  minute  vertical 
canals  lined  with  a  deposit  of  calcium  carbonate.  These 
canals  are  supposed  to  give  the  soil  its  vertical  cleavage 
and  its  tenacity.  The  particles  of  loess  are  usually  un- 
weathered  and  angular.  Quartz  seems  to  predominate, 
but  large  quantities  of  feldspar,  mica,  hornblende,  augite, 
calcite,  and  other  substances  are  found. 

A  few  typical  analyses  2  are  given  below : 

1  Richthofen,  F.  Chinese  Loess.  Geo!.  Mag.,  May,  1882, 
p.  293. 

2  Clark,  F.  W.  The  Data  of  Geochemistry.  U.  S.  Geol.  Sur- 
vey, Bui.  491,  p.  486.     1911. 

A.  From  near  Dubuque,  Iowa. 

B.  From  Vicksburg,  Mississippi. 

C.  From  Kansas  City,  Missouri. 

D.  From  Cheyenne,  Wyoming. 


60 


SOILS:    PROPERTIES   AND    MANAGEMENT 


Si02 

A1203 

Fe203 

MgO 

CaO 

Na20 

K20 

P205 

C02 

H20 


A 

B 

c 

72.68 

60.69 

74.46 

12.03 

7.95 

12.26 

3.53 

2.61 

3.25 

1.11 

4.56 

1.12 

1.59 

8.96 

1.69 

1.68 

1.17 

1.43 

2.13 

1.08 

1.83 

.23 

.13 

.09 

.39 

9.64 

.49 

2.50 

1.14 

2.70 

67.10 
10.26 
2.52 
1.24 
5.88 
1.42 
2.68 
.11 
3.67 
5.09 


It  is  immediately  noticeable  that  the  lime  content  of 
these  soils  is  high,  as  is  also  the  phosphoric  acid.  In 
fact,  all  the  more  soluble  constituents  are  present  in 
relatively  large  quantities,  as  would  naturally  be  ex- 
pected from  the  mode  of  origin  of  such  soils  —  they  having 
been  subjected  to  aridity  and  then  deposited  by  the  wind 
at  a  relatively  recent  period.  It  is  maintained  by  some 
geologists  *  that  the  deposition  of  loess  is  still  going  on 
in  certain  parts  of  the  world,  but  that  the  rate  of  accumu- 
lation is  so  exceedingly  slow  that  it  escapes  the  notice 
of  all  but  trained  observers.  The  lack  of  fossils,  par- 
ticularly those  of  plants,  is  accounted  for  by  this  slow 
rate  of  formation,  which  allows  sufficient  time  for  all 
organic  matter  to  become  fully  oxidized  before  being 
covered  by  the  drifting  material.  Snail  shells  are  often 
found,  but  as  they  are  of  land  species  they  argue  against 
a  water  origin  of  loess. 


1  Merzbacher,  G.  The  Question  of  the  Origin  of  Loess.  Mitt. 
Justus  Perthes'  Geogr.  Anst.,  59,  pp.  16-18,  69-74,  and  126-130. 
1913. 


GEOLOGICAL   CLASSIFICATION  OF  SOILS  61 

47.  Distribution  of  loess.  —  Not  only  is  loess  found 
over  thousands  of  square  miles  in  the  central  part  of  the 
United  States,  but  it  occurs  elsewhere  in  large  areas. 
It  is  greatly  developed  in  northern  France  and  Belgium, 
and  along  the  Rhine  in  Germany,  where  it  is.  an  important 
soil  in  all  the  valleys  that  are  tributary  to  that  river. 
Silesia,  Poland,  southern  Russia,  Bohemia,  Hungary, 
and  Roumania,  all  have  deposits  of  this  highly  fertile 
material.  In  Europe  it  extends  from  sea  level  to  eleva- 
tions of  5000  feet,  showing  its  independence  of  water 
as  a  formative  agent.  In  China  it  is  found  over  a  very 
large  part  of  the  valley  of  the  Hoangho,  a  region  prob- 
ably larger  in  area  than  France  and  Germany  combined. 
The  thickness  of  the  deposit  is  variable,  ranging  from  a 
few  feet  to  several  thousand  feet  in  certain  places.  The 
depth  is  practically  always  sufficient  for  any  form  of 
agricultural  operations. 

Wherever  moisture  relations  are  favorable  loess  is  an 
exceedingly  fertile  soil,  due  to  its  rich  stores  of  potash, 
phosphorus,  and  lime.  Its  organic  content  is  usually 
medium  to  high,  depending  on  conditions.  In  general 
it  may  be  classified  as  the  richest  soil  in  the  world, 
considering  its  wide  extension  and  the  great  variety 
of  climate  and  of  crops  to  which  it  is  subjected.  In 
the  United  States  it  occurs  in  the  Corn  Belt  region, 
and  might  be  called  the  great  corn  soil  of  the  Mississippi 
Valley. 

48.  Adobe  soils.  —  The  term  adobe  is  a  name  applied 
to  a  fine  calcareous  clay  or  silt  formed  in  a  manner  some- 
what like  that  in  which  loess  is  formed.  It  is  supposed 
that,  while  part  of  the  deposit  came  from  the  waste  of 
talus  slopes  as  mountains  were  weathered  under  conditions 
of  aridity,  the  remainder  had  an  origin  similar  to  that  of 


62 


SOILS:    PROPERTIES  AND  MANAGEMENT 


loess.1  Certain  characteristics  also  seem  to  indicate  that 
the  valley  adobe  might  have  been  deposited  by  water.2 
It  appears,  therefore,  that,  while  the  physical  characters 
of  all  adobe  are  somewhat  similar,  its  mode  of  origin  and 
chemical  composition  may  be  variable.  Below  are  the 
analyses 3  to  two  typical  adobe  soils :  — 


Si02  .    .    . 

A1203  .    .     . 

Fe203  .     .     . 

CaO  .     .     . 

MgO  .     .     . 

K20  ... 

Na^O  .     .     . 

C02  ... 

P2Os  .     .     . 
Organic  matter 


66.69 

44.64 

14.16 

13.19 

4.38 

5.12 

2.49 

13.91 

1.28 

2.96 

1.21 

1.71 

.67 

.59 

.77 

8.55 

.     .29 

.94 

2.00 

3.43 

Like  the  loess,  adobe  is  an  exceedingly  rich  soil,  but 
it  occurs  in  an  arid  or  a  semiarid  region.  When  irrigated, 
its  fertility  seems  inexhaustible.  It  is  found  in  Colorado, 
Utah,  southern  California,  Arizona,  New  Mexico,  and 
Texas.  It  has  an  especially  wide  distribution  in  New 
Mexico.  Like  loess  its  elevation  is  variable,  ranging 
from  sea  level  in  California  and  Arizona  to  6000  feet 
along  the  eastern  border  of  the  Rocky  Mountains.  Its 
maximum  thickness  cannot  be  estimated,  as  it  is  very 


1  Russell,   I.   C.      Subaerial   Deposits  of    the  Arid   Regions 
of  North  America.     Geol.  Mag.,  August,  1889,  pp.  342-350. 

2  Hilgard,  E.  W.    Relations  of  Soil  to  Climate.    U.  S.  Weather 
Bur.  Bui.  3.     1892. 

3  Merrill,  G.  P.    Rocks,  Rock  Weathering,  and  Soils,  p.  321. 
New  York.     1896. 


GEOLOGICAL   CLASSIFICATION   OF  SOILS  63 

little  eroded  and  is  supposed  to  be  still  accumulating. 
Some  valleys  are  known  to  be  filled  to  a  depth  of  3000 
feet  with  this  material.  Its  characteristics  are  its  fine 
texture,  its  great  depth,  its  wide  distribution,  and  its 
great  fertility  when  moisture  conditions  are  suitable  for 
crop  growth. 

49.  Sand  dunes.  —  Sand  dunes  are  the  outgrowth 
of  two  conditions  —  a  large  quantity  of  sand  and  a 
wind  that  blows  in  a  more  or  less  prevailing  direction. 
Under  such  conditions  the  sand  and  other  fine  material 
not  only  is  blown  into  heaps,  but  also  tends  to  move  in 
the  direction  of  the  prevailing  wind.  Such  heaps  or 
mounds  of  sand  may  travel  several  feet  a  day  by  the 
continual  movement  of  the  sand  grains  up  the  windward 
side  of  the  dune,  only  to  be  deposited  again  on  the  lee- 
ward side.  Sand  dunes  may  often  assume  gigantic 
proportions,  being  sometimes  several  hundred  feet  high 
and  twenty  or  thirty  miles  long.  In  such  proportions 
they  become  a  grave  menace  to  agriculture,  not  only 
because  they  are  an  absolutely  valueless  medium  for  plant 
growth,  but  also  because  they  cover  fertile  lands  and 
entirely  blot  out  all  plant  growth.  The  particles  of  this 
wind-blown  sand  are  usually  round,  from  the  continual 
abrasion  that  they  receive.  A  great  many  minerals 
may  be  represented,  but  quartz  is  the  commonest,  es- 
pecially if  the  dune  originally  had  its  origin  on  a  lake  or 
a  seashore. 

50.  Volcanic  dust.  —  From  early  geologic  times  de- 
posits of  the  very  fine  material  that  is  continually  being 
ejected  from  volcanoes  have  been  distributed  over  the 
earth's  surface.  These  deposits  are  usually  flour-like, 
and  while  at  one  time  they  probably  covered  many 
square   miles   of   territory,   they   have   succumbed    very 


64         SOILS:    PROPERTIES  AND  MANAGEMENT 

largely  to  erosion  and  denudation,  and  only  remnants 
are  found  at  the  present  time.  Such  material  may  be 
found  in  Montana,  Nebraska,  and  Kansas.  iEolian 
deposits  of  this  character  are  usually  rather  porous  and 
light,  and  are  likely  to  be  highly  siliceous.  They  are 
not  of  great  agricultural  importance. 


CHAPTER  V 

CLIMATIC   AND   GEOCHEMICAL   RELATION- 
SHIPS  OF  SOILS 

Although  during  the  process  of  weathering  the  tendency 
of  all  soil  is  toward  a  common  composition,  such  a  con- 
dition is  never  reached,  due  to  different  kinds  and  varying 
intensities  of  decay  and  disintegration.  Soils  lend  them- 
selves readily  to  a  geological  classification  because  of  this 
difference  in  mode  of  formation.  Such  a  classification 
really  signifies  a  variation  in  composition.  A  difference 
in  age,  a  preponderance  of  physical  agencies  over  chemi- 
cal or  vice  versa,  a  difference  in  the  transportive  agencies, 
or  a  variation  in  climatic  conditions  after  a  soil  is  once 
formed,  will  assuredly  give  a  different  product,  not  only 
chemically,  but  physically  and  biologically  as  well. 

51.  Climatic  relationships.  —  It  is  evident  that  climate 
is  a  factor  in  all  geochemical  relationships  of  soils.  Not 
only  does  climate  determine  the  kind  of  weathering  and 
its  intensity,  but  in  many  ways  it  influences  very  largely 
the  characteristics  of  the  soils  of  different  provinces  and 
sections.  Climate  must  be  considered  also  in  the  geo- 
logical classification  of  soils,  since  it  plays  such  an  im- 
portant role  in  determining  the  kind  and  intensity  of  the 
formative  agents.  In  any  scheme  of  grouping  for  the 
systematic  survey  and  mapping  of  soils,  climate  is  the 
very  first  factor  to  be  considered.  It  gives  three  great 
groups  —  tropical,  subtropical,  and  temperate.  These 
may  in  turn  be  subdivided  into  arid,  semiarid,  and  humid. 
f  65 


66 


SOILS:    PROPERTIES  AND  MANAGEMENT 


In  the  utilization  of  soil,  climate,  particularly  as  regards 
rainfall  and  temperature,  plays  an  important  part.  Crop 
adaptation  is  really  more  of  an  adaptation  to  climate  than 
to  soil,  although  the  latter  also  should  be  very  carefully 
studied.  The  climatic  relationships  in  soil  formation, 
in  soil  chemistry,  and  in  geochemistry  in  general,  cannot 
be  too  strongly  emphasized,  whether  the  viewpoint  be 
technical,  practical,  or  merely  educational. 

52.  Geochemical  relationships  of  residual  and  marine 
soils.  —  It  is  evident  from  the  above  that  coastal  plain, 
residual,  and  glacial  soils  should  exhibit  certain  well- 
defined  general  differences  due  to  their  mode  of  formation. 
The  following  analyses,  which  are  representative  of  the 
provinces  in  question,  illustrate  the  chemical  differences 
of  coastal  plain  and  residual  soils :  — 

Analyses  of  Typical  Coastal  Plain  and  Residual  Soils 


Si02 

A1203 

Fe203 

P205 

CaO 

C02 

MgO 

Na20 

K20 


Light  Sandy 
Loam  from 
Maryland 

Average  of 
5  Samples  1 

Corn  and 
Wheat  Clay 

Loam  Soil 
Average  of 

3  Samples1 

Residual 

Soil  from 

Virginia 

Gneiss2 

92.30 

80.55 

45.31 

3.20 

8.82 

26.55 

.91 

2.67 

12.18 

.05 

.42 

.47 

.41 

.47 

trace 

.08 

.05 

trace 

.35 

.29 

.40 

.50 

.49 

.22 

.70 

1.22 

1.10 

Residual 

Soil  from 

Virginia 

Limestone3 


57.57 

20.44 

7.93 

.10 

.51 

.38 

1.20 

.23 

4.91 


1  Veitch,  F.  P.     The  Chemical    Composition  of    Maryland 
Soils.     Maryland  Agr.  Exp.  Sta.,  Bui.  70,  pp.  71  and  73.     1901. 

2  Merrill,  G.  P.     Weathering  of  Micaceous  Gneiss  in  Albemarle 
County,  Virginia.     Bui.  Geol.  Soc.  Amer.,  Vol.  8,  p.  160.     1879. 

3  Diller,  G.  S.     The  Educational  Series  of  Rock  Specimens. 
U.  S.  Geol.  Survey,  Bui.  150,  p.  385.     1898. 


CLIMATIC  AND   GEOCHEMICAL   RELATIONSHIPS      61 

It  is  to  be  noted,  in  the  first  place,  that  silica  exists  in 
large  quantities  in  the  coastal  plain  soils,  due  to  the  fact 
that  quartz  is  such  a  resistant  mineral.  The  constant 
washing  that  this  soil  has  undergone  has  very  largely 
decomposed  the  silicates.  The  aluminium  and  iron  are 
rather  low  on  the  average  in  such  soils,  even  in  those  of 
the  richer  type.  It  is  to  be  noted  also  that  the  amounts 
of  phosphoric  acid,  calcium,  potash,  magnesia,  and  sodium 
are  much  less  in  the  marine  soils,  due  to  the  excessive 
washing  that  they  have  received.  These  figures  would 
lead  to  the  belief  that  in  general  the  marine  soils  are 
lower  in  the  mineral  plant-food  constituents  than  soils 
formed  in  situ.  The  amount  of  organic  matter  that  they 
may  contain  depends  entirely  on  their  location  and 
climatic  conditions.  They  may  or  may  not  be  rich  in 
humus,  according  to  circumstances.  It  is  generally  con- 
sidered, however,  that  they  are  not  so  well  supplied  with 
the  organic  elements  as  are  other  soils. 

53.  Residual  and  glacial  soils.  —  A  comparison  of 
residual  and  glacial  provinces  cannot  be  made  with  such 
assurance,  because  of  the  many  kinds  of  rocks  that  may 
have  been  parent  to  the  soils  and  because  of  the  great 
variety  of  climatic  conditions  under  which  the  soil-form- 
ing processes  may  have  gone  on.  Such  a  comparison  is 
best  made  in  a  region  where  both  residual  and  glacial  soils 
are  found,  as  nearly  as  it  is  possible  to  judge,  coming 
from  the  same  rocks.  Analyses  of  soils  under  such  con- 
ditions are  available,  from  the  driftless  and  glaciated 
parts  of  Wisconsin.  The  original  rock  was  limestone. 
The  analyses  1  are  as  follows  :  — 

1  Chamberlin,  T.  C,  and  Salisbury,  R.  D.  The  Drift- 
less  Area  of  the  Upper  Mississippi.  Sixth  Ann.  Rept.,  U.  S. 
Geol.   Survey,  pp.  249-250.     1885. 


68 


SOILS:    PROPERTIES  AND  MANAGEMENT 


Analyses  of  Residual  and  Glacial  Clays  from  the  Drift- 
less  and  Glaciated  Areas  of  \Vis< oxsin 


Residual 

Glacial 

1 

2 

3 

4 

SiO: 

A1203 

Fe203 

MgO 

CaO 

Na20 

K20 

P2Os 

C02 

H20 

71.13 
12.50 

5.52 
.38 
.85 

2.19 

1.61 
.02 
.43 

4.63 

49.13 

20.08 

11.04 

1.92 

1.22 

1.33 

1.61 

.04 

.39 

11.72 

40.22 

8.47 

2.83 

7.80 

15.65 

.84 

2.36 

.05 

18.76 

1.95 

48.81 
7.54 
2.53 
7.95 

11.83 

.92 

2.60 

.13 

15.47 
2.02 

These  analyses  illustrate  to  very  good  advantage  the 
beliefs  entertained  by  Chamberlain  and  Salisbury  regard- 
ing the  differences  between  residual  and  glacial  clays. 
Residual  clay  is  designated  by  them  as  "  rock  rot,"  and 
glacial  clay  as  "  rock  flour."  The  latter,  being  less 
weathered,  retains  a  larger  proportion  of  its  easily  soluble 
materials.  It  is  to  be  noted  here,  as  in  the  comparison 
of  marine  and  residual  soils,  that  silica,  aluminium,  and 
iron  are  lower  in  the  soil  subjected  to  the  less  amount  of 
leaching,  which  in  this  case  is  the  glacial  clay.  This  in 
itself  would  serve  to  indicate  that  the  important  plant- 
food  constituents  are  generally  present  in  larger  quanti- 
ties in  the  glacial  clay.  In  fact,  it  would  be  expected 
that  the  glacial  soils  would  approximate  very  closely  the 
rock  or  rocks  from  which  they  came.  The  phosphoric 
acid,  lime,  sodium,  magnesium,  and  potash  of  the  residual 
soils  in  this  case  amount  on  the  average  to  5.73  per  cent, 
while  that  of  the  glacial  clays  reaches  the  high  figure  of 
24.61  per  cent.     This  is  due  largely  to  the  great  amount 


CLIMATIC  AND   GEOCHEMICAL   RELATIONSHIPS      69 

of  lime  present,  and  again  emphasizes  the  point  that, 
while  a  glacial  soil  from  a  limestone  is  rich  in  lime,  a 
residual  soil  from  the  same  rock  is  usually  poor  in  that 
constituent.  Even  loess,  which  has  been  subjected  to 
some  washing  before  being  deposited,  is  a  considerably 
richer  soil  than  those  of  residual  origin. 

It  must  be  remembered,  however,  that  these  comparisons 
are  of  a  general  character  and  do  not  apply  to  all  cases, 
since  many  glacial  soils  may  be  very  much  poorer  in  the 
plant-food  constituents  than  some  of  the  representative 
residual  soils.  Moreover,  the  physical  condition  of  a 
soil  is  a  great  factor  in  productivity.  As  a  matter  of 
fact,  the  mere  presence  of  plant-food  is  but  one  of  a 
considerable  number  of  factors  that  determine  the  crop- 
producing  power  of  a  soil.  Also,  the  humus  content  of 
the  soils  of  various  provinces  may  be  variable,  due  to 
climatic  conditions.  Neither  are  all  glacial  soils  rich  in 
lime,  as  that  constituent  is  determined  largely  by  the 
amount  in  the  parent  minerals.  A  rock  poor  in  lime, 
therefore,  must  from  necessity  give  rise,  when  glaciated, 
to  a  soil  deficient  in  lime.  This  is  well  illustrated  by  the 
average  analyses  l  of  the  loam  soils  of  Ashtabula  County, 
Ohio,  originating  from  the  glaciation  of  the  lime-poor 
shales  of  that  region  :  — 

CaO    .     .     . 25 

MgO 61 

P205 04 

K20 1.87 

N 15 

Humus 1.70 

1  Ames,  J.  W.,  and  Gaither,  E.  W.  Soil  Investigations.  Ohio 
Agr.  Exp.  Sta.,  Bui.  261.     1913. 


70 


SOILS:     PHOPERTIES   AND   MANAGES!  EST 


However,  our  major  premise  does  seem  to  stand  in  a 
general  way  —  that  a  glacial  soil,  other  things  being 
equal,  contains  a  larger  amount  of  the  mineral  plant-food 
constituents,  and  ordinarily  a  smaller  amount  of  such 
materials  as  silica,  iron,  and  aluminium,  than  does  a 
corresponding  soil  of  residual  origin. 

The  following  data  !  bring  out  the  points  already  dealt 
with  in  their  fullest  significance:  — 

Percentage  of  P2O5,  CaO,  MgO,  and  K20  in  Soils  of  Dif- 
ferent Provinces 


Soils 

PiOs 

CaO 

MgO 

KiO 

Total 

7  Coastal  plain   .... 

3  Residual  (crystalline)    . 

10  Glacial 

.07 
.25 

.22 

.14 

.67 

1.36 

.16 
.75 

.79 

.70 
2.08 
2.08 

1.07 

3.75 

4.45 

54.  Effect  of  glaciation  on  agriculture.  —  These  differ- 
ences between  residual  and  glacial  soils  reflect  on  the 
general  fertility  of  the  soils.  In  a  comparison  of  the 
driftless  area  of  Wisconsin  with  the  glaciated  parts/ 
only  43  per  cent  of  the  former  is  improved  as  against 
61  per  cent  of  the  latter,  while  the  value  of  the  farms 
on  the  glaciated  soil  averages  50  per  cent  higher.  The 
same  general  differences  appear  between  the  glacial  and 
residual  soils  of  Indiana3  and  Ohio.4 


1  Failyer,   G.   H.,   and    others.      The  Mineral    Composition 
of  Soil  Particles.     U.  S.  D.  A.,  Bur.  Soils,  Bui.  54.     1908. 

2  Whitbeck,  R.  H.     The  Glaciated  and  Driftless  Portions  of 
Wisconsin.    Bui.  Geog.Soc.  Phil.,  Vol.  IX,No.3,pp.  10-20.     1911. 

3  Von  Engeln,  O.  D.      Effects  of  Continental  Glaciation  on 
Agriculture.     Bui.  Amer.  Geog.  Soc,  Vol.  XLVI,  p.  246.     1914. 

4  Ames,  J.  W.,  and    Gaither,    E.    W.       Soil    Investigations. 
Ohio  Agr.  Exp.  Sta.,  Bui.  261.     1913. 


CLIMATIC  AND   GEOCIIEMICAL   RELATIONSHIPS      71 

Von  Engeln,1  in  a  comparison  of  glaciated  soils  with 
corresponding  residual  areas,  was  able  to  point  out  cer- 
tain general  differences.  The  agricultural  condition 
within  the  zone  of  glaciation  was  always  consistently 
higher  than  that  beyond  the  regions  of  drift  accumulation. 
The  extensive  leveling  due  to  glacial  erosion  and  dep- 
osition had  almost  always  resulted  favorably  for  agri- 
cultural operations.  Even  the  thickness  of  the  drift 
was  found  to  conserve  the  ground  water  supply.  Not 
only  did  this  author  conclude  that  glacial  soils  were 
richer  in  soluble  plant-food  constituents  than  residual 
soils,  but  he  also  showed  that  glacial  soils  had  a  greater 
crop-producing  power  and  a  higher  agricultural  value. 
The  dominant  textural  quality  of  glacial  soils  seems 
adapted  to  certain  staple  food  crops,  and,  due  to  their 
intermingling,  a  considerable  opportunity  for  diversified 
and  intensified  farming  is  offered.  It  is  therefore  evident 
that  in  any  study  of  soils,  particularly  those  of  the  United 
States,  a  careful  consideration  of  the  effects  of  glaciation 
is  necessary.  The  great  ice  sheet  has  been  responsible 
in  some  cases  for  the  rejuvenation  of  our  soils,  in  others 
for  the  production  of  an  entirely  new  soil  mantle.  Even 
the  alterations  in  topography  are  factors  not  to  be  ignored. 

55.  Arid  and  humid  soils.2  —  This  distinction  between 
soils  due  to  differences  in  the  formative  process  is  always 
evident,  but  is  particularly  striking  in  a  comparison  of  arid 
and  humid  regions.  In  areas  of  light  rainfall  the  physical 
agents  are  dominant,  and  disintegration  goes  on  very 
largely  without  decomposition.     Under  humid  conditions, 

1  Von  Engeln,  O.  D.  Effects  of  Continental  Glaciation  on  Agri- 
culture.    Bui.  Amer.  Geog.  Soc,  Vol.  XLVI,  pp.  353-355.    1914. 

2  For  a  more  complete  discussion  of  this  subject,  see  Hilgard, 
E.  W.     Soils,  Chapters  XX  and  XXI.     New  York.     1911. 


72 


SOILS:    PROPERTIES  AND  MANAGEMENT 


however,  the  chemical  forces  are  the  determining  factor 
as  to  the  character  of  the  soil.  Arid  soils  are  therefore 
usually  coarser  soils  and  their  color  is  very  likely  to  be 
light.  Such  soils  are  deep  and  uniform,  there  being 
but  little  difference  between  the  surface  and  the  subsoil. 
The  soils  of  the  humid  regions  are  usually  of  fine  texture, 
particularly  in  residual  regions,  since  the  chemical  agencies 
have  been  so  active.  Various  colors  may  develop  because 
of  oxidation,  hydration,  and  the  presence  of  organic  matter. 
Such  soils  usually  are  not  excessively  deep,  and  are  likely 
to  be  underlaid  by  subsoils  heavier  than  the  surface. 
The  general  physical  condition  and  tilth  of  arid  soil  is 
uniformly  better  than  that  of  regions  of  plentiful  rainfall. 
/Chemically,  because  of  less  leaching  the  arid  soils  con- 
4ain_jriQre  ol  the  important  -mineral  plant-food  elements. 
The  following  analyses  bring  out  the  differences  in  a 
striking  manner :  — 


Arid  Soils 
Average  op 
573  Samples1 

Humid  Soils 
Average  op 
696  Samples1 

75.87 

88.21 

7.21 

3.66 

5.48 

3.88 

.16 

.12 

1.43 

.13 

1.27 

.29 

.35 

.14 

.67 

.21 

5.15 

4.40 

1.13 

1.22 

Average 
Composition  of 
l.ithosphere1 


Insoluble  residue  and  soluble 

Si02 

A1203 

Fe203 

P2O5 

CaO 

MgO 

Na20 

K20 

Water  and  ignition      .     .     . 
Humus 


59.36  (Si02) 
14.81 
6.34 
.29 
4.78 
3.74 
3.35 
2.98 


1  Hilgard,  E.  W.     The  Relation  of  Soil  to  Climate.     U.  S. 
Weather  Bur.,  Bui.  3.     1892. 

2  Clarke,    F.    W.       Data    of    Geochemistry.       U.    S.    Geol. 
Survey,  Bui.  491,  p.  33.     1911. 


CLIMATIC  AND   GEOCHEMICAL   RELATIONSHIPS      73 

It  is  immediately  apparent  that  the  arid  soil  is  poorer 
in  silica  than  the  humid  soil,  but  richer  in  iron  and  alumin- 
ium, indicating  a  less  weathered  condition  of  the  feldspars. 
Due  to  a  greater  amount  of  leaching,  the  humid  soil  is 
much  lower  in  phosphoric  acid,  lime,  magnesium,  sodium, 
and  potassium.  The  humus  in  arid  soils  is  somewhat 
lower  than  in  the  soils  under  better  conditions  of  rainfall, 
as  one  would  naturally  expect.  The  amount  of  easily 
soluble  material  is  higher  in  arid  regions,  due  to  the  lack 
of  heavy  rain  and  the  tendency  for  soluble  salts  to  accumu- 
late. A  comparison  of  the  analyses  above  with  Clarke's 
estimate  regarding  the  composition  of  the  earth  shows 
that  the  humid-region  soil  has  moved  farther  away  from 
the  average  soil-forming  rock  than  the  soil  produced 
under  conditions  of  aridity. 

Biologically,  organisms  are  found  active  at  greater 
depths  l  in  arid  regions  than  in  humid  regions,  because 
of  the  loose  structure  of  arid  soils  and  because  of  their 
good  aeration.  Such  soils  are  seldom  water-logged.  In 
humid  regions  bacterial  action  is  limited  very  largely 
to  the  surface  foot  of  soil,  since  only  there  are  the  aeration 
and  the  food  conditions  adequate.  The  intensity  of 
biological  activity  in  arid  soils  is  very  largely  governed 
by  moisture,  and  when  moisture  conditions  are  satisfied 
bacterial  changes  may  be  expected  to  take  place  rapidly. 
Cases  are  on  record  in  which  the  soluble  salts  due  to 
bacterial  action  have  become  of  such  concentration  as 
to  be  toxic  to  plants. 

56.  Soil  color.  —  Another  characteristic  of  soil  is  its 
color,  which  has  originated  during  the  processes  of  soil 

1  Lipman,  C.  B.  The  Distribution  and  Activities  of  Bacteria 
in  Soils  of  the  Arid  Region.  Univ.  of  Calif.,  Pub.  in  Agr.  Sci., 
Vol.  I,  No.  1,  pp.  1-20.     1912. 


74 


SOILS:    PROPERTIES   AND   MANAGEMENT 


formation,  largely  through  natural  weathering  agencies. 
This  is  really  a  phase  of  geochemistry,  particularly  as 
regards  those  tints  that  originate  from  the  oxidation  of 
the  iron.  Color  has  long  occupied  the  attention  of 
geologists  and  agriculturists,  in  the  first  place  because 
it  gives  a  clew  to  the  mode  of  soil  formation,  and  in  the 
second  place  because  it  is  to  a  certain  extent  an  index  to 
agricultural  value.  At  the  outset  it  must  be  understood 
that  soil  colors  are  not  pure  colors,  although  spoken  of 
as  such,  but  tints  and  shades.  In  soils  it  is  possible  to 
find  almost  any  conceivable  color,  ranging  from  white 
sands  to  black  swamp  muds  or  the  blood-red  clays  of  the 
Piedmont  region.  The  three  coloring  matters  of  soil 
may  be  classified  as  (1)  the  color  arising  from  the  mineral, 
(2)  the  color  given  by  the  humus  present  in  the  soil  and 
around  the  particles,  and  (3)  the  reds  or  the  yellows  due 
to  oxidization  of  the  iron.  These  three  primary,  or 
basal,  colors  may  be  represented  for  convenience  as 
follows :  — 


WH/TE 


0LACX 


BROWM/SH 


R5D 


Fig.  9.  —  A  triangular  representation  of  the  three  primary  soil  colors 
and  their  mixtures. 


CLIMATIC  AND   GEOCHEMICAL   RELATIONSHIPS      75 

A  soil  low  in  humus,  and  with  the  iron  either  absent  or 
unoxidized,  will  be  of  a  light  color.  Sea  sands  are  good 
illustrations  of  this  condition.  A  well-drained  soil  con- 
taining large  quantities  of  organic  matter  will  present  a 
deep  black  color  in  spite  of  the  oxidized  iron,  as  the  latter 
will  be  masked  to  a  large  extent.  If  humus  is  low  or 
lacking  and  the  iron  is  oxidized,  a  red  or  a  yellow  color 
may  characterize  the  soil.  As  might  be  expected,  there 
are  blendings  of  these  three  primary  colors,  and  grays, 
browns,  and  yellows  of  varying  intensities  are  common. 

57.  White  and  black  soils.  —  The  light  colors  in  soils 
are  not  due  to  the  agencies  of  weathering,  but  rather  to 
a  lack  of  such  action.  The  cause  of  such  coloration  is 
therefore  not  hard  to  explain.  The  development  of  the 
black  or  dark  colors  and  tints,  being  due  to  the  accumula- 
tion of  organic  matter,  indicates  the  operation  of  two 
favoring  conditions:  first,  climatic  agencies  that  stimu- 
late the  luxuriant  development  of  plants ;  and,  secondly, 
sufficient  aeration  to  promote  a  favorable  decay  of  such 
tissue.  It  is  a  well-recognized  fact  that  in  order  to\ 
develop  a  black  color  from  decaying  vegetable  matterj 
fairly  good  aeration  must  be  provided.  If  such  a  coi/ 
dition  does  not  prevail,  the  decayed  material  has  a  lighter 
hue  and  may  exhibit  toxic  properties  which  will  check 
or  inhibit  plant  growth.  The  development  of  the  black 
color,  therefore,  in  a  normal  well-drained  soil,  is  an  in- 
dication of  good  soil  sanitation. 

58.  Red  and  yellow  soils.  —  The  presence  of  iron,  as 
already  noted,  is  a  very  important  factor  in  rock  weather- 
ing, and  the  discoloration  due  to  its  presence  is  an  unfailing 
indication  of  chemical  decay.  The  iron  in  minerals 
occurs  usually  as  ferrous  oxide,  which  is  soluble,  especially 
if  the  water  circulating  among  the  rock  fragments  carries 


76         SOILS:    PROPERTIES  AND  MANAGEMENT 

carbon  dioxide.  As  this  water  comes  in  contact  with  the 
air,  its  excess  of  carbon  dioxide  is  discharged  and  the 
oxides  and  carbonates  of  iron  are  deposited.  Under 
this  condition  oxidation  goes  on  rapidly,  and  the  iron 
passes  to  the  ferric  state  and  becomes  insoluble.  Thus 
it  may  be  seen  that  iron  imparts  a  fatal  weakness  to  rocks 
and  minerals  in  which  it  may  exist,  due  to  its  solubility ; 
yet  from  the  oxidation  that  it  undergoes,  it  tends  to 
persist  and  accumulate  in  soils.  A  corollary  might  be 
added  to  the  law  of  mineral  resistance,  to  the  effect  that 
"  the  more  iron  a  mineral  contains,  the  more  susceptible 
it  is  to  the  weathering  agencies." 

Therefore,  from  the  geochemical  standpoint,  the  de- 
velopment of  the  red  and  yellow  colors  in  soils  has  been 
the  subject  of  considerable  dispute  from  time  to  time. 
The  red  and  yellow  soils  of  the  Cotton  States  frequently 
excite  comment,  especially  as  a  difference  in  fertility  is 
popularly  recognized ;  the  red  surface  soil  with  a  red 
subsoil  being  considered  more  fertile  than  a  similar  soil 
with  a  yellow  subsoil.  Crosby  1  believes  that  the  differ- 
ence in  color  is  due  to  a  difference  in  hydration  of  the 
iron  oxides.  The  soil  temperatures,  particularly  in 
tropical  and  subtropical  regions,  have  first  tended  to 
fully  oxidize  and  hydrate  the  iron,  and  then  to  dehydrate 
the  soil  at  the  surface  into  the  deep  red  color,  leaving  the 
subsoil  yellow  and  causing  the  contrasts  so  markedly 
evident.  The  ultimate  product  of  both  oxidation  and 
hydration  would  be  limnite,  a  yellow  mineral;  while 
if  only  oxidation  were  active,  hematite,  which  imparts 
a  red  color,  would  result  as  a  final  product.  A  dehydra- 
tion of  the  limnite  Avould  cause  the  formation  of  hematite 

1  Crosby,  W.  O.  Colors  of  Soils.  Proc.  Boston  Soc.  Nat.  Hist., 
Vol.  23,  pp.  219-222.     1875. 


CLIMATIC  AND   GEOCHEMICAL   RELATIONSHIPS      77 

or  some  intermediate  product.  The  composition  of  the 
common  iron  oxides  found  in  soils  tends  to  support 
Crosby's  explanation :  — 

Hematite   ....  Fe203  Red 

Turgite       ....  2  Fe203 .  H20 

Goethite     ....  Fe203 .  H20 

Limonite    ....  2  F203 .  3  H20 

Xanthosiderite    .     .  Fe203 .  2  H20 

Limnite      ....  Fe203 . 3  H20  Yellow 

Merrill l  holds  the  same  idea,  but  thinks  that  the  sur- 
face soil  may  contain  relatively  more  iron  than  the  sub- 
soil. He  considers  that  the  ferric  iron  oxides,  because 
of  their  insoluble  nature,  tend  to  accumulate  at  the 
surface,  and  because  of  their  large  quantities  and  because 
they  are  there  subjected  to  more  vigorous  weathering 
action  the  vivid  red  colors  tend  to  develop. 

The  iron  coloring  matter  usually  exists  as  a  coating  2 
on  the  soil  particles,  although  it  may  sometimes  occur 
as  concretions.  It  is  found  also  that  in  general,  but  not 
always,  the  intensity  of  the  color  varies  with  the  amount 
of  iron  present.  From  a  large  number  of  analyses  com- 
piled by  Robinson  and  McCaughey,3  the  following  figures 
may  be  obtained  showing  the  authority  for  such  a  state- 
ment :  — 

Average  Iron  Content  op  Percent  of 

Soils  Ferric  Iron 

Deep  reds  to  light  reds  .     . 14.40 

Ochre  yellow  to  yellow  .......       8.85 

1  Merrill,  G.  P.  Rocks,  Rock  Weathering,  and  Soils,  p.  375. 
New  York.     1906. 

2  Van  Bemmelen,  J.  M.  Beitrage  zur  Kenntnis  der  Ver- 
witterungsprodukte  der  Silicate  in  Ton-,  Vulkanischen-,  und  Lat- 
erite-Boden.     Zeit.  Anorg.  Chem.,  Bd.  42,  Seite.  290-298.     1904. 

3  Robinson,  W.  O.,  and  McCaughey,  W.  J.  The  Color  of 
Soils.     U.  S.  D.  A.,  Bur.  Soils,  Bui.  79,  p.  21.     1911. 


78         SOILS:    PROPERTIES  AND   MANAGEMENT 

This  being  true,  the  thicker  the  film,  the  greater  is  the 
intensity  of  the  color.  The  same  quantity  of  iron,  there- 
fore, would  make  a  greater  showing  in  a  sandy  soil  than 
in  clay,  as  the  amount  of  internal  surface  of  the  former  is 
comparatively  low  and  the  film  of  iron  oxide  would  there- 
fore be  thicker. 

It  is  evident  from  the  data  already  presented  that 
the  intensity  of  color  arising  from  iron  in  the  soil  is  due 
to  several  conditions.  Without  a  doubt  the  oxidation 
that  occurs  is  of  primary  importance,  but  the  hydration 
that  very  often  takes  place  is  a  powerful  modifying  agent. 
The  thickness  of  the  film,  as  determined  by  the  amount 
of  iron  present  or  by  the  texture  of  the  soil,  is  probably 
a  factor  having  to  do  particularly  with  the  intensity  of 
the  coloration,  although  the  color  or  tint  itself  may  be 
modified  to  a  certain  extent  thereby. 

59.  Agricultural  significance  of  color.  —  The  white  or 
the  black  color  of  a  soil  indicates  the  lack  or  the  presence 
of  an  important  constituent,  namely,  organic  matter. 
This  matter  not  only  tends  to  keep  the  soil  in  good 
physical  condition,  but  also  acts  both  as  a  plant-food 
and  as  a  source  of  energy  for  bacteria  and  other  soil 
organisms.  A  dark  soil,  provided  its  drainage  and 
climatic  conditions  are  favorable,  is  usually  a  rich  soil. 
The  dark  color  is  no  mean  factor  in  temperature  relation- 
ships, since  not  only  does  a  dark  soil  absorb  heat  faster 
than  a  light  soil,  but  the  tendency  of  the  former  toward 
reflection  and  radiation  is  much  restricted.  This  is 
important  with  crops  which  must  go  into  the  soil  early 
in  the  season,  or  which  need-  to  be  pushed  rapidly  to 
maturity.  A  dark  color,  with  virgin  soils  especially,  is 
an  excellent  guide  to  fertility  and  general  agricultural 
value. 


CLIMATIC  AND   GEOCHEMICAL   RELATIONSHIPS      79 

Red  colors  in  soil  often  show  low  or  medium  organic 
content.  Besides  this,  the  presence  of  oxidized  iron  is 
always  an  indication  of  age.  The  residual  soils  of  the 
Piedmont  Plateau  are  especially  characterized  in  this 
way.  Age  gives  opportunities  for  leaching,  and  conse- 
quently a  lack  of  the  soluble  bases  may  be  expected  in 
such  soils.  The  reds  and  the  yellows  are  characteristic 
of  residual  soils,  or  of  soils  that  have  arisen  from  them 
by  erosion  or  glaciation.  A  red  color  is  almost  as  efficient 
in  the  absorption  of  heat  as  is  black;  so  that  the  early- 
growth  and  quick-maturing  tendencies  of  crops  on  a 
red  soil,  other  things  being  equal,  are  about  the  same  as 
on  a  dark  soil.  Hilgard,1  who  lays  great  stress  on  the 
agricultural  significance  of  color,  considers  the  mottled 
yellows  and  reds  as  indications  of  poor  drainage,  since 
such  a  condition  shows  that  oxidation  has  been  both 
unequal  and  insufficient.  A  soil  that  has  a  heavy  blue 
or  mottled  blue  clay  as  a  subsoil  will  in  most  cases  be 
greatly  benefited  by  some  form  of  drainage. 

60.  Soil  and  subsoil.  —  A  common  distinction  is  made 
between  the  surface  soil  and  that  which  is  some  distance 
below  the  surface.  This  is  natural,  as  the  forces  of  soil 
formation  have  served  to  bring  about  certain  distinctions, 
especially  in  humid  regions,  which  are  of  importance  in 
any  consideration  of  soil  fertility  and  crop  growth.  Cli- 
matic agencies  acting  on  soil  after  it  has  been  formed  have 
served  to  intensify  these  distinctions.  The  term  soil 
is  used  to  designate  the  top  layer  of  earth,  which  usually 
extends  to  the  plow  line  or  even  deeper.  The  soil  below 
is  spoken  of  as  the  subsoil,  and  may  be  rather  variable  in  its 
depth  (Fig.  10) .    Often  the  subsoil  is  divided  into  the  upper 

1  Hilgard,  E.  W.     Soils,  pp.  283-285.     New  York.     1906. 


80 


SOILS:    PROPERTIES  AND  MANAGEMENT 


and  the  lower  subsoil,  the  upper  subsoil  being  considered 
to  extend  to  about  the  depth  of  three  feet  below  the  sur- 
face.    Usually,   especially  in  humid  regions,  there  is  a 


Fig.   10.  —  Soil  and  subsoil,     (a),  Surface  soil  with  many  plant  roots 
(b),  subsoil ;  (c),  country  rock. 


sharp  line  of  demarcation  between  the  soil  and  the  sub- 
soil, due  to  differences  in  the  humus  content.  As  organic 
matter  accumulates  faster  at  the  surface,  the  soil  there 
tends  to  assume  a  darker  color.  Whether  the  land  has 
been  tilled  or  not,  this  line  of  separation  is  fairly  marked 
and  can  usually  be  located  with  little  difficulty.  On 
tilled  land,  where  the  surface  soil  extends  to  about  the 
depth  of  plowing,  the  plow  line  marks  the  separation  of 
surface  and  subsoil.  Where  soil  samples  are  being  taken 
for  soil  survey  or  soil  analysis,  some  arbitrary  depth, 
depending  on  circumstances,  is  usually  established  for 
the  surface  soil.  This  depth  varies  from  six  to  twelve 
inches. 

61.    Soil  and  subsoil  of  humid  regions.  —  In  humid  cli- 


CLIMATIC  AND   GEOCHEMICAL   RELATIONSHIPS      81 

mates  there  are  usually  certain  well-defined  differences  be- 
tween the  surface  soil  and  the  subsoil,  besides  the  organic 
content  already  spoken  of.  The  subsoil  is  usually  of  a  finer 
and  heavier  character  than  the  surface  soil,  due  to  the 
downward  movement  of  the  small  particles.  This  tends 
to  give  the  subsoil  high  retentive  power,  and  may  make 
it  rather  impervious  to  water.  Poor  drainage  conditions 
may  result.  A  certain  amount  of  retentive  power  in  a 
subsoil  is  of  considerable  advantage,  in  that  it  aids  in 
the  storage  of  water  and  prevents  the  excessive  leaching 
away  of  soluble  plant-food.  Moreover,  almost  all  the 
bacterial  activities  so  important  in  the  simplification  of 
compounds  carrying  food  constituents  are  restricted 
to  the  surface  soil.  The  subsoil,  being  protected  by  the 
layers  above,  has  not  been  subjected  to  such  vigorous 
weathering,  and  as  a  consequence  its  mineral  constituents 
are  not  so  available  for  the  use  of  the  crop.  The  deepen- 
ing of  the  plow  line  and  a  consequent  turning-up  of  the 
subsoil  must  be  carried  out  very  cautiously  for  the  above 
reason.  The  cropping  power  of  a  soil  may  be  markedly 
reduced  by  the  presence  of  too  much  of  such  material  on 
the  surface  at  one  time. 

The  root  distribution  is  restricted  largely  to  the  surface 
soil,  and  this  condition  determines  to  some  extent  the 
larger  accumulation  of  humus  therein  and  also  its  better 
aeration  and  drainage.  Experiments  conducted  in  Utah  l 
show  that  with  barley,  corn,  and  clover,  from  90  to  96 
per  cent  of  the  roots  grow  in  the  upper  seven  inches  of 
soil.     From  experiments  made  in  Kansas  2  and  in  North 

1  Sanborn,  J.  W.  Roots  of  Farm  Crops.  Utah  Agr.  Exp. 
Sta.,  Bui.  32.     1894. 

2  Ten  Eyck,  A.  M.  The  Roots  of  Plants.  Kansas  Agr.  Exp. 
Sta.,  Bui.  127.     1904. 


82       SOILS:   properties  and  management 

Dakota,1  the  roots  of  such  crops  as  alfalfa  were  found 
to  penetrate  to  a  depth  of  ten  feet,  while  the  small  grains 
often  showed  an  extension  of  their  roots  to  four  feet 
below  the  surface.  It  must  be  borne  in  mind,  however, 
that,  while  some  plant  roots  may  penetrate  far  into  the 
subsoil,  the  main  feeding  rootlets  are  restricted  largely 
to  the  surface  soil.  This  is  natural,  as  there  they  find 
the  aeration  and  drainage  essential  to  normal  growth. 
Hilgard  2  has  shown  that  plants  in  arid  regions  have  a 
root  extension  far  beyond  that  of  the  same  crops  under 
humid  conditions.  The  physical  conditions  of  the  arid 
subsoil,  the  larger  amount  of  plant-food,  and  the  better 
aeration,  account  for  such  differences. 

62.  Soil  and  subsoil  of  arid  regions.  —  The  subsoils 
in  arid  or  semiarid  regions  do  not  exhibit  such  marked 
contrast  to  the  surface  soil  as  are  observed  in  humid 
climates.  In  arid  soils  there  is  generally  no  sharp  line 
of  demarcation  between  soil  and  subsoil,  the  latter  being 
as  high  in  humus  and  in  agricultural  value  as  the  former. 
Nor  is  any  great  textural  variation  to  be  observed.  The 
latter  condition  is  due  to  the  fact  that  physical  weather- 
ing is  dominant  in  such  a  region.  As  a  consequence,  arid 
soils  may  be  leveled,  often  excessively,  in  establishing 
an  even  surface  for  the  application  of  irrigation  water, 
without  any  danger  of  lowering  the  fertility  thereby. 
Such  a  practice  in  humid  regions  would  be  fatal  to  the 
further  growing  of  successful  crops,  at  least  for  a  con- 
siderable period  of  years. 

1  Sheppard,  J.  H.  Root  Systems  of  Field  Crops.  N.  Dak. 
Agr.  Exp.  Sta.,  Bui.  64.     1905. 

2  Hilgard,  E.  W.  Soils,  Chapter  X,  pp.  161-187.  New  York. 
1911. 


CHAPTER  VI 
THE  SOIL  PARTICLE 

The  soil  formed  by  the  grind ing-up  of  rocks  and  the 
intermixing  therewith  of  small  quantities  of  organic 
matter  must  be  studied  physically  from  the  standpoint 
of  its  particles.  These  particles,  varying  in  size  from 
coarse  gravel  easily  discernible  by  the  naked  eye  to 
particles  so  fine  as  to  be  invisible  under  the  ultramicro- 
scope,  determine  very  largely  the  different  relationships 
of  the  soil  to  the  plant.  The  movement  of  air  in  the  soil, 
the  circulation  of  water,  the  rate  of  oxidation  and  hydra- 
tion, and  the  presence  and  virility  of  various  organisms, 
are  determined  very  largely  by  the  size  of  the  particles 
making  up  the  soil.  Texture  is  the  term  used  to  express 
this  size  of  particle.  Thus  a  soil  texture  may  be  coarse, 
medium,  or  fine,  indicating  that  the  particles  making  up 
that  soil  conform  in  general  to  such  description.  Tex- 
ture is  of  great  importance  in  soil  study  and  utilization. 

There  is  hardly  any  condition  exhibited  by  the  soil 
that  is  not  influenced,  if  not  directly  determined,  by  the 
size  of  the  soil  particles.  A  study  of  plant  conditions, 
whether  physical  or  chemical,  ultimately  leads  either 
directly  or  indirectly  to  a  consideration  of  soil  texture. 
Texture,  however,  is  an  element  which  can  be  but  little 
modified  under  normal  conditions.  We  have  seen  how 
a  rock  can  be  disintegrated  and  decomposed  into  a  soil. 
A  change  in  texture  has  been  wrought,  but  such  a  process 
demands  geologic  ages  for  its  fulfillment.     In  the  time 

83 


84  SOILS:    PROPERTIES  AND  MANAGEMENT    . 

covered  by  the  life  of  man  the  necessary  forces  are  not 
active  enough  to  have  this  effect ;  consequently,  as  far 
as  the  farmer  is  concerned  the  texture  of  the  soil  in  his 
field  is  subject  to  but  slight  alteration.  A  sand  remains 
a  sand  and  a  clay  remains  a  clay,  as  far  as  practical  con- 
siderations are  concerned.  Changes  in  texture  may  be 
made  on  a  small  scale  by  mixing  two  soils,  but  this  is  not 
practicable  in  the  field. 

63.  Soil  separates  and  mechanical  analysis.  —  The 
soil  particles,  varying  in  size  as  they  do,  may  be  separated 
into  arbitrary  divisions,  according  to  their  diameters. 
The  various  groups  are  designated  as  soil  separates,  and 
the  process  of  making  the  separation  and  determining 
the  percentage  of  each  group  present  is  called  mechanical 
analysis.  There  are  a  large  number  of  classifications,  or 
groupings,  of  the  soil  particles,  as  well  as  several  methods 
of  bringing  about  the  actual  separation.  The  grouping 
and  method  of  mechanical  analysis  most  generally  used 
in  this  country  is  that  devised  by  the  United  States  Bureau 
of  Soils.1  Other  methods 2  are  more  nearly  accurate, 
but  speed  as  well  as  precision  is  necessary  in  this  work. 
A  Swedish  classification 3  of  soil  particles  has  been  adopted 
by  the  Committee  on  Mechanical  Soil  Analysis,4  appointed 

1  Briggs,  L.  J.,  and  others.  The  Centrifugal  Method  of  Soil 
Analysis.     U.  S.  D.  A.,  Bur.  Soils,  Bui.  24.     1904. 

2  For  a  detailed  discussion  of  all  methods  of  mechanical 
analysis,  see  Wiley,  H.  W.  Agricultural  Analysis,  Vol.  I, 
pp.  195-276.     Easton,  Pa.     1906. 

8  Atterberg,  A.  Die  Mechanische  Bodenanalyse  und  die 
Klassifikation  der  Mineralboden  Schwedens.  Internat.  Mitt, 
f.  Bodenkunde,  Band  II,  Heft  4,  Seite  312-342.     1912. 

4  Schucht,  F.  Uber  die  Sitzung  der  Internationalen  Kom- 
mission  fur  die  Mechanische  und  Physikalische  Bodenunter- 
suchung  in  Berlin  am  31,  October  1913.  Internat.  Mitt.  f. 
Bodenkunde,  Band  IV,  Heft  I,  Seite  1-31.     1914. 


THE  SOIL  PARTICLE 


85 


by  the  Second  International  Agro-Geological  Congress, 
which  met  in  Stockholm  in  1910.  In  simplicity  and 
facility  of  interpretation  the  last-named  grouping  seems 
at  least  equal  to  that  of  the  Bureau  of  Soils.  Since  a 
number  of  methods  of  mechanical  analysis  have  been 
devised  during  the  evolution  and  study  of  soil  separation, 
it  is  necessary  to  be  conversant  with  the  principles  in- 
volved and  with  at  least  two  or  three  of  the  most  successful 
modes  of  procedure. 

64.  Principles  of  mechanical  analysis.  —  The  various 
methods  of  mechanical  analysis  may  be  grouped  according 
to  the  agents  employed  in  the  separation.  The  outline  is 
as  follows :  — 


1.  Sieve 


Outline  of  systems  of  mechanical  analysis 

Wet 

(Used  to  separate  sands  in  practically  all 

j)  methods) 


3.  Water 


In  motion 


At  rest 


2.  Air  (Cushman's  air  elutriator) 

Gravity   (Schone's  elutriator  and 

Hilgard's  churn  elutriator) 
Centrifugal     (Yoder's    centrifugal 

elutriator) 
Gravity  (Osborne's  beaker  method 

and    Atterberg's    modified    silt 

cylinder) 
Centrifugal      (Bureau      of      Soils 

method) 

In  the  consideration  of  such  an  outline,  certain  of  the 
general  methods  proposed  may  be  dismissed  without 
further  parley  since  they  are  inadequate  for  the  separation 


86         SOILS:    PROPERTIES  AND  MANAGEMENT 

in  question.  Sieves  of  all  kinds  have  the  one  great  dis- 
advantage that  their  meshes  cannot  be  made  small  enough 
to  separate  the  finer  grades  of  soil.  When  one  considers 
that  many  soil  particles  are  less  than  .005  millimeter  in 
diameter,  the  inadequacy  of  sieve  separation  becomes 
apparent.  However,  sieves  may  be  used  in  connection 
with  other  methods  as  an  easy  way  of  dealing  with  the 
larger  soil  particles.  Air  in  motion  1  is  inadequate,  as 
it  can  be  used  only  for  very  fine  particles.  Even  with 
these  the  separation  is  slow  and  inaccurate,  because  of 
the  tendency  of  the  dry  particles  to  cohere.  These  two 
methods  have  therefore  been  largely  abandoned  as  dis- 
tinct methods,  and  water  is  used  as  the  medium  of  separa- 
tion in  all  the  modern  systems  of  mechanical  analysis. 

The  principle  involved  in  the  subsidence  of  soil  particles 
in  water,  whether  the  force  of  gravity  or  centrifugal 
force  is  utilized,  is  recognized  by  every  one.  When 
fragments  of  rock  or  soil  are  suspended  in  water,  they 
tend  to  sink  slowly,  and  it  is  a  well-recognized  fact  that 
other  things  being  equal,  the  rate  of  settling  depends 
on  the  size  of  the  particle.  As  the  particle  is  decreased 
in  size,  its  weight  decreases  faster  than  the  surface  ex- 
posed to  the  buoyant  force  of  the  water.  As  a  conse- 
quence, the  rapidity  with  which  the  soil  particles  settle 
is  proportional  to  their  size.  The  suspension  of  a  sample 
of  soil  would  therefore  be  the  first  step  in  mechanical 
separation  by  water ;  the  second  step  would  be  subsidence 
and  the  withdrawal  of  each  successive  grade  of  particles 
as  it  slowly  settled ;  the  third  step  would  be  determina- 
tion of  the  percentage  of  each  grade,  or  group,  of  particles 

1  Cushman,  A.  S.,  and  Hubbard,  P.  Air  Elutriation  of 
Fine  Powders.  Jour.  Amer.  Chem.  Soc.,  Vol.  29,  No.  4,  pp. 
589-597.     1907. 


THE  SOIL   PARTICLE 


87 


as  based  on  the  original  sample.     This  is 
every  method  of  mechanical  analysis  in 
utilized  aims  to  do,  although  often  the 
technique  are  excessively  complicated. 

65.  Mechanical  analysis  by  water  in 
motion.  Schone's  elutriator.  —  Any  ap- 
pliance that  is  designed  to  separate 
particles  of  different  sizes  by  water  in 
motion  may  be  designated  as  an  elutria- 
tor. One  of  these,  commonly  used  in 
Europe,  is  called  Schone's  elutriator.1 
This  utilizes  hydraulic  force.  In  it  the 
upward  current  of  water  ascends  a  coni- 
cal glass  tube  (see  Fig.  11)  from  a 
narrow  curved  inlet  tube  below.  The 
soil  sample  present  in  the  inlet  tube  is 
kept  agitated  by  the  current.  It  is  evi- 
dent that  by  regulating  the  rate  of  flow 
of  the  water,  different  sizes  of  particles 
will  be  carried  away  over  the  top  of 
this  conical  glass  tube.  Thus  by  a  gentle 
flow  only  fine  grades  will  be  separated, 
while  by  increasing  the  current  larger 
and  still  larger  particles  will  be  carried 
upward  against  the  force  of  gravity. 

There  are  three  objections  to  this 
method :  first,  the  entrance  tube  may 
become  clogged,  and,  unless  a  very  small 


precisely  what 
which  water  is 
apparatus  and 


Fig.  11.  —  Schone's 
elutriator  for  me- 
chanical soil 
analysis  with 
water  in  motion. 


1  Schone,  E.  Ueber  Schlammanalyse.  Bui.  Soc.  imperiale 
des  Naturalistes  de  Moscow,  40,  Part  1,  p.  324.  1867.  Uber 
Schlammanalyse  und  einen  neuen  Schlammapparat.  Berlin, 
1867.  Also  see  Wiley,  H.  W.  Agricultural  Analysis,  Vol.  I, 
pp.  231-241.     Easton,  Pa.     1906. 


88 


SOILS:    PROPERTIES  AND  MANAGEMENT 


f 


quantity  of  soil  is  used,  the  mass  is  not  kept  properly 
agitated ;  secondly,  convex  currents  are  set  up  in  the 
conical  glass  tube,  which  vitiates  the  results ;  and,  thirdly, 
the  separate  particles  tend  to  coalesce  into  granules.  It 
is  evident  that  in  any  separation  of  soil  particles  all 
granulation  must  be  avoided.  This  is  usually  accom- 
plished by  shaking  or  boiling  the 
,/jL  sample  previous  to  the  determi- 

nation.     The   tendency  toward 
granulation  during   the   process 
of   separation   itself  is  fatal   to 
JL^Y_— j!  accuracy,  as  compound  particles 

carrying  a  large  number  of  small 
grains  would  fail  to  pass  over 
at  water-current  velocities  cor- 
responding to  their  component 
parts. 

66.  Hilgard's  churn  elutriator. 
—  The  errors  of  the  Schone  ap- 
paratus are  obviated  to  some 
extent  by  Hilgard's,1  the  prin- 
ciple of  operation  remaining  ex- 
actly the  same.  In  Hilgard's 
elutriator  the  deflocculated  soil 
sample  is  introduced  into  the 
base  of  a  cylindrical  tube  (see 
Fig.  12)  in  which  is  placed  a 
rapidly  revolving  stirrer.  This 
is  designed  to  counteract  convex 
currents    and    to    prevent    the 


Fig.  12.  —  Hilgard's  churn 
elutriator  for  mechanical 
soil  analysis  of  particles 
above  .01  mm.  in  diameter, 
(e),  Intake;  (p),  stirrer; 
(c),  screen;  (a),  separating 
chamber ;  (o),  outlet  tube. 


1  Hilgard,  E.  W.  Methods  of  Physical  and  Chemical  Soil 
Analysis.  Ann.  Rept.  California  Agr.  Exp.  Sta.,  pp.  241-257. 
1891-1892. 


*  THE  SOIL  PARTICLE  89 

formation  of  compound  particles.  A  screen  placed  just 
above  the  stirrer  serves  to  prevent  the  whirling  motion 
from  being  communicated  to  the  ascending  column  of 
water  in  which  the  separation  occurs.  The  various  grades 
in  the  separation  are  regulated  by  the  rate  of  water  flow. 
With  this  apparatus  it  is  necessary  to  remove  the  finer 
particles  below  .01  mm.  in  diameter  by  subsidence 
previous  to  the  determination. 

While  this  method  is  very  nearly  accurate  and  will 
give  a  separation  of  the  various  grades  such  as  is  impossible 
with  most  other  methods,  it  is  impracticable  in  ordinary 
soil  work.  The  large  quantity  of  water  which  is  used 
in  carrying  over  each  grade,  and  which  of  course  must 
be  evaporated  before  the  sample  can  be  weighed,  is  the 
first  objection.  The  length  of  time  necessary  for  the 
separation,  and  the  cost  of  the  apparatus,  are  two  addi- 
tional objections  urged  against  it.  As  mechanical  analysis 
is  used  largely  in  determining  soil  texture,  rapidity  and 
ease  of  operation  are  of  more  importance  than  the  ex- 
tremely accurate  separation  of  the  particles. 

67.  Yoder's  centrifugal  elutriator.  —  One  of  the  ob- 
jections to  the  methods  already  described  is  the  length 
of  time  necessary  for  a  determination.  This  is  due  to 
the  fact  that  very  fine  particles  subside  in  water  very 
slowly.  In  order  to  hasten  the  separation,  Yoder x 
devised  a  machine  in  which  hydraulic  force  may  be 
supplemented  by  a  centrifugal  pull.  This  ingenious 
apparatus  consists  of  an  elutriator  bottle  (see  Fig.  13) 
mounted  in  a  centrifuge.  The  muddy  water  is  introduced 
into  the  bottle  at  the  center  of  the  centrifuge.  It  then 
passes  to  the  bottom  of  the  bottle  and  back  again  to  the 

1  Yoder,  P.  A.  A  New  Centrifugal  Soil  Elutriator.  Utah 
Agr.  Exp.  Sta.,  Bui.  89.     1904. 


90         SOILS:    PROPERTIES  AND  MANAGEMENT 

outlet,  carrying  with  it  a  sediment  the  size  of  which 
depends  on  the  rate  .of  water  flow  and  the  strength  of 
the  centrifugal  force.  The  bottle  is  so  designed  that 
particles  in  all  parts  of  the  separating  chamber  are  sub- 
jected to  the  same  force,  no  matter  what  their  distance 


Fig.  13.  —  Separatory  bottle  of  Yoder's  centrifugal  elutriator.  (B),  Bottle  ; 
(e),  intake ;  (a),  tube  for  conducting  liquid  to  bottom  of  separatory 
bottle;  (o),  outlet;  (C),  centrifuge ;  (u>),  counterpoise. 

from  the  center  of  the  centrifuge  may  be.  The  apparatus 
can  be  used  only  for  separating  particles  less  than  .03 
millimeter  in  diameter.  It  is  open  to  the  same  objections 
that  apply  to  Hilgard's  machine,  besides  being  very  much 
more  complicated  and  delicately  adjusted.  It  is  too 
costly  an  apparatus  for  ordinary  work. 

68.  Mechanical  analysis  by  water  at  rest.  Osborne's 
beaker  method.  —  One  of  the  earliest  and  most  nearly 
accurate  methods  to  be  perfected  was  the  separation  of 
the  various  grades  of  soil  by  simple  subsidence  in  a  column 
of  still  water.  This  is  commonly  spoken  of  as  the  Os- 
borne beaker  method.1  The  determination  is  very 
simple.  The  soil  sample  is  first  fully  deflocculated  and 
thrown  into  suspension,  each  particle  functioning  sepa- 
rately.    Beakers  are  commonly  used  as  containers,  but 

1  Osborne,  T.  B.  Methods  of  Mechanical  Soil  Analysis.  Ann. 
Rept.  Connecticut  Agr.  Exp.  Sta.,  1886,  pp.  141-158;  1887,  pp. 
144-162;    1888,  pp.  154-157. 


THE  SOIL   PARTICLE  91 

any  vessel  that  is  relatively  deep  will  do  for  the  deter- 
mination. The  larger  particles,  or  sand  grains,  will  of 
course  settle  first,  and  the  finer  silts  and  clays  may  be 
decanted  off.  As  the  sands  carry  finer  particles  down 
with  them,  the  suspension  and  subsidence  must  be  re- 
peated a  number  of  times.  The  finer  particles,  separated 
thus  and  decanted,  may  be  further  subdivided  in  the 
same  manner.  The  time  necessary  for  such  decantation 
as  will  leave  in  suspension  only  particles  below  a  given 
size  is  determined  by  the  examination  of  a  drop  of  the 
suspension  under  a  microscope  fitted  with  an  eyepiece 
micrometer.  In  this  way  the  size  of  the  particles  decanted 
may  be  accurately  measured. 

The  three  steps  in  this  method  of  separation  are: 
deflocculation  of  the  sample;  separation  by  successive 
subsidence  and  decantation;  and  evaporation  to  dryness 
of  the  separates  and  their  calculation  to  a  percentage 
based  on  the  original  sample.  The  method,  however,  is 
slow,  as  the  time  necessary  for  each  subsidence  of  the 
finer  particles  is  very  great  and  the  number  of  individual 
subsidences  is  large.  Neither  is  the  method  capable  of 
the  refinement  of  separation  which  is  possible  with  cer- 
tain of  the  elutriators.  As  a  consequence  it  has  been 
superseded  by  methods  that  utilize  centrifugal  force  for 
the  finer  separations  while  retaining  gravity  for  removing 
the  various  grades  of  sand. 

69.  Atterberg's  modified  Appiani 1  silt  cylinder  (Fig. 
14). —  This  method2 is  similar  to  the  beaker  method  in 

1  Appiani,  G.  Ueber  einen  Schlammapparat  fur  die  Analyse 
der  Boden-  und  Thonarten.  Forsch.  a.  d.  Gebiete  d.  Agri- 
Physik,  Band  17,  Seite  291-297.     1894. 

2  Atterberg,  A.  Die  Mechanische  Bodenanalyse  und  die 
Klassifikation  der  Mineralboden  Schwedens.  Internat.  Mitt, 
f.  Bodenkunde,  Band  II,  Heft  4,  Seite  312-342.     1912. 


92 


SOILS:    PROPERTIES  AND  MANAGEMENT 


general  principle,  but  a  special  apparatus  is  employed  by 
means  of  which  the  various  grades  obtained  by  sedimenta- 
tion are  siphoned  off  instead  of  decanted.  The  cylinder  is 
really  a  modified  Wahnschaffe  cylinder,1  such  as  was 
used  in  early  soil  analyses  for  drawing  off  the  various 
suspensions  except  that  the  siphon  is  placed  outside  the 
cylinder  instead  of  inside. 

The  cylinder  (die  Schlammapparat) 
as  used  by  Atterberg  is  about  25  cen- 
timeters high,  with  a  glass  pedestal 
and  a  ground  glass  stopper.  It  is 
graduated  at  5,  10,  15,  and  20  cen- 
timeters upward  from  the  bottom. 
The  same  distance  is  divided  also 
into  16  divisions  at  the  left  of  the 
first  graduation.  The  latter  gradua- 
tion is  used  in  the  separation  of  the 
clay  (Schlamm),  so  that  the  height 
of  the  sedimenting  column  may  be 
regulated  according  to  the  time 
available  for  the  settling  process. 
An  outside  siphon,  4  to  5  milli- 
meters wide,  is  attached  to  the  cylin- 
der at  the  bottom  for  the  drawing 
of  the  liquid  when  the  sedimenta- 
tion is  complete.  The  top  of  this 
siphon  is  opposite  the  5-centimeter 
mark  on  the  cylinder.  Cylinders 
of  this  size  are  used  only  for  the  separation  of  particles 
below  .2  millimeter  in  diameter;  for  larger  particles  a 
somewhat   taller  cylinder   is  used,  with  a  siphon  of  the 

1  Wiley,  H.  W.     Agricultural  Analysis,  pp.  205-207.     Easton, 
Pa.     1906. 


Fig.  14.  —  Atterberg's 
silt  cylinder  for  the 
mechanical  analysis 
of  soil  by  subsidence. 


THE  SOIL   PARTICLE  93 

same  width  as  for  the  finer  particles.  The  graduation  of 
the  cylinder  and  its  diameter  are  the  same  as  described 
above. 

A  20-gram  sample  of  soil  is  used  with  this  apparatus, 
and  deflocculation  is  brought  about  by  means  of  a  stiff 
brush.  The  sample  is  reduced  to  a  paste  in  a  porcelain 
dish,  and  then,  by  alternate  working  with  the  brush 
and  decanting,  all  the  particles  are  thrown  into  separate 
suspension.  A  deflocculating  chemical  is  used  in  humus 
soils,  in  order  to  hasten  the  process  and  counteract  the 
effect  of  the  organic  matter.  As  in  the  beaker  method, 
the  size  of  the  various  grades  of  separation  may  be  varied 
according  to  the  will  of  the  operator. 

70.  Centrifugal  soil  analysis.  —  Of  the  centrifugal 
methods  used  in  mechanical  analysis,  that  employed  by 
the  United  States  Bureau  of  Soils  l  is  the  most  successful. 
A  5-gram  sample  of  well-pulverized  soil  is  put  into  a 
shaker  bottle  of  about  250  cubic  centimeters  capacity 
(see  Fig.  15).  This  bottle  is  filled  about  two-thirds 
full  of  water,  so  that  in  shaking  the  disintegrating  force 
of  the  liquid  may  be  utilized.  A  few  drops  of  ammonia 
are  added,  to  dissolve  the  organic  matter  and  to  make 
deflocculation  easier.  The  sample  is  then  agitated  in 
the  bottle  until  disintegration  is  complete.  This  period 
ranges  from  five  to  twenty  hours,  depending  on  the 
sample. 

The  separation  of  the  silt  and  the  clay  from  the  sands 
is  made  in  the  shaker  bottle  by  simple  subsidence,  the 
time  for  decantation  being  determined  by  a  microscopic 
examination  of  a  drop  of  the  suspension.     The  silt  and 

1  Fletcher,  C.  C,  and  Bryan,  H.  Modifications  of  the 
Method  of  Soil  Analysis.  U.  S.  D.  A.,  Bur.  Soils,  Bui.  S4. 
1912. 


94 


SOIL  S  :    PROPER  TIES  AND  MANAGEMENT 


the  clay  are  decanted  directly  into  a  test  tube  fitted  into 
a  centrifuge  (see  Fig.  15).  Whirling  at  the  rate  of  800  to 
1000  revolutions  a  minute  will  cause  the  subsidence  of 
the  silt  to  the  bottom  of  the  test  tube  in  a  few  minutes.  / 
The  clay  is  then  decanted.  The  microscope  is  necessary 
here,  in  order  to  determine  when  the  settling  of  the  silt 
is  complete.  As  small  particles  tend  to  cling  to  the 
larger  particles,  the  entire  operation  must  be  repeated 


Fig.  15.  —  Apparatus  for  centrifugal  mechanical  analysis  of  soil,  show- 
ing shaker  bottle,  shaker,  centrifuge,  and  test  tube. 


several  times;  therefore  the  processes  of  gravity  sub- 
sidence and  centrifugal  subsidence  are  carried  on  side  by 
side,  material  being  constantly  poured  from  the  shaker 
bottle  into  the  centrifuge  tubes  and  from  the  test  tubes 
into  the  receptacles  for  the  clay. 

The  centrifuge  is  usually  large  enough  to  allow  the 
separation  of  several  duplicate  samples  at  once.  The 
various  separates  made  by  this  method  are  dried  and 


THE  SOIL   PARTICLE  95 

weighed.  The  sands,  which  are  obtained  in  bulk,  are 
»  further  separated  by  sieves  into  the  grades  desired. 
Where  a  large  quantity  of  organic  matter  is  present,  it 
must  be  determined  and  included  in  the  final  report  on 
the  sample. 

This  method  of  mechanical  analysis  as  perfected  by 
the  Bureau  of  Soils  has  been  very  generally  adopted  by 
soil  workers.  It  has  many  advantages  over  other  methods. 
In  the  first  place,  it  is  rapid,  often  requiring  only  hours 
where  other  methods  take  days  for  completion ;  secondly, 
it  is  simple,  and  the  technique  of  the  separation  is  easily 
acquired ;  thirdly,  in  the  decantations  no  very  large 
amount  of  water  is  accumulated  with  the  separates, 
except  for  the  clay,  and  thus  the  time  and  cost  of  evapora- 
tion is  eliminated.  The  clay,  moreover,  may  be  as  ac- 
curately determined  by  difference  as  by  direct  methods, 
thus  allowing  a  further  saving  of  time.  The  cost  of  the 
equipment  for  this  method  is  low.  The  apparatus  itself 
is  simple,  and  is  carried  by  all  standard  chemical  com- 
panies. The  same  cannot  be  said  of  the  various  elutriator 
mechanisms.  While  the  method  is  accurate  only  within 
one  per  cent,  it  is  sufficiently  precise  for  practical  pur- 
poses, especially  in  class  determination,  for  which  me- 
chanical analysis  is  generally  utilized. 

71.  Classification  of  soil  particles.  —  With  the  large 
number  of  different  methods  of  mechanical  soil  analyses 
there  has  arisen  a  large  variation  in  textural  groupings 
expressed  in  diameter  of  particles.  This  would  naturally 
occur  because  of  the  differences  in  degree  of  refinement 
which  the  various  methods  of  separation  allow,  and  also 
because  of  the  uses  which  the  investigators  wished  to 
make  of  such  analyses.  Some  of  the  best-known  group- 
ings are  given  below  :  — 


96 


SOILS:    PROPEPTIES  AND  MANAGEMENT 


Various  Textural  Classifications  used  in  the  Mechani- 
cal Analyses  of  Soils.  Expressed  in  Diameter  of 
Particles  in  Millimeters 


Separate 

Osborne ' 

HlLGARD* 

Bureau  op 
Soils3 

English4 

Atterberg6 

1 

3.000 

3.000 

2.000 

1.000 

20.000 

2 

1.000 

1.000 

1.000 

.200 

2.000 

3 

.500 

.500 

.500 

.040 

.200 

4 

.250 

.300 

.250 

.010 

.020 

5 

.050 

.160 

.100 

.002 

.002 

6 

.010 

.120 

.050 

7 

.072 

.005 

8 

.047 

9 

.036 

10 

.025 

11 

.016 

12 

.010 

Of  these  classifications  only  three  need  claim  our  atten- 
tion —  that  of  the  Bureau  of  Soils,  that  of  Hall  and 
Russell  (the  English  classification),  and  that  devised  by 
Atterberg.  These  represent  the  groupings  used  in  ex- 
pressing mechanical  soil  analyses  in  the  United  States, 


1  Osborne,  T.  B.  Methods  of  Mechanical  Soil  Analysis. 
Ann.  Rept.  Connecticut  Agr.  Exp.  Sta.,  1886,  pp.  141-158; 
1887,  pp.  144-162 ;    1888,  pp.  154-157. 

2  Hilgard,  E.  W.  Methods  of  Physical  and  Chemical'  Soil 
Analysis.  Ann.  Rept.  California  Agr.  Exp.  Sta.,  1891-1892, 
pp.  241-257. 

3  Briggs,  L.  J.,  and  others.  The  Centrifugal  Method  of 
Soil  Analysis.     U.  S.  D.  A.,  Bur.  Soils,  Bui.  24.     1904. 

4  Hall,  A.  D.,  and  Russell,  E.  J.  Soil  Surveys  and  Soil 
Analyses.  Jour.  Agr.  Science,  Vol.  IV,  part  2,  pp.  182-223. 
1911. 

5  Atterberg,  A.  Die  Mechanische  Bodenanalyse  und  die 
Klassiflkation  der  Mineralboden  Schwedens.  Internat.  Mitt, 
f.  Bodenkunde,  Band  II,  Heft  4,  Seite  312-342.     1912. 


THE  SOIL   PARTICLE 


97 


4  in  England,  and  in  Continental  Europe,  respectively. 
As  to  which  is  the  best  for  an  interpretation  and  ex- 
pression of  textural  qualities  it  is  difficult  to  say.  They 
are  all  arbitrary,  yet  they  are  all  extremely  useful. 
It  therefore  seems  immaterial  which  one  is  employed. 
It  would  be  better,  of  course,  if  the  classification  were 
uniform  for  all  countries;  correlation  of  soil  properties 
would  then  be  easier. 

72.  Bureau  of  Soils  classification.  —  As  the  grouping 
established  by  the  United  States  Bureau  of  Soils  is  met 
with  in  all  of  our  soil  literature,  and  as  it  is  really  the 
standard  classification  for  this  country,  a  closer  considera- 
tion of  it  may  be  profitable.  The  discussion  of  the 
properties  exhibited  by  the  various  separates,  and  of  the 
interpretation  and  value  of  a  mechanical  analysis,  will 
therefore  be  made  with  this  classification  as  a  basis.  By 
way  of  illustrating  the  grouping  and  the  mode  of  ex- 
pressing a  mechanical  analysis  the  results  obtained  from 
two  distinctly  different  soils  are  given  below :  — 

Mechanical  Analyses  l  of   a  Dunkirk  Fine   Sandy   Loam 
and  a  Dunkirk  Clay 


Separate 


Fine  gravel  . 
Coarse  sand  . 
Medium  sand 
Fine  sand 
Very  fine  sand 
Silt  .  .  . 
Clay      .     .     . 


Size  in 
Millimeters 


2-1 

1-.5 

.5-.25 

.25-.  10 

.10-.05 

.05-.005 

below  .005 


Fine  Sandy 
Loam 


% 
1 

2 

3 

22 

35 

27 
10 


% 
1 
2 
2 
6 
7 
39 
43 


1  Soil  Survey  Field  Book,  pp.  152,  154.     U.  S.  D.  A.,  Bur. 
Soils.     1906. 


98         SOILS:    PROPERTIES  AND  MANAGEMENT 

73.  Physical  character  of  the  separates.  —  It  is  im- 
mediately apparent  that  as  these  groups  vary  in  size  they 
must  exhibit  properties,  especially  physical  ones,  which 
are  widely  different.  These  properties  should  in  turn 
be  imparted  to  the  soil  of  which  the  separates  form  a 
part.  If  we  are  conversant  with  these  various  values,  a 
mechanical  analysis  should  reveal  to  us  at  a  glance  cer- 
tain soil  conditions  which  may  or  may  not  be  conducive 
to  the  best  plant  growth. 

The  clay  particles  are  very  minute ;  many  of  them  are 
so  small  as  to  be  invisible  under  the  ultramicroscope. 
They  are  really  shreds  and  fragments  of  minerals,  and 
are  jagged  and  angular  in  outline.  They  are  highly 
plastic,  and  when  rubbed  together  they  become  sticky 
and  impervious.  They  shrink  much  on  drying,  with  the 
absorption  of  considerable  heat.  On  being  wet  again 
they  swell  with  the  evolution  of  the  heat  already  taken 
up.  Many  of  the  particles  exhibit  the  Brownian  move- 
ment and  will  remain  in  suspension  for  an  indefinite 
period.  The  finer  part  of  the  clay  makes  up  a  portion  of 
that  indefinite  group  of  material  in  the  soil  called  colloids, 
which  because  of  their  fineness  of  division  (molecular 
complexes)  exhibit  certain  well-defined  properties,  of 
which  adsorption  of  moisture  and  salts  in  solution,  and 
high  plasticity*  and  cohesion,  are  the  most  important 
from  a  soil  standpoint.  Silt  exhibits  the  same  qualities 
as  clay,  but  to  a  much  less  marked  extent.  The  presence 
of  clay  in  a  soil  imparts  to  it  a  heavy  texture,  with  a 
tendency  to  slow  water  and  air  movement.  Such  a  soil 
is  highly  plastic*  but  becomes  sticky  when  too  wet  and 
hard  and  cloddy  when  too  dry.  The  expansion  and  the 
contraction  on  wetting  and  drying  are  very  great.  The 
water-holding  capacity  of  a  clay  soil  is  high. 


THE  SOIL   PARTICLE  99 

The  sands  and  the  gravel,  because  of  their  sizes,  func- 
tion as  separate  particles.  They  aFe  irregular  and  rounded, 
the  continual  rubbing  that  they  have  received  being 
sufficient  to  have  effaced  their  angular  character.  They 
exhibit  very  low  plasticity  and  cohesion,  and  as  a  con- 
sequence are  little  influenced  by  changes  in  water  content. 
Their  water-holding  capacity  is  low,  and  because  of  the 
large  size  of  the  spaces  between  each  separate  particle 
the  passage  of  water  is  rapid.  They  therefore  facilitate 
drainage  and  encourage  good  air  movement.  In  all 
the  grades  of  sand  the  separate  particles  are  visible  to 
the  naked  eye,  a  condition  impossible  with  the  silt  and 
clay  groups.  Soil  containing  much  sand  or  gravel, 
therefore,  is  of  an  open  character,  possessing  good 
drainage  and  aeration,  and  is  usually  in  a  loose  friable 
condition. 

74.  Mineralogical  characteristics  of  the  separates.  — 
From  the  mineralogical  standpoint  there  are  usually 
considerable  differences  in  the  soil  separates.  These 
differences  would  naturally  be  expected  to  occur  par- 
ticularly in  residual  soils,  because  of  the  differentiating 
tendencies  of  weathering.  Quartz  would  naturally  per- 
sist, and  because  of  its  slow  solubility  would  very  soon 
make  up  most  of  the  larger  soil  grains.  Other  minerals, 
such  as  the  feldspars,  hornblende,  augite,  and  the  like, 
being  less  persistent  as  the  law  of  mineral  resistance 
has  already  taught  us,  would  be  worn  to  fine  shreds 
and  be  found  as  the  main  constituents  of  the  silts 
and  the  clays.  The  following  data  sustain  this  as- 
sumption regarding  the  mineralogical  characteristics  of 
some  of  the  soil  groups  as  designated  by  the  Bureau  of 
Soils  as  well  as  furnish  some  interesting  comparisons  of 
some  important  soil  provinces  :  — 


100       SOILS:    PROPERTIES  AND  MANAGEMENT 

General   Mineralogical   Composition   of   the   Sands   and 
Silts  of  Various  Soil  Provinces  of  the  United  States  -'■ 


Soil 

No.  op 
Samples 

Minerals  other  than 
Quartz  in 

Sands 

Silts 

Residual 

Glacial  and  loessial  .     . 

Marine 

Arid 

12 
6 
4 
3 

15% 

12% 

5% 

37% 

21% 
15%     . 

8% 

42% 

It  is  to  be  seen  immediately  that  in  every  case  the 
silt  carries  a  larger  quantity  of  the  important  soil-forming 
minerals  and  a  smaller  quantity  of  quartz  than  does  the 
sand.  This  reveals  at  least  one  of  the  reasons  for  the 
greater  fertility  and  lasting  qualities  of  fine-textured  soils 
as  far  as  agricultural  operations  are  concerned.  It  is 
important  to  note,  however,  that,  although  quartz  is 
the  predominating  mineral  in  sands,  all  the  common 
soil-forming  minerals  are  usually  accessory.  This  merely 
serves  to  again  emphasize  the  fact  that  all  soils  contain 
all  the  common  minerals  found  in  soil-forming  rocks. 

It  is  also  interesting  to  note  the  general  differences 
exhibited  by  the  various  soil  provinces.  The  residual, 
glagial,  and  coastal  plain  soils  possess  minerals  other 
than  quartz  in  the  order  named.  In  the  marine  soils, 
in  particular,  this  difference  has  largely  come  about  by 
the  disintegration  and  leaching  that  these  soils  have 
undergone  during  their  formation.  The  arid  soils,  due 
to  the  suppression  of  chemical  weathering  and  the  activity 


1  McCaughey,  W.  G.,  and  William,  H.  F.  The  Microscopic 
Determination  of  Soil-Forming  Minerals.  U.  S.  D.  A.,  Bur. 
of  Soils,  Bui.  91.     1913. 


THE   SOIL  PARTICLE 


101 


of  the  physical  agents,  exhibit  smaller  quantities  of  free 
quartz.  The  silica  in  such  soils  is  held  as  complex  sili- 
cates, which  very  largely  carry  the  elements  that  are  so 
important  in  plant  development. 

Although  these  data  are  based  on  but  a  few  samples, 
they  are  so  concordant  with  what  would  naturally  be 
expected  that  these  general  conclusions  cannot  be  avoided. 

75.  The  chemical  constitution  of  soil  particles.  —  The 
mineralogical  examination  of  soils  has  revealed  a  larger 
percentage  of  such  minerals  as  feldspars,  mica,  horn- 
blende, and  the  like,  in  the  finer  separates.  A  larger 
percentage  of  the  important  plant-food  elements  would 
therefore  be  expected  in  those  groups.  The  following 
data,1  compiled  from  work  performed  by  the  United 
States  Bureau  of  Soils,  substantiate  this  assumption :  — 

Chemical  Composition  of  Various  Soil  Separates 


Soils 

Num- 

BEROF 

Sam- 
ples 

Percentage  of 

P2O5  IN 

Percentage  of 

K2O  IN 

Percentage  of 
CaO  in 

Sand 

Silt 

Clay 

Sand 

Silt 

Clay 

Sand 

Silt 

Clay 

Crystalline  residual 
Limestone  residual 
Coastal  plain 
Glacial  and  loessial 
Arid  soils 

3 
3 

7 

10 

2 

.07 
.28 
.03 
.15 
.19 

.22 
.23 
.10 
.23 
.24 

.70 
.37 
.34 
.86 
.45 

1.60 
1.46 
.37 
1.72 
3.05 

2.37 
1.83 
1.33 
2.30 
4.15 

2.86 
2.62 
1.62 
3.07 
5.06 

.50 
12.26 

.07 
1.28 
4.09 

.82 
10.96 

.19 
1.30 
9.22 

.94 
9.92 

.55 
2.69 
8.03 

It  is  seen  that  on  the  average  the  soils  with  finer  particles 
are  richer  in  phosphoric  acid,  potash,  and  lime,  than 
those  of  coarser  texture,  the  only  exception  in  this  case 
being  in  the  lime  of  the  residual  limestone  soils.  The 
arid  soils  present  a  less  marked  difference  in  the  sands, 


1  Failyer,    G.   H.,   and  others.     The    Mineral    Composition 
of  Soil  Particles.     U.  S.  D.  A.,  Bur.  Soils,  Bui.  54.     1908. 


i02 


SOILS:    FROPERTJES  AND   MANAGEMENT 


silts,  and  clays  than  the  representatives  of  the  other 
soil  provinces;  this  is  true  also  of  the  glacial  soils,  but 
to  a  less  degree.  Under  such  conditions  of  weathering 
the  sands  have  not  as  yet  been  depleted  of  their  stores 
of  essential  elements.  Average  data  compiled  from  a 
number  of  soil  analyses  by  Hall,1  presented  below,  tend 
to  corroborate  the  data  already  noted  and  that  obtained 
by  Loughridge  2  of  California  :  — 

Composition  of  Soil  Separates 


*SiOt 

AW, 

FejOj 

CaO 

MgO 

K20 

PjOi 

Coarse  sand  (1-.2  mm.)     . 

93.9 

1.6 

1.2 

.4 

.5 

.8 

.05 

Fine  sand  (.2-.04  mm.) 

94.0 

2.0 

1.2 

.5 

.1 

1.5 

.1 

Silt  (.04-.01  mm.)     .     .     . 

89.4 

5.1 

1.5 

.8 

.3 

2.3 

.1 

Fine  silt  (.01-002  mm.)     . 

74.2 

13.2 

5.1 

1.6 

.3 

4.2 

.2 

Clay  (Below  .002  mm.)      . 

53.2 

21.5 

13.2 

1.6 

1.0 

4.9 

.4 

76.  Value  of  a  mechanical  analysis.  —  It  is  now  evi- 
dent that  the  proper  interpretation  of  a  mechanical 
analysis  throws  considerable  light  on  the  probable  physical 
and  chemical  properties  of  a  soil.  To  the  trained  ob- 
server the  preponderance  of  sand  or  clay  signifies  certain 
physical  properties  which  may  affect  the  plant  not  only 
mechanically,  but  physiologically  as  well,  through  varia- 

1  Hall,  A.  D.,  and  Russell,  E.  J.  Soil  Surveys  and  Soil 
Analyses.  Jour.  Agr.  Science,  Vol.  IV,  Part  2,  p.  199.  1911. 
Also  a  Report  of  the  Agriculture  and  Soils  of  Kent,  Surrey,  and 
Sussex.     Board  of  Agriculture  and  Fisheries.     1911. 

2  Loughridge,  R.  H.  On  the  Distribution  of  Soil  Ingredi- 
ents among  Sediments  Obtained  in  Silt  Analyses.  Amer. 
Jour.  Sci.,  Vol.  VII,  p.  17.     1874. 

In  this  connection  see  also  Puchner,  Dr.  Uber  die  Ver- 
tielung  von  Niihrstoffen  in  den  Verschieden  Feinen  Bestandteilen 
des  Boden.     Landw.  Ver.  Stat.,  Band  66,  Seite  463-470.     1907. 


THE   SOIL   PARTICLE  103 

tions  in  air  and  water  movements.  The  chemical  phases 
of  such  an  interpretation  are  also  worthy  of  considera- 
tion, as  the  proportion  of  the  various  separates  determines 
whether  the  essential  plant-food  elements  will  be  present 
in  sufficient  quantities  to  permit  normal  crop  growth. 
Thus  in  a  general  way  the  mechanical  analysis  of  a  soil 
not  only  enlightens  us  as  to  the  general  properties  of  a 
given  soil,  but  is  to  some  extent  a  criterion  of  agricultural 
value  and  crop  adaptation.  Some  authors l  maintain 
that  in  the  investigation  of  any  soil  a  mechanical  analysis 
should  first  be  made,  as  such  an  analysis  throws  so  much 
light  on  the  general  qualities  of  a  soil. 

77.  Soil  class.  —  Class  is  a  term  used  in  relation  to 
the  texture,  or  size  of  particles,  of  a  soil.  Class  differs 
from  texture,  however,  in  that  it  has  reference  rather  to 
the  particular  properties  exhibited  by  a  soil  than  to  any 
absolute  grain  size.  As  soils  are  not  made  up  of  particles 
of  the  same  size,  a  blanket  term  is  needed  which  will  not 
only  give  an  idea  of  the  texture  of  the  soil,  but  also  name 
it  in  such  a  manner  as  to  reveal  general  peculiarities  and 
properties.  We  may  have  any  number  of  classes,  de- 
pending on  the  sizes  of  the  soil  grains  carried. 

These  class  names  have  originated  through  long  cen- 
turies of  agricultural  operations,  but  of  late  they  have 
been  more  or  less  standardized  because  of  the  necessity 
of  a  definite  nomenclature.  In  general  the  names  used 
for  the  soil  classes  are  the  same  as  are  used  in  mechanical 
analyses  to  designate  the  soil  separates.  This  is  rather 
unfortunate,  but  it  obviates  the  increase  of  technical  terms 
and  a  little  care  will  prevent  confusion  in  this  regard. 

Another  word  introduced  by  common  usage  is  loam. 

iHall,  A.  D.,  and  Russell,  E.  J.  Soil  Surveys  and  Soil 
Analyses.     Jour.  Agr.  Science,  Vol.  IV,  Part  2,  p.  199.     1911. 


104       SOILS:    PROPERTIES  AND  MANAGEMENT 

Loam,  from  the  technical  standpoint,  refers  to  a  soil 
possessing  in  about  equal  amounts  the  properties  im- 
parted by  the  various  separates.  If,  however,  we  have 
practically  the  same  condition  but  with  one  size  of  particle 
predominating,  the  name  of  that  particular  separate  is 
prefixed,  giving  still  more  data  regarding  the  soil  in 
question.  Thus  a  loam  in  which  clay  is  dominant  will 
be  classified  as  a  clay  loam.  In  the  same  way  we  may 
have  a  sandy  loam,  a  sandy  clay  loam,  a  gravelly  sandy 
clayey  loam,  and  so  on.  The  number  of  soil  classes  that 
may  occur  is  therefore  rather  large,  ranging  from  coarse 
gravel,  through  the  various  grades  of  sands,  to  silts  and 
clays. 

A  few  of  the  common  classes,  with  their  mechanical 
analyses,  are  listed  below :  — 

Mechanical  Composition  of  Various  Soil  Classes  x 


| 

a 

g 

o 

> 
0 

go 

GO 

5 

1 

g§ 

a 

S 

< 

GO 

£ 

Si 

m 

S 

I 

1 

S 
i 

3 

£  . 

PS  z 
6 

a 

GO 

7 

3 

Coarse  sands  .     .     . 

135 

12 

31 

19 

20 

5 

Sands     

401 

2 

15 

23 

37 

11 

7 

5 

Fine  sands       .     .     . 

511 

1 

4 

10 

57 

17 

7 

4 

Sandy  loams    .     .     . 

1141 

4 

13 

12 

25 

13 

21 

12 

Fine  sandy  loams     . 

934 

1 

3 

4 

32 

24 

24 

12 

Loams 

659 

2 

5 

5 

15 

17 

40 

16 

Silt  loams   .... 

1268 

1 

2 

1 

5 

11 

65 

15 

Sandy  clays     .     .     . 

162 

2 

8 

8 

30 

12 

13 

27 

Clay  loams      .     .     . 

718 

1 

4 

4 

14 

13 

38 

26 

Silty  clay  loams  .     . 

765 

0 

2 

1 

4 

7 

61 

25 

Clays 

1970 

1 

3 

2 

8 

8 

36 

42 

1  Whitney,  M.     The  Use  of  Soils  East  of  the  Great  Plains 
Region.     U.  S.  D.  A.,  Bur.  Soils,   Bui.  78,  p.  12.  1911. 


THE  SOIL   PARTICLE  105 

It  is  evident  that  a  mechanical  analysis  of  a  soil  is 
nothing  more  or  less  than  an  expression  of  class,  and 
the  inferences  that  may  be  derived  from  either  are 
the  same.  This  leads  to  a  consideration  of  class  deter- 
mination. 

78.  Determination  of  class.  —  The  common  method 
of  class  determination  is  that  employed  in  the  field.  It 
consists  in  examination  of  the  soil  as  to  color,  an  estima- 
tion of  its  humus  content,  and,  especially,  a  testing  of 
the  "  feel  "  of  the  soil.  Probably  as  much  can  be  judged 
as  to  the  texture  and  class  of  a  soil  by  merely  rubbing  it 
between  the  thumb  and  the  fingers  as  by  any  other 
superficial  method.  This  is  a  method  used  in  all  field 
operations,  especially  in  soil  survey  work.  The  accuracy 
of  the  determination  depends  largely  on  experience. 
Inaccuracies  are  likely  to  occur  in  distinguishing  between 
the  various  finer  grades  of  soil;  for  this  reason  more 
nearly  exact  methods  are  necessary  at  times,  especially 
in  checking  soil  survey  work  or  in  carrying  out  investiga- 
tions in  which  absolute  accuracy  is  required. 

%As  a  mechanical  analysis  of  a  soil  is  really  a  percentage 
expression  of  texture,  it  presents  an  exact  method  for 
class  determination.  For  detailed  work  somewhat  com- 
plicated tables  *  have  been  arranged ;  but  the  following 
diagram  (Fig.  16),  devised  by  Whitney,2  presents  a  simple 
method  for  the  identification  of  a  soil  from  a  mechanical 
analysis.  The  convenience  of  this  triangular  representa- 
tion may  be  tested  by  the  use  of  the  average  analyses, 
already  presented  on  a  previous  page. 

JBur.  of  Soils,  Soil  Survey  Field  Book,  p.  17,  U.  S.  D.  A., 
Bur.  Soils.     1906.     Also,  Bur.  Soils,  Bui.  78,  p.  12.     1911. 

2  Whitney,  M.  The  Use  of  Soils  East  of  the  Great  Plains 
Region.     U.  S.  D.  A.,  Bur.  Soils,  Bui.  78,  p.  13.     1911. 


106       SOILS:    PROPERTIES  AND  MANAGEMENT 


ci^r 


to       zo       jo      40       so       eo       70       80       so       1009b 


Fig.   16.  —  Diagram  for  the  determination  of  soil  class  from  a  mechani- 
cal analysis. 


79.  The  significance  of  texture  and  class.  —  Soil 
texture  and  class  are  the  basis  for  all  soil  consideration, 
whether  regarding  some  specific  property  or  a  general 
condition  such  as  crop  adaptation.  No  matter  what  the 
phase  of  soil  study  may  be,  texture  and  class  are  sure 
to  have  some  important  influence  and  must  be  included 
in  the  investigation.  From  observations  in  practice, 
certain  crops  have  been  found  to  be  adapted  to  certain 
kinds  of  soil  —  as  clay  loam  for  wheat,  silt  loam  for 
corn,  loam  or  sandy  loam  for  potatoes,  clay  or  clay  loam 
for  timothy,  and  so  on.  Two  authors  x  have  determined 
the  mechanical  qualities  of  soils  well  adapted  to  certain 
crops.     An  average  of  their  analyses  is  given  below:  — 


1  Hall,  A.    D.,    and    Russell,    E.    J.     Soil    Surveys   and   Soil 
Analyses.     Jour.  Agr.  Science,  Vol.  IV,  Part  2,  p.  207.     1911. 


THE  SOIL   PARTICLE 


107 


The  Mechanical  Analysis  of  Specific  Crop  Soils 


Wheat 

Barley 

Potato 

Hop 

Fruit 

(9  samples) 

(9  samples) 

(8  samples) 

(7  samples) 

(6  samples) 

Fine  gravel 

1.4 

1.2 

.9 

1.2 

1.0 

Coarse  sand     . 

3.7 

18.3 

20.1 

4.8 

6.8 

Fine  sand   .     . 

24.5 

32.0 

43.5 

33.8 

42.0 

Silt    .... 

23.0 

18.2 

11.0 

28.8 

23.3 

Fine  silt      .     . 

12.8 

8.0 

6.4 

8.7 

7.3 

Clay       .     .     . 

20.0 

11.9 

9.7 

12.1 

10.9 

The  soil  particle  can  thus  be  seen  to  function  in  no 
insignificant  manner  regarding  plant  conditions.  Its 
size,  its  physical  relationships,  its  chemical  composition, 
and  the  conditions  imposed  by  a  preponderance  or  a 
limitation  in  its  various  grades,  are  of  vital  importance. 
Soil  texture  and  soil  class,  therefore,  are  terms  of  constant 
usage  in  soil  discussion  and  study,  whether  the  viewpoint 
is  practical  or  purely  theoretical. 


CHAPTER  VII 
SOME  PHYSICAL  PROPERTIES  OF   THE  SOIL 

While  texture  is  of  great  importance  in  the  determina- 
tion of  the  physical  and  chemical  nature  of  a  soil,  it  is 
evident  that  the  arrangement  of  the  particles  also  exerts 
considerable  influence.  The  term  texture  refers  to  the 
size  of  the  soil  particles;  the  term  structure  is  used  in 
reference  to  their  arrangement,  or  grouping.  It  is  at 
once  apparent  that  certain  conditions  —  such,  for  ex- 
ample, as  air  and  water  movement,  heat  transference, 
and  the  like  —  will  be  as  much  affected  by  structure 
as  by  texture.  As  a  matter  of  fact,  the  great  changes 
wrought  by  the  farmer  in  making  his  soil  better  suited 
as  a  foothold  for  plants  are  structural  changes  rather 
than  changes  in  texture.  The  compacting  of  a  light 
soil  or  the  loosening  of  a  heavy  soil  is  merely  a  change 
in  arrangement  of  the  soil  grains.  It  is  of  interest,  there- 
fore, to  ascertain  the  probable  arrangement  of  the  particles 
in  any  soil. 

80.  Arrangement  of  soil  particles  (Fig.  17). —  In  any 
consideration  it  is  the  easier  way  to  advance  from  the 
simple  to  the  complex.  Therefore  in  the  explanation  of 
structural  relationships  a  theoretical  condition  will  be  dealt 
with  first,  after  which  the  discussion  will  proceed  to  the 
intricate  condition  existing  in  the  soil.  Assuming  that 
this  theoretical  condition  consists  in  spherical  particles 
all  of  the  same  size,  we  find  these  particles  susceptible 

108 


SOME  PHYSICAL  PROPERTIES   OF  THE  SOIL      109 

to  two  different  arrangements:  (1)  in  columnar  order, 
with  each  particle  touched  in  four  points  by  its  neighbors; 
and  (2)  the  oblique,  in  which  each  particle  is  in  contact 
with  six  of  its  neighbors.  The  possible  pore  space  in 
the  first  case  is  47.64  per  cent,  while  that  in  the  second 
case  is  25.95  per  cent.  The  amount  of  this  pore  space 
is  uninfluenced  by  the  size  of  the  particles,  provided  they 
are  round  and  all  of  the  same  volume. 


Fig.   17.  —  Ideal  arrangements  of  spherical  particles,  showing,  from  left 
to  right,  columnar,  oblique,  compact,  and  granular  orders. 

Tew  any  one  of  practical  experience  it  is  a  well-known 
fact  that  the  soil  particles  are  not  homogeneous  as  to 
size,  and  neither  do  all  the  particles  function  as  simple 
grains,  being  gathered  together  in  groups  called  granules, 
or  crumbs.  A  small  particle  of  soil  may  be  made  up  of 
a  number  of  very  small  grains.  This  will  modify  the 
ideal  condition  as  described  above,  giving  two  additional 
conditions  —  first,  a  mixture  of  spherical  grains  of  differ- 
ent sizes,  and,  secondly,  a  condition  in  which  the  large 
grains  are  complexes  made  up  of  numerous  small  particles. 
A  mixture  such  as  is  presented  by  the  first  of  these  con- 
ditions, in  which  the  small  grains  fit  in  between  the 
larger  ones,  will  result  in  a  reduction  of  pore  space.  The 
pore  space  will  fall  below  25.95  per  cent  and  approach 
zero.     A  real  soil  having  such  restricted  pore  space  is 


110       SOILS:    PROPERTIES  AND  MANAGEMENT 

designated  as  being  in  a  puddled  condition.  This  con- 
dition is  detrimental  to  plant  growth,  for  it  not  only 
impedes  root  development  and  extension,  but  also  pre- 
vents the  circulation  of  air  and  water,  a  function  necessary 
for  proper  soil  sanitation.  In  the  second  condition  an 
increase  in  pore  space  must  occur,  as  each  large  grain 
presents  considerable  internal  air  space.  If  the  granules 
as  well  as  their  component  particles  were  arranged  in 
columnar  order,  the  pore  space  would  reach  the  high 
percentage  of  72.58.  Under  natural  conditions,  then, 
the  pore  space  might  range  from  zero  plus  to  about  72 
per  cent. 

However,  not  only  are  the  particles  of  a  normal  soil 
not  of  the  same  size,  but  they  are  far  from  round.  A 
soil,  as  already  demonstrated,  ordinarily  presents  varying 
amounts  of  particles,  ranging  in  size  from  stone  and 
coarse  gravel  to  the  very  finest  clay.  These  particles 
may  also  differ  in  shape,  varying  from  almost  perfect 
spheres  to  flakes,  chips,  and  fragments  of  every  con- 
ceivable form.  Therefore  the  laws  that  apply  to  the 
ideal  condition  will  hold  only  in  a  general  way  in  a  normal 
soil.  It  is  evident,  first,  that  the  more  compact  the 
soil,  the  less  is  the  pore  space ;  secondly,  that  it  is  possible 
to  so  manipulate  a  soil  as  to  work  the  small  particles  in 
between  the  larger  ones  and  create  an  impervious  or 
puddled  condition ;  and,  thirdly,  that  by  the  forming 
of  granules  the  pore  space  of  a  soil  may  be  increased  to 
a  high  percentage. 

From  the  standpoint  of  size  and  arrangement  of  particles 
there  are  really  two  classes  of  soils,  those  of  single  grain 
structure  and  those  that  are  granular.  In  the  former 
each  particle  functions  separately.  In  order  to  do  this 
the  particles  must  be  large.     This  condition  is  found  in 


SOME  PHYSICAL  PROPERTIES   OF  THE  SOIL    '  111 

sand.  In  such  soil  would  naturally  be  found  also  a 
medium  to  low  pore  space,  just  as  has  been  exemplified 
in  the  ideal  condition  described  above.  In  granular 
soils,  the  granules,  being  made  up  of  small  particles, 
present  much  internal  pore  space.  This  condition  occurs 
only  in  fine  soils,  such  as  loams,  silts,  and  clays,  since 
large  particles  will  not  cohere  firmly  enough  to  produce 
a  crumb  structure.  A  fine-textured  soil,  which  will 
puddle  more  readily  than  a  coarser  one,  is  thus  saved 
from  a  semi-impervious  condition  by  this  tendency  toward 
granulation. 

The  ideal  soil  condition  might  be  considered  to  be  most 
likely  to  occur  in  a  loam  (Fig.  18).  In  loamy  soil  some 
of  the  particles  are  large  and 
function  separately;  others  are 
medium  in  size  and  tend  to  form 
the  nuclei  around  which  smaller 
particles  may  cluster  to  form 
granules,  or  crumbs.  There  are 
thus  a  few  large  pore  spaces 
which  facilitate  drainage,  and 
numberless  small  openings  in 
which  water  is  retained.  Air 
therefore  finds  easy  movement 
and  sanitation  is  promoted.  In 
such  a  condition  the  organic  mat- 
ter plays  an  important  part. 
This  exists  usually  as  dark,  par- 
tially decayed  material.   It  pushes 

apart  the  grains  and  lightens  the  soil,  and  contributes 
much  in  bringing  about  the  loamy  condition  so  favorable 
to  plant  development.  Because  of  its  water-holding 
capacity  also  it  proves  a  valuable  addition.     Thus  with 


Fig.  18.  —  The  arrange- 
ment of  particles  in  loamy 
soil  of  good  structural 
condition.  (a).  Large 
granule  ;  (b) ,  small  sand 
particle ;  (c) ,  large  pore 
space  ;  (d),  small  granules 
with  small  interstitial 
spaces;  (e),  large  sand 
grain. 


112       SOILS:    PROPERTIES  AND  MANAGEMENT 


particles  of  varying  sizes,  of  a  structure  partly  single- 
grain  and  partly  granular,  to  which  has  been  added  by 
natural  means  sufficient  organic  matter  in  an  advanced 
stage  of  decomposition,  we  have  the  ideal  soil  conditions 
for  plant  development.  Yet  the  same  laws  govern  here 
in  a  general  way  as  were  found  to  function  with  homo- 
geneous grains  of  spherical  shape. 

81.  The  absolute  specific  gravity  of  the  soil.  —  The 
structural  condition  of  any  soil,  be  it  single-grain,  granular, 
or  a  favorable  combination  of  the  two,  has  considerable 
influence  on  certain  other  physical  conditions.  One  of 
those  most  affected  is  the  weight.  The  weight  of  a  soil 
is  determined  by  two  factors :  the  weight  of  the  individ- 
ual particles,  or  the  absolute  specific  gravity;  and  the 
amount  of  actual  space  taken  up  by  such  particles  in  any 
given  volume.  The  latter  is  really  a  structural  condition 
and  is  independent  to  some  extent  of  the  size  of  particles. 
The  absolute  specific  gravity,  or  weight  compared  with 
an  equal  volume  of  water,  of  some  of  the  common  minerals 
is  as  follows :  — 

Quartz    ....  2.7 

Orthoclase       .     .  2.6 

Plagioclase       .     .  2.7 

Mica       ....  3.0 

Olivine    ....  3.4 

Calcite    ....  2.7 

Dolomite     ...  2.9 

Although  a  great  range  is  observed  in  the  absolute 
specific  gravities  of  these  common  soil-forming  minerals, 
it  must  be  remembered  that  such  minerals  as  quartz 
and  feldspar  usually  make  up  the  bulk  of  a  soil.  As  a 
consequence  it  has  been  found  that  the  absolute  specific 


Apatite 

.     3.2 

Gypsum    . 

.    2.3 

Hematite 

.     5.2 

Limonite   . 

.    4.0 

Serpentine 

.    2.6 

Chlorite     . 

.     .     2.2 

Talc      .     . 

.    2.7 

SOME  PHYSICAL  PROPERTIES   OF  THE  SOIL      113 

gravity  of  a  purely  mineral  soil  varies  only  between 
narrow  limits,  these  being  from  2.6  to  2.8.  Nor  has 
fineness  any  appreciable  effect,  as  shown  below  by  Whit- 
ney's l  determinations  on  the  various  separates :  — 

Specific  Gravity 

Fine  gravel  (2-1  mm.) 2.647 

Coarse  sand  (1-.5  mm.) 2.655 

Medium  sand  (.5-25  mm.)    .     .     .     .     .  2.648 

Fine  sand  (.25-.  10  mm.) 2.659 

Very  fine  sand  (.10-.05  mm.)      ....  2.680 

Silt  (.05-.005  mm.) 2.698 

Clay  (below  .005  mm.) 2.837 

The  only  marked  variation  here  observed  is  in  the  clay 
separate,  and  this  may  be  due  to  the  concentration  of  the 
iron-bearing  silicates  in  this  grade.  However,  for  all 
practical  purposes  the  average  absolute  specific  gravity 
of  a  mineral  soil  may  be  placed  at  about  2.70.  One 
condition  that  may  vary  this  is  the  quantity  of  organic 
matfer  present.  As  the  specific  gravity  of  the  soil  humus 
usually  ranges  from  1.2  to  1.7,  the  more  humus  there  is 
present,  the  lower  will  be  the  absolute  figure  for  a  given 
soil.  A  purely  organic  soil,  such  as  muck  or  peat,  pre- 
sents a  variable  absolute  specific  gravity  ranging  from 
1.5  to  2.0,  according  to  the  amount  of  wash  it  has  received 
from  external  sources.  Some  humus-loam  soils  may 
drop  as  low  as  2.1.  Nevertheless  for  general  calculations 
the  average  arable  soil  may  be  considered  to  have  an 
absolute  specific  gravity  of  about  2.70. 

82.  Apparent  specific  gravity.  —  Since  all  soils  contain 
mqre  or  less  pore  space,  depending  on  textural  and  struc- 

1  Whitney,  M.  Some  Physical  Properties  of  Soils.  U.  S. 
D.  A.,  Weather  Bur.,  Bui.  4,  p.  34.     1892. 

i 


114       SOILS:    PROPERTIES  AND  MANAGEMENT 


tural  conditions,  the  actual  weight  of  the  absolutely  dry 
soil  in  any  volume  is  of  great  importance.  This  is  ex- 
pressed as  is  the  absolute  specific  gravity  of  any  material, 
the  weight  of  an  equal  volume  of  water  being  used  as  a 
unit.  Because  of  their  tendency  to  granulate,  fine  soils 
have  a  very  large  amount  of  pore  space,  as  has  been  shown 
in  the  discussion  of  structure ;  it  is  to  be  expected,  there- 
fore, that  they  will  weigh  less  in  any  particular  volume 
than  will  soils  made  up  of  large  particles.  Coarse  soils 
are  heavy  soils,  as  far  as  weight  is  concerned.  Mineral 
soils  may  range  in  apparent  specific  gravity1  from  1.10 
to  1.20  for  clay  to  1.65  to  1.75  for  sand.  Humous  loams 
may  drop  as  low  as  1.00,  and  muck  often  reaches  the 

low  figure  of  .40.  The  appar- 
ent specific  gravity  is  always 
expressed  on  the  basis  of  abso- 
lutely dry  soil. 

In  the  field  the  apparent 
specific  gravity  of  a  soil  may 
be  determined  by  driving  a 
cylinder  of  known  volume  into 
the  ground  and  obtaining 
thereby  a  core  of  natural  soil 
(see  Fig.  19).  By  weighing 
the  soil  and  then  determining 
the  amount  of  water  that  it 
holds,  the  amount  of  absolutely 
dry  soil  may  be  ascertained. 
Dividing  this  by  the  weight 
of  an  equal  volume  of  water 
gives    the    apparent     specific 

1  See    Whitney,    M.     Some    Physical    Properties    of    Soils. 
U.  S.  D.  A.,  Weather  Bur.,  Bui.  4.     1892. 


Fig.  19. —  Cylinder  for  deter- 
mining the  apparent  specific 
gravity  of  soil  in  the  field. 
The  cutting  edge  at  (6)  is 
drawn  in  somewhat  to  pre- 
vent excessive  friction  be- 
tween the  sides  of  the  cylin- 
der and  the  entering  soil  core. 


\        SOME  PHYSICAL   PROPERTIES   OF  THE  SOIL      115 

gravity  for  that  soil.  A  laboratory  determination  may  be 
made  by  putting  the  soil  into  a  receptacle  of  known  volume 
and  weighing  it.  From  the  weight  of  the  absolutely 
dry  soil  and  the  weight  of  an  equal  volume  of  water,  the  ap- 
parent specific  gravity  may  be  calculated.  This  method 
will  give  only  approximate  results,  however,  as  the  struc- 
tural relationships  are  more  or  less  artificial.  The  only 
reliable  method  is  the  one  first  described. 

83.  Actual  weight  of  a  soil.  —  With  the  apparent 
specific  gravity  of  a  soil  known,  its  weight  in  pounds  to 
the  cubic  foot  may  be  found  by  multiplying  by  62.42. 
Soils  may  vary  in  weight  from  68  to  80  pounds  for  clays 
and  silts  to  100  to  110  pounds  for  sand.  (The  greater  the 
humus  content,  the  less  is  this  weight  to  the  cubic  foot. 
A  muck  soil  often  weighs  as  little  as  25  or  30  pounds. 
This  weight,  of  course,  is  for  absolutely  dry  soil  and  does 
not  include  the  water  present,  which  may  be  much  or 
little,  according  to  circumstances.  The  actual  weight 
of  *  soil  is  often  expressed  in  acre-feet.  An  acre-foot  of 
soil  refers  to  a  volume  of  soil  one  acre  in  extent  and  one 
foot  deep.  In  the  same  way  we  may  have  an  acre-eight- 
inches  or  an  acre-six-inches.  The  weight  of  an  acre-foot 
of  soil  usually  varies  from  3,500,000  to  4,000,000  pounds ; 
granulation  and  organic  matter  may  modify  this  con- 
siderably./The  value  of  knowing  the  actual  weight  of 
a  soil  lies  in  the  possibility  of  calculating  thereby  the 
amount  of  water,  the  amount  of  humus,  or  the  actual 
number  of  pounds  of  the  mineral  constituents  present  in 
the  soil.  Such  information  affords  a  ready  means  of 
comparing  two  soils  as  to  their  crop-producing  capabilities. 

84.  Pore  space  in  soil.  —  The  pore  space  in  soil  is 
due  largely  to  structural  conditions.  As  already  em- 
phasized, the  coarser  the  soil,  the  smaller  is  the  aggregate 


116       SOILS:    PROPERTIES  AND  MANAGEMENT 

amount  of  internal  space.  Each  individual  space  is 
larger  under  such  conditions,  and  this  accounts  for  the 
ready  movement  of  water  and  air  through  such  soils. 
A  clay  soil,  while  containing  a  very  large  amount  of  pore 
space,  has  the  disadvantage  of  very  minute  individual 
pores.  The  large  amount  of  space  occurs  because  of 
the  lightness  of  the  particles  and  the  tendency  toward 
granulation.  The  small  size  of  the  individual  spaces 
is  a  direct  function  of  size  of  particle,  or  texture. 

A  very  simple  formula  may  be  used  for  a  determination 
of  the  percentage  of  pore  space  in  any  soil,  provided  the 
absolute  and  the  apparent  specific  gravities  are  known  : — 

,™     TAp.  Sp.  Gr.  ^  1001 
Percentage  of  pore  space  =  100  —    .  — ~ — —  X  — — 

Thus  a  soil  having  an  apparent  specific  gravity  of  1.60 
and  an  absolute  specific  gravity  of  2.60  has  38.5  per  cent 
of  pore  space ;  while  in  a  soil  in  which  the  above  figures 
are  1.10  and  2.50,  respectively,  the  percentage  of  pore 
space  is  56.  The  following  figures,  taken  from  King,1 
illustrate  the  relation  that  texture  holds  to  total  pore 
space  in  soils  :  Percentage  of 

Pore  Space 

Sandy  soil 32.49 

Loam 34.49 

Heavy  loam       44.15 

Loamy  clay  soil 45.32 

Clayey  loam 47.10 

Clay     . 48.00 

Very  fine  clay 52.94 

1  King,  F.   H.     Physics  of  Agriculture,  p.   124.     Published 
by  the  author,  Madison,  Wisconsin.     1910. 


SOME  PHYSICAL   PROPERTIES   OF  THE  SOIL      117 

The  pore  space  in  any  of  these  soils  is,  of  course,  subject 
to  considerable  fluctuation,  especially  in  the  surface 
soil,  due  to  tillage  and  the  incorporation  of  organic  matter ; 
hence  a  sandy  loam  might  under  certain  conditions  present 
more  pore  space  than  a  silt  or  a  clay  loam.  When  soils 
are  in  the  physical  condition  for  the  best  plant  growth, 
however,  the  rule  holds  that,  the  finer  the  soil,  the  greater 
is  the  pore  space.  The  differences  in  pore  space  between 
the  surface  soil  and  the  subsoil  in  Wisconsin  are  shown 
by  King  *  as  follows :  — 


Weight  per 

Percentage  of 

Cubic  Foot 

Pore  Space 

First  foot        

79.0 

52.2 

Second  foot 

92.6 

44.0 

Third  foot 

104.6 

36.8 

Fourth  foot 

106.2 

35.8 

Fifth*foot 

111.0 

32.9 

Sixth  foot 

111.1 

32.8 

The  pore  space  in  a  normal  soil  is  occupied  by  water 
and  air.  If  the  water  content  is  low,  the  air  space  is 
large,  and  vice  versa.  Thus  the  relationships  of  the  total 
pore  space  and  the  size  of  the  individual  spaces  to  the 
amount  of  air  and  water  contained,  to  their  movement 
through  the  soil,  to  soil  sanitation,  to  root  extension,  to 
bacterial  action,  and  to  cropping  conditions  in  general, 
become  apparent.  It  is  the  regulation  of  this  pore  space 
that  is  really  studied  in  any  structural  consideration. 
The  effect  on  plant  growth  of  a  change  in  pore  space  is 
the  final  test  of  its  advisability. 


1  King,   F.   H.     Physics   of  Agriculture,   p. 
by  the  author,  Madison,  Wisconsin.     1910. 


111.     Published 


118       SOILS:    PROPERTIES  AND  MANAGEMENT 

85.  The  number  of  soil  particles.  —  The  number  of 
particles  in  any  given  volume  of  soil  is  really  determined 
by  texture,  and,  as  this  number  determines  very  largely 
the  probable  arrangement  of  the  soil  grains,  structure 
becomes  in  turn  dependent  on  the  size  of  grain.  Since 
soil  particles  run  to  very  small  diameters,  the  number 
in  any  given  volume  is  very  large,  especially  when  we 
are  dealing  with  fine-textured  soils  or  with  soils  of  com- 
pact structural  condition.  Any  calculation  of  the  number 
of  particles  present  in  a  soil  is  open  to  considerable  in- 
accuracy, first,  because  it  is  impossible  to  get  a  correct 
figure  for  the  average  diameter  of  the  particles  of  any 
soil  or  of  the  various  groups  of  separates  that  go  to  make 
it  up,  and,  secondly,  because  it  must  be  assumed  in  the 
calculation  that  the  particles  are  spherical.  This  assump- 
tion is  of  course  incorrect,  as  has  already  been  demon- 
strated ;  but  it  must  be  entertained  in  order  to  obtain 
approximate  ideas  as  to  the  number  of  grains  in  any  soil. 

The  number  of  particles  in  any  soil  sample  may  be 
arrived  at  from  a  mechanical  analysis  and  the  diameters 
that  limit  each  group.  Using  the  average  diameter  of 
each  group  together  with  the  percentage  of  the  groups 
in  a  given  sample,  the  number  of  particles  may  be  cal- 
culated by  the  following  formula  :  — 

Number  of  particles  in  a  _    Weight  of  sample  in  grams 
sample  of  soil  1/6  tt  Z)3  X  2.70 

The  formula  1/6  tt  D3  is  that  used  for  determining  the 
volume  of  a  sphere,  the  diameter  in  this  case  being  ex- 
pressed in  centimeters.  The  volume  of  the  sphere,  then, 
is  obtained  in  cubic  centimeters,  which  must  be  multiplied 
by  the  absolute  specific  gravity  of  soil  minerals,  or  2.70, 


SOME  PHYSICAL   PROPERTIES   OF  THE  SOIL      119 

in  order  that  the  weight  in  grams  of  a  single  soil  grain 
may  be  obtained.  A  calculation  by  this  method  of  the 
number  of  particles  in  a  sandy  loam  is  given  below :  — 


I'd    g 

Approximate 

Separates 

Limits 

Number  op  Parti- 
cles to  the  Gram 

£    ,2  H 

Number  of 
Particles  in  one 

of  Each  Separate 

^S     to 

Gram  of  Sandy 
Loam 

Fine  grav- 
el  .     . 

2-1  mm. 

209 

1 

2 

Coarse 

sand    . 

1-.5  mm. 

1,670 

4 

67 

Medium 
sand    . 

.5-.2S  mm. 

13,410 

25 

3,352 

Fine 

sand    . 

.25-.  10  mm. 

131,900 

35 

46,165 

Very  fine 

sand    . 

.10-.05  mm. 

1,676,500 

20 

335,300 

Silt    .     . 

.05-.005  mm. 

35,934,000 

10 

3,593,400 

Clay*.     . 

below  .005  mm. 

45,632,000,000 

5 

2,281,600,000 

2,285,578,286 

A  very  great  error  is  introduced  by  this  method,  es- 
pecially in  assuming  that  the  average  size  of  particles 
for  the  clay  separate  is  .0025  millimeter.  As  the  clay 
particles  may  become  molecular  complexes  and  conse- 
quently are  very,  very  small,  it  stands  to  reason  that 
such  an  assumption  is  far  from  correct.  Nevertheless, 
it  gives  a  very  good  idea  as  to  the  immense  number  of 
grains  that  we  have  to  deal  with,  even  in  the  coarsest  of 
soils.  A  few  figures  as  to  the  approximate  number  of 
particles  in  various  average  soil  classes  of  the  United 
States,1  as  reported  by  the  Bureau  of  Soils,  are  given 
below :  — 

1  For  mechanical  analysis  of  the  classes,  see  Chapter  VI,  p.  104. 


120       SOILS:    PROPERTIES  AND  MANAGEMENT 


Approximate  Number  of  Particles  to  the  Gram  in 
Various  Classes  of  Soil  in  the  United  States 


CLA8S 


Coarse  sands  . 
Sands     .... 
Fine  sands       .     . 
Sandy  loams 
Fine  sandy  loams 
Loams    .... 
Silt  loams  .     .     . 
Sandy  clays    .     . 
Clay  loams      .     . 
Silty  clay  loams 
Clays     .     .     .     . 


Approximate  Number  op 
Particles 


2,299,145,360 

2,287,251,842 

1,826,176,893 

5,483,797,920 

5,485,069,147 

7,332,679,042 

6,868,546,664 

12,324,914,033 

11,877,875,092 

11,430,037,544 

19,177,571,994 


86.  Surface  exposed  by  soil  particles.  —  Besides  giving 
an  actual  numerical  figure  and  an  insight  into  the  prob- 
able structural  relationships  of  a  soil,  the  approximate 
number  of  particles  may  serve  still  another  purpose  — 
that  of  enabling  us  to  calculate  the  aggregate  internal 
surface  exposed  by  the  soil  grains.  The  surfaces  of  the 
grains  hold  more  or  less  water  according  to  their  area, 
and  they  increase  the  amount  of  chemical  and  biological 
activities  —  functions  so  necessary  to  a  continuous  re- 
placement in  the  soil  solution  of  the  elements  withdrawn 
by  the  plant.  The  minerals  in  the  soil  are  all  very  re- 
sistant to  solution ;  if  they  were  not,  they  would  long 
ago  have  been  leached  away.  Such  materials,  while 
almost  insoluble,  allow  the  amount  of  material  going 
into  solution  to  be  notably  increased  by  fineness  of  tex- 
ture, although  their  solubility  remains  the  same.  The 
fineness  of  the  particles,  then,  presents  another  significant 
feature  besides  those  already  pointed  out. 


SOME  PHYSICAL   PROPERTIES   OF  THE  SOIL      121 

Another  important  property  of  the  surface  of  the 
grains  is  the  tendency  toward  the  retention  of  soluble 
material  in  a  partially  or  wholly  available  condition  for 
plant  use.  This  power,  designated  as  adsorption,  is 
one  exhibited  to  a  high  degree  by  fine  soils,  in  which 
the  individual  pore  spaces  are  small  and  the  amount  of 
surface  exposed  is  large.  It  is  an  important  factor  to 
be  observed  in  the  addition  to  the  soil  of  soluble  fertilizing 
constituents.  Adsorption  may  also,  by  bringing  materials 
into  closer  contact,  hasten  or  retard  certain  chemical 
actions.  Reactions  may  thus  be  expected  to  go  on  in  the 
soil  that  would  not  take  place  in  the  laboratory  beaker. 
The  relation  of  this  adsorption  to  bacterial  activity  also 
cannot  be  overlooked. 

The  aggregate  area  presented  by  soil  particles  is  very 
large,*  even  for  the  coarser  soils.  With  the  finer  soils, 
because  of  the  immense  number  of  particles,  a  figure  is 
reached  that  is  almost  beyond  comprehension.  When 
the  approximate  number  of  particles  and  their  sizes 
in  any  given  weight  of  soil  are  known,  the  internal  surface 
may  be  calculated  by  the  following  formula :  — 

Surface  =  it  D2  X  number  of  particles 

As  the  estimation  of  the  number  of  particles  in  a  soil  is 
so  inaccurate,  it  is  evident  that  a  calculation  of  the 
surface  exposed  based  on  such  a  figure  must  be  more  in 
error. 

However,  to  give  some  idea  of  the  internal  surface  ex- 
posed by  ordinary  soils,  the  calculations  made  on  a  few  of 
the  average  soil  classes  of  the  United  States,  already 
presented,1  are  given  in  the  table  on  the  following  page. 

1  See  Chapter  VI,  p.  104. 


122       SOILS:    PROPERTIES  AND  MANAGEMENT 


Approximate  Internal  Area  exposed  by  Average  Classes 
op  United  States  Soils 


Square  Inches 
per  Gram 

Square  Feet 
per  Pound 

Acres  per 

Acre-foot  of 

3,500,000  Pounds 

Coarse  sands  .     . 
Sands     .... 

91 
89 
79 
213 
222 
294 
307 
417 
430 
458 
653 

286 

280 

248 

671 

699 

926 

967 

1313 

1354 

1442 

2057 

23,055 
22,549 

Fine  sands       .     . 
Sandy  loams   . 
Fine  sandy  loams 
Loams    .... 

20,014 
53,965 
56,180 
74,410 

Silt  loams  . 

77,700 

Sandy  clays     .     . 
Clay  loams      .     . 
Silty  clay  loams  . 
Clays      .... 

105,540 
108,830 
115,910 
165,270 

It  is  at  once  apparent  that  the  amount  of  surface 
exposed  by  the  soil  grains  of  even  a  sand  is  tremendous. 
It  is  not  to  be  wondered  at  that  the  slowly  soluble  minerals 
are  able  to  supply  sufficient  food  to  the  crop  growing  on 
the  soil,  when  such  a  large  amount  of  surface  is  continually 
available  for  chemical  action.  The  figures  presented  for 
an  acre-foot  of  soil  are  almost  too  large  for  adequate  com- 
prehension. It  is  quite  evident  that  the  finer  the  soil, 
the  greater  is  the  amount  of  internal  surface.  For  ex- 
ample, a  sandy  loam  weighing  90  pounds  to  the  cubic 
foot  would  present  60,390  square  feet  of  surface,  while 
a  clay  weighing  75  pounds  to  the  cubic  foot  would  expose 
about  154,275  square  feet.  This  is  equivalent  to  1.39 
and  3.54  acres,  respectively. 

87.  The  effective  mean  diameter  of  soil  particles.  — 
It  is  very  evident  that  the  calculations  presented  above, 
both  as  to  the  number  of  particles  and  as  to  the  internal 


SOME  PHYSICAL   PROPERTIES   OF  THE  SOIL      123 

surface  exposed,  are  far  from  correct,  as  we  can  arrive 
at  no  definite  figure  as  to  the  average  size  of  grain.  Neither 
do  we  know  the  actual  structural  conditions.  In  con- 
sidering these  inaccuracies  King  l  decided  that  we  were 
in  need  of  a  single  term  which  not  only  would  give  an 
indication  regarding  the  size  of  grain,  but  also  would 
carry  with  it  definite  ideas  as  to  the  arrangement  of  the 
particles,  particularly  as  to  the  rate  at  which  they  would 
allow  air  and  water  to  pass  through.  This  would  bring 
the  considerations  nearer  to  the  plant,  as  permeability 
very  largely  determines  the  conditions  for  plant  develop- 
ment. King,  while  he  could  obtain  neither  the  mean 
diameter  of  particle  nor  the  actual  internal  surface,  found 
that  he  could  determine  with  considerable  accuracy, 
particularly  in  sands,  the  diameter  of  grain  which  if  sub- 
stituted for  the  actual  one  would  permit  under  like  con- 
ditions the  same  rate  of  air  and  water  movement.  This 
size  of  grain  he  designated  as  the  effective  mean  diameter 
of  particle  for  that  particular  soil. 

The  theory  of  the  method  is  presented  by  Schlicter,2 
and  is  based  on  the  flow  of  fluids  through  capillary  tubes. 
From  the  observed  rate  of  the  flow  of  air  through  a  soil 
column  under  controlled  conditions,  it  is  possible  to 
calculate  the  effective  diameter  of  the  interstitial  spaces. 
From  these  data  the  size  of  the  spherical  grains  which 
would  be  necessary  to  form  such  pore  spaces,  or  capillary 
tubes,  is  computed  by  appropriate  formulae.  Such  a 
figure  represents  the  effective  mean  diameter  of  the  soil, 


1  King,  F.  H.  Physics  of  Agriculture,  pp.  119-120.  Pub- 
lished by  the  author,  Madison,  Wisconsin.     1910. 

2  Schlicter,  C.  S.  Theoretical  Investigation  of  the  Move- 
ment of  Ground  Waters.  U.  S.  Geol.  Survey,  19th  Ann.  Rept., 
Part  II,  pp.  301-384.     1899. 


124       SOILS:    PROPERTIES  AND  MANAGEMENT 


^% 


from  which  the  effective  surface  exposed  can  be  deter- 
mined.  Thus,  designating  a  soil  as  having  an  effective 
mean  diameter  of  particle  of  .0052  millimeter  merely 
indicates  that  this  particular  soil  shows  an  air  and 
water  movement  the  same  as  would  be  shown  by  a 
homogeneous  soil  with  spherical  particles  of  that  diameter. 

The  apparatus  x  for  the  de- 
termination consists  of  a  cylin- 
der in  which  is  placed  a  sample 
of  air-dry  soil,  the  pore  space 
being  carefully  determined  by 
weighing.  The  rate  of  air 
movement  is  then  determined 
by  connecting  with  an  aspi- 
rator, the  temperature  and 
the  pressure  being  constantly 
under  control.  The  reading  is 
usually  calculated  to  a  tem- 
perature of  20°  C.  The  fact 
that  the  structural  condition 
of  the  soil  is  likely  to  be  dis- 
turbed in  placing  the  sample 
in  the  aspirator  tube  detracts 
from  the  accuracy,  especially 
in    fine     soils.      Nevertheless, 

Fig.  20.  — King's  aspirator  for      £ing    foun(J    fa    results    fairlv 
the  determination  of  the  rate  j        1  1         1     . 

of  air  movement    through     accurate,     and    showed     that 
soils.    (G),  Pressure  gauge;     tne    calculated    and    the    ob- 

(<S),  soil  column ;   (L),  water;  in*  1  1 

U),  aspirator;  (HO,  weight.  .  served  flow  ot  water  through 


1  King,  F.  H.  Principles  and  Conditions  of  the  Movements 
of  Ground  Water.  U.  S.  Geol.  Survey,  19th  Ann.  Rept.,  Part 
II,  pp.  222-224.  1899.  A  complete  discussion  is  given  of 
King's  ideas  in  this  article,  pp.  67-294. 


SOME  PHYSICAL  PROPERTIES   OF  THE  SOIL      125 

sands  1  agreed  rather  closely  (see  Fig.  20).  The  effective 
diameter  of  the  particles  of  some  of  our  common  soils,  to- 
gether with  the  effective  surface  exposed,  is  given 
below : 2  — 


Soil 


Coarse  sandy  soil 
Sandy  soil  . 
Sandy  loam     . 
Loam      .     .     . 
Loamy  clay  soil 
Fine  clay  son*  . 
Very  fine  clay 


Effective 
Diameter 


.1432  mm. 
.0755  mm. 
.0303  mm. 
.0219  mm. 
.0140  mm. 
.008(1  mm. 
.0049  mm. 


Percentage 

of  Pore 

Space 


34.9 
34.4 
38.8 
44.1 
45.3 
48.0 
52.9 


Effective  Sur- 
face Exposed  in 
One  Cubic  Foot 
of  Soil 


8,318  sq.  ft. 

15,870  sq.  ft. 

36,880  sq.  ft. 

46,510  sq.  ft. 

71,316  sq.ft. 
110,500  sq.  ft. 
173,700  sq.  ft. 


The  method  of  King  has  certain  advantages,  besides 
giving  an  idea  as  to  the  number  of  particles,  their  internal 
•surface,  and  the  relation  of  this  internal  surface  to  soil 
conditions.  In  the  first  place,  a  single  figure  is  used 
to  express  the  size  of  particle ;  secondly,  from  this  effec- 
tive size  of  particle  the  probable  rate  of  air  and  water 
movement  may  be  calculated ;  and,  thirdly,  the  number 
of  particles  and  the  internal  surface  calculated  therefrom 
have  a  fairly  definite  relationship  to  the  plant,  as  such 
figures  are  so  closely  correlated  to  the  circulation  of  air 
and  water. 


1  King,  F.  H.      Physics  of  Agriculture,  p.   123. 
by  the  author,  Madison,  Wisconsin.     1910. 

mid.,  d.  124. 


Published 


CHAPTER  VIII 
THE   ORGANIC  MATTER  OF    THE  SOIL 

One  of  the  essential  differences  between  a  soil  and  a 
mass  of  rock  fragments  lies  in  the  organic  content  of  the 
former.  Organic  matter  is  a  necessary  constituent  in 
order  that  finely  ground  mineral  material  may  be  desig- 
nated as  a  soil  and  that  it  may  grow  crops  successfully. 
Physical  condition  depends  largely  on  the  presence, 
and  chemical  reaction  is  greatly  accelerated  by  the  de- 
cay,  of  organic  matter.  In  the  process  of  soil  formation 
its  addition  is  more  or  less  a  secondary  step.  In  residual 
debris  the  amount  of  organic  matter  held  by  the  growing 
soil  increases  as  the  process  of  weathering  goes  on ;  in 
glacial  soils,  however,  the  matrix,  or  skeleton  of  the  soil, 
is  already  formed  before  there  is  an  opportunity  for  humus 
to  become  incorporated  therein.  The  final  result  from 
the  mixing  of  the  minerals  carrying  numerous  weathered 
and  altered  products  with  the  decayed  or  partially  de- 
cayed organic  matter  that  is  sure  to  accumulate,  must 
be  a  mass  much  more  complicated  than  either  of  the 
original  constituents.  It  is  hardly  necessary  to  further 
emphasize  the  complexity  of  the  average  soil,  the  reasons 
therefor,  and  the  difficulties  in  studying  the  question. 

88.  The  source  and  distribution  of  organic  matter.  — 
The  source  of  practically  all  soil  organic  matter  is  plant 
tissue.  Some  of  this  matter  accumulates  from  the  above- 
ground  parts  of  plants  that  have  died  and  fallen  down 

126 


THE  ORGANIC  MATTER    OF  THE  SOIL 


127 


to  become  mixed  with  the  surface  soil;  the  remainder 
is  a  result  of  root  extension  and  subsequent  decay.  The 
organic  matter  of  the  surface  soil  is  derived  from  the 
tops  and  the  roots  of  plants  growing  on  it,  while  that  of 
the  subsoil  is  very  largely  a  result  of  root  extension  and 
subsequent  decomposition.  The  relationship  between  the 
humus  content  of  three  soils  and  the  roots  developed  is 
shown  by  the  following  data  presented  by  Kostytscheff  1 
and  quoted  by  Hilgard  2  and  Wollny  3  :  — 

Root  Content  and  Percentage  of  Humus  in  Three 
Russian  Soils 


l 

2 

3 

Depth 

(inches) 

Roots 

Humus 

Roots 

Humus 

Roots 

Humus 

6 

100 

5.4 

100 

8.1 

100 

9.6 

12 

89 

4.8 

64 

5.2 

80 

7.7 

18 

67 

3.6 

48 

3.9 

70 

6.7 

24 

47 

2.5 

35 

2.8 

58 

5.6 

30 

47 

2.5 

26 

2.1 

38 

3.6 

36 

35 

1.8 

18 

1.5 

33 

3.1 

42 

24 

1.3 

6 

.5 

16 

1.5 

48 

14 

.8 

54 

7 

.3 

89.  Composition  of  plants.  —  It  is  usual,  in  classifying 
the  materials  composing  plant  tissue,  to  group  them  under 
three  heads  —  carbohydrates,  fats  and  oils,  and  proteins. 


1  Kostytscheff,  M.  P.     Les  Terres  Noires  de  Russie.     Annales 
de  la  Sci.  Agron.,  Tome  II,  pp.  165-191.     1887. 

2  Hilgard,  E.  W.     Soils,  p.  130.     New  York.     1906. 

3  Wollny,  E.     Die  Zersetzung  der  Organischen  Stoffe,  p.  194. 
Heidelberg.     1897. 


128       SOILS:    PROPERTIES   AMD    MANAGEMENT 

The  carbohydrates,  having  the  general  formula  of 
Cx(H20),„  include  such  compounds  as  glucose,   starch. 

cellulose,  dextrose,  cane  sugar,  and  the  like.  The  fats 
and  oils  may  be  represented  in  plants  by  such  glyceridea 
as  butyrin,  stearin,  olein,  palmitin,  and  the  like.  The 
proteins  arc  l>y  far  the  most  complicated  of  the  three 
principal  compounds,  as  they  may  carry  not  only  carbon, 
hydrogen,  oxygen,  and  nitrogen,  but  also  mineral  elements 
such  as  sulfur,  phosphorus,  lime,  iron,  and  other  elements. 
They  are  compounds  of  high  molecular  weight  and  are 
mostly  of  unknown  constitution.  Simple  proteins,  such 
as  albumin,  globulin,  protamins,  and  others,  are  found 
in  plants,  besides  certain  derived  proteins  such  as 
proteoses  and  peptones.  In  addition  to  all  these,  there 
is  a  host  of  other  compounds  that  have  no  small  influence 
on  the  composition  of  the  soil  organic  matter.  Among 
these  are  the  alkaloids,  waxes,  tannins,  phenols  and  their 
derivatives,  hydrocarbons,  resins,  acids,  aldehydes,  and 
others. 

The  original  plant  tissue,  therefore,  while  fairly  well 
known,  as  to  chemical  constitution,  is  far  from  simple. 
The  degradation  of  such  material,  especially  in  the  pres- 
ence of  complex  mineral  products,  will  evidently  give  rise 
at  first  to  compounds  no  simpler;  in  fact,  the  chances 
are  that  the  resulting  compounds  will  be  much  more  com- 
plicated. It  is  only  later  in  the  processes  of  decay  that 
simple  products  result. 

90.  Decay  of  organic  matter  in  soils.  —  From  the  fact 
that  weathering  in  general  is  a  process  of  simplification, 
and  since  it  is  evident  that  the  plant  tissue  as  it  enters 
the  soil  is  so  very  complex,  the  general  change  that  the 
organic  matter  undergoes  must  be  one  of  simplification. 
This  simplification,  however,  is  very  slow,  and  many  of 


THE   ORGANIC  MATTER    OF  THE  SOIL  129 

the  products  built  up  are  more  complex  than  the  original 
tissue.  Most  of  this  decay  and  simplification  is  due  to 
that  great  group  of  organisms  so  universally  present  in 
soil,  called  bacteria.  Some  of  these  are  putrefactive  in 
their  action,  while  others  deal  to  a  large  extent  with  the 
products  of  the  decomposition.  All,  however,  exert  a 
general  simplifying  influence.  The  action  of  such  or- 
ganisms may  be  direct,  but  is  more  likely  to  be  enzymotic 
in  its  nature,  and  may  take  place  either  within  or  outside 
of  the  cell.  A  cycle  is  therefore  set  up,  in  which  the  higher 
plants  and  animals  are  occupied  in  building  up,  while 
bacteria  are  tearing  «down  and  reducing  the  residue  of 
plant  action  to  simple  forms,  such  as  can  be  ultimately 
utilized  again  in  plant  nutrition.  The  great  importance 
of  bacteria  is  thus  evident,  and  the  encouragement  of 
their  growth  and  function  is  clearly  a  part  of  good  soil 
management. 

When  the  complex  molecules  that  make  up  plant 
tissue  break  down,  they  split  along  definite  lines  of  cleav- 
age, depending  on  the  structure  of  the  original  molecule. 
These  bodies,  which  are  usually  simpler  in  nature  than 
those  from  which  they  have  sprung,  are  called  cleavage 
products,  and  without  a  doubt  they  are  the  primary 
products  of  the  first  step  in  organic  decay.  These,  com- 
pounds are  subject  to  still  further  change,  and  because 
of  the  great  number  of  agencies  at  work  the  secondary 
products  that  result  may  be  simpler  or  more  complex, 
according  to  conditions.  Bacteria  have  a  tendency, 
while  tearing  down  organic  matter,  to  construct  certain 
built-up  products  which  present  a  very  complicated 
molecule  until  they  are  in  turn  degraded.  The  secondary 
products  therefore  vary  widely  because  of  differences  in 
temperature,   moisture,    aeration,   and   other   conditions. 


130       SOILS:    PROPERTIES  AND  MANAGEMENT 

The  character  of  the  secondary  products  probably  ex- 
hibits a  greater  variation  than  does  that  of  the  original 
plant  tissue.  In  the  process  of  decay  these  products 
become  black  or  brown  in  color,  and  are  usually  designated 
as  humous  materials  in  the  soil.  Organic  matter,  then, 
covers  all  the  material  of  organic  origin  in  the  soil,  and 
may  refer  not  only  to  the  original  plant  tissue,  but  also 
to  that  which  has  lost  its  identity  in  the  secondary  prod- 
ucts. Humus  refers  specifically  to  the  primary  and  the 
secondary  products  of  decay,  and  may  be  simple  or  com- 
plex, according  to  conditions. 

As  the  process  of  decay  goes  on,  certain  end  products 
result.  These  are  partially  solid  and  partially  gaseous. 
Carbon  dioxide  is  a  universal  product  of  bacterial  activity 
of  all  kinds,  as  is  also  water.  Besides  these,  urea,  am- 
monia, nitrites,  and  nitrates  may  result  from  nitrogenous 
decay.  The  three  general  classes  of  organic  matter  found 
in  soil  may  be  illustrated  by  the  following  diagram :  — 

PLANT   TISSUE   I  HUMUS  [END   PRODUCTS 

UNDECOMPOSED  MATTER  |  SECONDARY  AND  INTERMEDIATE  |  SIMPLE    MATERIAL 

Fig.  21.  —  Diagram  illustrating  the   three   general   classes   of   organic 
matter  found  in  soils. 

It  is  therefore  possible  to  have  present,  besides  the 
original  organic  constituents  which  are  mostly  of  plant 
origin,  not  only  their  primary  and  secondary  degradation 
products,  but  also  compounds  either  torn  down  or  built 
up  from  these.  An  attempt  to  enumerate  even  the 
original  compounds  in  the  plant  tissue,  or  even  the  simple 
end  products  of  complete  decay,  would  result  in  a  long 
list  of  materials  representing  almost  every  known  class 
of  organic  compound.  Such  a  procedure  is  possible, 
but  is  unnecessary  as  the  important  ones  have  already 


THE  ORGANIC  MATTER    OF  THE   SOIL  131 

Jbeen  mentioned.  It  is  to  be  kept  in  mind  that  the  simpler 
products  of  decay  are  the  ones  utilized  by  crops,  although 
it  is  a  well-established  fact  that  some  of  the  secondary 
and  intermediate  compounds  may  be  taken  up  by  certain 
plants  and  probably  are  of  some  importance  from  the 
standpoint  of  use  as  plant-food. 

91.  Composition  of  the  soil  humus.  —  It  is  evident 
that  the  most  complicated  parts  of  the  organic  matter 
in  the  soil  are  the  primary  and  the  secondary  products 
of  decay,  or  the  so-called  soil  humus.  The  study  of  this 
matter  is  difficult  and  calls  for  the  very  highest  knowledge 
of  organic  chemistry.  This  is  true  for  two  reasons: 
first,  because  of  the  complexity  of  these  compounds; 
and,  secondly,  because  they  are  continually  changing. 
A  certain  compound  present  in  the  soil  one  week  may 
be  altered  the  next  week.  Moreover,  while  some  of  the 
soil  humus  is  soluble  in  water  and  may  circulate  in  the 
soil  solution,  the  bulk  of  it  is  insoluble.  This  in  itself 
presents  difficulties.  When  the  soil  humus  is  treated 
with  the  various  extractive  agents,  reactions  may  be 
induced  which  would  not  take  place  in  a  normal  soil. 
Compounds  are  then  formed  which  not  only  would  be 
abnormal,  but  would  probably  not  exist  under  natural 
conditions. 

A  great  many  chemists  have  worked  on  the  problem  of 
the  constitution  of  the  organic  matter  of  the  soil  and 
have  published  their  results.  The  ideas  of  the  early 
workers  are  really  embodied  in  the  conclusions  advanced 
by  Mulder,1  who  was  in  many  ways  far  in  advance  of  his 

1  Mulder,  T.  J.  Die  Organischen  Bestandtheile  im  Boden. 
Chemie  der  Ackerkrume,  I,  pp.  308-360.  Berlin,  1863.  Also, 
Wiley,  H.  W.  Agricultural  Analysis,  Vol.  I,  p.  53,  Easton, 
Pa.     1906. 


182       SOILS:    PROPERTIES   AND  MANAGEMENT 

time.  Mulder  contended  that  the  organic  matter  con- 
sisted of  seven  distinct  compounds,  as  follows :  — 

1  and  2.  Ulmic  acid   and   ulmin     5.  Geic  acid 
3  and  4.  Humic  acid  and  humin     6.  Apocrenic  acid 
7.  Crenic  acid 

These  bodies  he  considered  as  arising  from  one  another 
by  oxidation ;  thus,  ulmic  acid  (C40H14O12)  gave  humic 
acid  (C40H12O12),  which  in  turn  yielded  geic  acid 
(C40H19O14),  followed  by  apocrenic  acid  (C48H12O24), 
and  finally  by  crenic  acid  (C24Hi20i6)-  Such  a  classi- 
fication seems  very  simple,  but  certain  flaws  are  at  once 
noticeable.  In  the  first  place,  nitrogen  does  not  find  a 
place  in  any  of  these  formulae ;  secondly,  the  compounds 
are  simpler  than  most  plant  tissue,  which  is  not  what 
would  be  expected,  especially  with  some  of  the  degrada- 
tion compounds;  thirdly,  none  of  these  products  have 
united  with  the  bases  in  the  soil,  a  reaction  that  would 
be  very  likely  to  take  place  especially  with  acid  compounds. 
Even  the  investigators1  of  Mulder's  time  obtained  dis- 
cordant results,  but  these  were  explained  for  the  time 
being  by  assuming  that  the  discrepancies  occurred  because 
of  added  molecules  of  water. 

Later  investigators,  while  progressing  only  slightly 
toward  definite  results,  did  accomplish  one  thing  of  im- 
portance, and  that  was  throwing  considerable  doubt 
on  the  old  ideas  of  the  Mulder  school  of  chemists.  This 
again  opened  up  the  question  as  to  the  composition  of 
the  soil  organic  matter,  especially  the  humous  constituents. 
Thus,  wThile  it  is  evident  that  no  such  compounds  as  geic 

1  See  Schreiner,  O.,  and  Shorey,  E.  C.  The  Isolation  of 
Harmful  Organic  Substances  from  Soils.  U.  S.  D.  A.,  Bur. 
Soils,  Bui.  53,  pp.  15-16.     1909. 


THE  ORGANIC  MATTER   OF  THE  SOIL  138 

acid,  humic  acid,  or  crenic  acid  exist  in  the  soil,  one  name 
has  persisted  in  soil  literature  —  that  of  humus  and 
humic  acid.  The  word  humus,  as  already  indicated, 
does  not  relate  to  any  definite  compound,  but  to  the 
great  mass  of  primary  and  secondary  products  of  bio- 
logical and  chemical  organic  decay  taking  place  in  the 
soil.  One  of  the  men  whose  work  established  beyond 
a  doubt  the  fact  that  humus  was  not  a  definite  compound 
was  Van  Bemmelen.1  His  investigations  still  further 
showed  that  the  soil  humus  was  largely  in  a  colloidal 
condition,  and  therefore  exhibited  properties  quite  dis- 
tinct from  those  shown  by  crystalloids. 

In  recent  years  investigation  has  again  been  directed 
toward  the  immense  field  opened  by  the  overthrow  of 
the  Mulderian  school.  Baumann,2  by  his  researches, 
has  shown  freshly  precipitated  humus  to  possess  properties 
which  are  largely  colloidal  in  nature.  Among  these  char- 
acteristics are  high  water  capacity,  great  adsorptive  power 
for  certain  salts,  ready  mixture  with  other  colloids,  power 
to  decompose  salts,  great  shrinkage  on  drying,  and  coagula- 
tion in  the  presence  of  electrolytes.  Jodidi 3  has  studied 
the  composition  of  the  acid-soluble  organic  nitrogen  in 
peat  and  in  mineral  soils.  The  nitrogenous  compounds 
thus  obtained  can  be  divided  into  the  following 
groups :  — 

1  Van  Bemmelen,  J.  M.  Die  Absorptions  Verbindungen 
und  das  Absorptionsvermogen  der  Ackererde.  Landw.  Ver- 
suchs.  Stat.,  Band  35,  Seite  67-136.     1888. 

2  Baumann,  A.  Untersuchungen  iiber  die  Humussauren. 
Mitt.  d.  K.  bayr.  Moorkulturanstalt,  Heft  3,  Seite  53-123. 
1909. 

3  Jodidi,  S.  L.  Organic  Nitrogenous  Compounds  in  Peat 
Soils  I.  Mich.  Agri.  Exp.  Sta.,  Tech.  Bui.  4,  November,  1909. 
Also,  The  Chemical  Nature  of  the  Organic  Nitrogen  in  Soil. 
Iowa  Agr.  Exp.  Sta.,  Research  Bui.  I,  June,  1911. 


134     soils:  properties  and  management 

1.  Nitric  nitrogen  3.  Diamine-  acids 

2.  Ammoniacal  nitrogen  4.  Acid  amides 

5.  Monamino  acids 

The  two  latter  constituents  were  found  to  make  up  the 
bulk  of  the  organic  nitrogen,  but  quantitative  determina- 
tions proved  uncertain.  These  compounds  produced 
ammonia  readily,  the  rate  depending  on  their  chemical 
structure. 

92.  The  work  of  Oswald  Schreiner.  — Of  the  chemists 
who  have  been  most  active  and  most  successful,  Schreiner  l 
deserves  especial  mention.  Our  present  knowledge  of  the 
chemical  constitution  of  the  organic  matter  of  the  soil  is 
very  largely  due  to  his  efforts.  While  he  realized  that 
the  isolation  of  specific  compounds  from  the  soil  was  likely 
to  present  insurmountable  problems,  and  that  the  iden- 
tification of  such  compounds  after  they  were  obtained 
might  be  very  difficult,  he  undertook  a  systematic  ex- 
traction of  the  soil.  As  a  result  of  several  years  of  work 
he  was  able  to  isolate  and  identify  a  number  of  com- 
pounds. The  complexity  and  varied  character  of  these 
compounds  is  revealed  by  the  following  list  of  the  more 
important  bodies  isolated  :  — 

List  of  Compounds  isolated  from  Soil  Organic 
Matter  by  Schreiner,  Shorey,  Skinner,  Reed, 
and  others,  of  the  u.  s.  bureau  of  soils 

Hentriacontane,  C3iH64  Picoline     carboxylic    acid, 

Dihydroxystearic   acid,  C7H702X 

Ci8H3664  Histidine,  C6H902N3 

1  Schreiner,  O.,  and  Shorey,  E.  C.  The  Isolation  of  Harm- 
ful Organic  Substances  from  Soils.  U.  S.  D.  A.,  Bur.  Soils, 
Bui.  53,  1909 ;  also,  Buls.  47,  70,  74,  77,  80,  83,  87,  88,  and  90. 


THE   ORGANIC  MATTER    OF  THE  SOIL 


135 


Monohydroxy  stearic  acid, 

C18H36U3 
Agroceric  acid,  C21H42O3 
Agrosteral,  C22H22O  .  H20 
Paraffinic  acid,  C24H48O2 
Lignoceric  acid,  C24H48O2 
Phytosterol,  C26H440  .  H20 
Pentosan,  C5H3O4 
Oxalic  acid,  C2H2O4 
Succinic  acid,  C4H604 
Sacharaic  acid,  CeHsOio 
Acrylic  acid,  C3H402 
Mannite,  C6Hi406 
Rhamnose,  CeHsOio 
Salicylic  aldehyde, 

C6H4OHCOH 


Arginine,  C6H14O2X4 
Cytosine,  C4H5OX3 .  H20 
Xanthine,  C5H402XT4 
Hypoxan thine,  C5H4ON4 
Tysine,  C6H1402N2 
Adenine,  C5H5N5 
Choline,  C5H1502N 
Trimethylamine,  C3H9N 
Quanine,  CH5N3 
Creatinine,  C4H7ON3 
Creatine,  C4H902X3 
Nucleic   acid    (constitution 

unknown) 
Trithiobenzaldehyde, 
(C6H6CSH)3 


From  a  chemical  standpoint  these  compounds  may  be 
classified  under  four  heads :  (1)  those  containing  carbon 
and  hydrogen;  (2)  those  containing  carbon,  hydrogen, 
and  oxygen ;  (3)  those  containing  carbon,  hydrogen,  and 
nitrogen,  or  carbon,  hydrogen,  oxygen,  and  nitrogen ; 
(4)  those  containing  sulfur  in  combination  with  the 
elements  listed  above.  With  the  possible  presence  in 
soils  of  compounds  containing  five  elements,  it  is  of 
little  wonder  that  the  subject  is  a  complicated  one.  It 
is  evident,  moreover,  that  the  list  given  above  is  only  a 
partial  one,  and  many  other  compounds  of  an  even  more 
intricate  composition  will  later  be  isolated. 

So  far  as  the  plant  is  concerned,  the  compounds  may 
be  divided  into  three  groups  —  those  that  are  beneficial, 
those  that  are  neutral,  and  those  that  are  toxic,  or  harm- 
ful, in  their  effects.     As  an  example  of  the  first  group, 


136       SOILS:    PROPERTIES   AND   MANAGEMEST 

histidine  and  creatinine l  may  be  mentioned.  Here  ib 
a  case  in  which  the  compounds  found  in  the  soil  humus 
may  exert  a  stimulating  effect  on  plant  growth,  and 
may  also  be  a  source  of  plant-food,  supplementing  the 
nitrates2  to  a  certain  extent.  That  the  nitrogen  of  the 
soil  organic  matter  may  be  utilized  by  plants  is  weli  sum- 
marized by  the  publications  of  Hutchinson  and  Miller.3 
As  an  example  of  a  harmful  compound  arising  from  the 
decomposition  of  the  organic  matter,  dihydroxy stearic 
acid4  may  be  mentioned  as  one  of  the  best  known.  This 
compound  was  the  first  to  be  isolated  and  identified  by 
Schreiner,  and  is  very  toxic. 

93.  Toxic  material  in  the  soil.  —  The  discovery  of 
such  compounds  in  the  soil  has  revived  the  old  theory 
of  toxicity,5  by  which  the  infertility  of  certain  soils  is 
accounted  for.  Root  excretions  are  also  held  to  be 
detrimental  to  succeeding  crops  of  the  same  kind.     The 

1  Skinner,  J.  J.  Effect  of  Histidine  and  Arginine  as  Soil 
Constituents.  Eighth  Internat.  Cong.  App.  Chem.,  Vol.  XV, 
pp.  253-264.  1912.  Also,  Beneficial  Effects  of  Creatinine 
and  Creatine  on  Growth.  Bot.  Gaz.,  Vol.  54,  No.  2,  pp.  152- 
163.     1912. 

2  Schreiner,  O.,  and  Skinner,  J.  J.  Nitrogenous  Soil  Con- 
stituents and  Their  Bearing  upon  Soil  Fertility.  U.  S.  D.  A., 
Bur.  Soils,  Bui.  87,  p.  68.  1912.  Also,  Schreiner,  O.,  and  others. 
A  Beneficial  Organic  Constituent  of  Soils ;  Creatinine.  U.  S. 
D.  A.,  Bur.  Soils,  Bui.  83,  p.  44.     1911. 

3  Hutchinson,  H.  B.,  and  Miller,  N.  H.  J.  The  Direct 
Assimilation  of  Inorganic  and  Organic  Forms  of  Nitrogen  by- 
Higher  Plants.  Jour.  Agr.  Sci.,  Vol.  4,  Part  3,  pp.  282-302. 
1912. 

4  Schreiner,  O.,  and  Skinner,  J.  J.  Some  Effects  of  a  Harm- 
ful Organic  Soil  Constituent.  U.  S.  D.  A.,  Bur.  Soils,  Bui.  70. 
1910. 

5  See  Schreiner,  O.,  and  Reed,  H.  S.  Some  Factors  Influ- 
encing Soil  Fertility.  U.  S.  D.  A.,  Bur.  Soils,  Bui.  40,  pp.  36- 
40.     1907. 


THE  ORGANIC  MATTER   OF  THE  SOIL  137 

>xic  materials  of  the  soil  humus  largely  originate  under 
conditions  of  poor  drainage  and  aeration,  and  conse- 
quently are  biological  in  their  genesis.  The  toxicity  of 
such  compounds  as  dihydroxystearic  acid,  picoline  car- 
boxy  lie  acid,1  and  aldehydes  2  may  therefore  be  overcome 
by  oxidation,3  so  that  good  soil  aeration  is  a  factor  in 
dealing  with  such  conditions.  Insufficient  sanitation 
of  the  soil  seems  to  account  very  largely  for  the  presence 
of  soil  toxines.  Fertilizers,  according  to  Schreiner  and 
Skinner,4  seem  to  decrease  the  harmful  effects  of  such 
compounds;  nitrogenous  fertilizers  overcoming  some 
toxic  materials,  and  phosphorus  or  potash  neutralizing 
others.  For  example,  in  water  solution  and  sand  culture, 
nitrogen  seems  especially  efficacious  in  correcting  such 
toxic  substances  as  dihydroxystearic  acid  and  vanillin, 
phosphorus  is  particularly  powerful  in  counteracting 
cumarine,  and  potash  has  considerable  influence  on 
quinone. 

While  the  real  importance  of  the  toxic  material  generated 
in  the  soil  cannot  be  fully  discussed  at  this  point,  it  is 
quite  evident  that  such  constituents  do  tend  to  develop 
under  insanitary  conditions  and  must  be  considered  in 


1  Schreiner,  O.,  and  Skinner,  J.  J.  The  Isolation  of  Harm- 
ful Organic  Substances  from  Soils.  U.  S.  D.  A.,  Bur.  Soils, 
Bui.  53,  pp.  46-49.     1909. 

2  Schreiner,  O.,  and  Skinner,  J.  J.  Harmful  Effects  of 
Aldehydes  in  Soils.  U.  S.  D.  A.,  Bui.  108  (Professional  Paper). 
1914. 

3  Schreiner,  O.,  and  others.  Certain  Organic  Constituents 
of  Soils  in  Relation  to  Soil  Fertility.  U.  S.  D.  A.,  Bur.  Soils, 
Bui.  47,  p.  52.  1907.  Also,  Schreiner,  O.,  and  Reed,  H.  S. 
The  Role  of  Oxidation  in  Soil  Fertility.  U.  S.  D.  A.,  Bur. 
Soils,  Bui.  56,  p.  52.     1906. 

4  Schreiner,  O.,  and  Skinner,  J.  J.  Organic  Compounds  and 
Fertilizer  Action.     U.  S.  D.  A.,  Bur.  Soils,  Bui.  77.     1911. 


138       SOILS:    PROPERTIES  AND  MANAGEMENT 

the  discussion  of  the  composition  of  that  great  group  oi 
intermediate  compounds,  called  humus,  arising  from  the 
decay  of  the  organic  matter  of  the  soil.  While  Schreiner 
found  twenty  soils,  out  of  a  group  of  sixty  taken  in  eleven 
States  of  this  country,  to  contain  dihydroxystearic  acid, 
this  does  not  necessarily  mean  that  this  compound  in 
itself  is  a  serious  detrimental  factor.  It  is  very  likely 
that  such  compounds  are  merely  products  of  improper 
soil  conditions,  and  are  to  be  considered  as  concomitant 
with  depressed  crop  yields.  When  such  conditions  are 
righted,  the  so-called  toxic  matter  will  disappear.  Good 
drainage,  lime,  tillage,  a  balanced  food  ration,  promoted 
aeration  and  oxidation,  are  so  efficacious  in  this  regard 
that  permanent  soil  toxicity  need  never  be  feared  by  the 
farmer. 

94.  End  products  of  humus  decay.  —  As  the  processes 
of  chemical  and  biological  decay  of  the  soil  organic  matter 
proceed,  the  simple  compounds  already  noted  begin  to 
appear.  This  change  is  of  course  coordinate  with  a 
certain  amount  of  synthetic  action,  but  compounds  thus 
built  up  must  ultimately  succumb  to  the  agencies  at 
work  and  suffer  a  splitting-up  and  reduction  to  simple 
bodies.  Carbon  dioxide  is  one  of  the  most  important  of 
these  compounds,  being  always  a  product  of  bacterial 
activity.  Its  importance  has  already  been  noted  in  the 
discussion  of  weathering.  Here  it  heightens  the  solvent 
power  of  water  and  tends  to  increase  the  amount  of  plant- 
food  carried  in  the  soil  solution.  Carbonation  is  a  direct 
result  of  its  presence.  Carbon  dioxide  may  also  tend  to 
flocculate  colloidal  matter  in  soils,  and  thus  benefit  the 
physical  conditions.  With  increased  organic  matter  in 
any  soil,  there  greater  bacterial  action  and  an  increase 
in  the  carbon  dioxide  evolved  may  well  be  expected.     In 


THE  ORGANIC  MATTER   OF  THE  SOIL 


139 


fact,  the  carbon  dioxide  production  of  a  soil  is  considered 
by  some  authors  *  to  be  a  measure  of  bacterial  activity. 
With  this  increase  in  carbon  dioxide  the  soil  air  becomes 
more  heavily  charged  and  an  alteration  in  bacterial  and 
plant  relationships  may  thereby  be  induced.  The  fol- 
lowing figures,  by  Wollny,2  show  the  composition  of  the 
soil  atmosphere  and  the  effects  of  additional  humous 
material  on  the  carbon  dioxide  content :  — 


Percentage  by 
Volume  of 


Soil  air  (average  of  19  analyses) 

Atmospheric  air 

A  sandy  soil        

A  sandy  soil  plus  manure 


18.33 
20.96 
19.72 
10.35 


While  carbon  dioxide  may  be  evolved  by  the  splitting-up 
of  both  carbohydrate  and  nitrogenous  bodies,  ammonia 
results  only  from  the  latter.  It  is  really  the  first  ex- 
tremely simple  nitrogenous  body  produced.  It  can  be 
utilized  by  some  plants  as  a  source  of  nitrogen,  as  is  also 
true  with  certain  simple  humic  bodies,  but  ordinarily 
it  must  undergo  oxidation.  This  oxidation  results  in 
nitrites  (N02)  and  ultimately  in  nitrates  (N03),  the 
latter  being  usually  considered  as  the  chief  source  of 
the  nitrogen  utilized  by  plants. 


1  Stoklasa,  J.,  and  Ernest,  A.  Ueber  den  Ursprung,  die 
Menge,  und  die  Bedeutung  des  Kohlendioxyds  im  Boden. 
Centrlb.  Bakt.,  II,  14,  Seite  723-736.     1905. 

2  Wollny,  E.  Die  Zersetzung  der  Organischen  Stoffe, 
Seite  2.     Heidelberg,  1897. 


140       SOILS:    PROPERTIES  AND  MANAGEMENT 

Other  end  products,  such  as  methane  (CH4),  hydrogen 
disulfide  (H2S),  free  nitrogen  (N),  sulfur  dioxide  (SO,), 
carbon  disulfide  (CS2),  and  the  like,  may  also  result. 
They  are  relatively  unimportant,  however,  as  regards 
the  plant,  in  comparison  to  the  role  played  by  carbon 
dioxide,  ammonia,  the  nitrites,  and  the  nitrates.  The 
production  of  the  nitrates  from  ammonia  particularly 
is  very  clearly  correlated  with  good  soil  conditions,  es- 
pecially optimum  moisture  and  adequate  aeration.  The 
proper  handling  of  the  soil,  then,  not  only  will  tend  to 
eliminate  toxic  matter  and  prevent  its  further  formation, 
but  will  encourage  the  proper  decay  of  the  soil  humus  and 
the  production  of  end  products  which  will  function  directly 
or  indirectly  as  plant  foods. 

Snyder  l  found  that  when  humus  was  extracted  with  an 
alkali  and  then  precipitated  with  an  acid,  it  yielded  from 
five  to  twenty-five  per  cent  of  a  reddish  brown  ash.  This 
ash  contained  silica,  iron,  and  alumina,  as  well  as  mag- 
nesia, potash,  phosphorus,  sulfur,  sodium,  and  calcium. 
While  part  of  these  mineral  constituents  may  be  chemically 
combined  with  humus,  it  is  probable  that  some  may  be 
present  because  of  the  adsorptive  capacity  of  the  organic 
colloids  which  are  always  present  in  humus  generated 
under  normal  conditions.  Snyder  has  estimated  that 
in  an  ordinary  soil  containing  a  fair  amount  of  organic 
matter,  one-sixth  of  the  phosphorus  and  one-twelfth  of 
the  potash  may  be  present  in  such  a  state.  They  are 
then  fairly  available,  and  are  yielded  much  more  readily 
to  the  plant  than  if  of  a  strictly  inorganic  nature. 

95.  Carbonized  materials  of  soil.  —  After  the  extrac- 
tion of  the  soil  for  the  study  of  the  ordinary  humus  com- 

1  Snyder,  Harry.  Production  of  Humus  from  Manures. 
Minnesota  Agr.  Ekp.  Sta.,  Bui.  53,  pp.  29-30.     1897. 


THE  ORGANIC  MATTER   OF  THE  SOIL  141 

pounds,  a  considerable  mass  of  material  remains,  which 
is  insoluble  in  water,  alkali,  and  other  ordinary  solvents. 
By  the  extraction  of  a  large  amount  of  soil,  Schreiner  1 
was  able  to  study  this  material.  He  found  it  susceptible 
to  division  into  six  groups,'  as  follows:  (1)  plant  tissue, 
(2)  insect  and  other  organized  material,  (3)  charcoal 
particles,  (4)  lignite,  (5)  coal  particles,  and  (6)  materials 
resembling  natural  hydrocarbons,  as  bitumen,  asphalt,  and" 
the  like.  Such  material  was  found  not  only  near  the  sur- 
face of  the  soil,  but  at  depths  of  fifteen  or  twenty  feet  be- 
low. All  the  groups  above  listec?  were  found  by  Schreiner 
to  be  represented  in  the  thirty-four  soils  collected  from 
all  parts  of  the  United  States  and  subjected  to  rigid  test. 

The  exact  origin  of  such  material  is  problematical.. 
Forest  and  prairie  fires,  infiltration,  mild  oxidation,  and 
lignification  might  be  mentioned.  Of  a  certainty,  the 
agencies  of  distribution  are  the  natural  forces  engaged 
in  physical  weathering.  This  carbonized  material  is 
important,  as  it  makes  up  no  inconsiderable  part  of  the 
soil  humus.  It  is  very  resistant,  and  consequently  lends 
stability  to  the  soil  organic  matter.  It  can  be  divided 
into  two  general  groups,  organized  and  unorganized; 
in  the  former  the  original  structure  remains  intact,  while 
in  the  latter  the  original  features  have  been  obliterated. 
The  study  of  such  material  and  the  changes  that  it  under- 
goes not  only  increases  the  list  of  known  organic  com- 
pounds existing  in  the  soil,  but  throws  considerable  light 
on  the  nature  of  the  soil  organic  matter  as  a  whole. 

96.  The  estimation  of  the  soil  organic  matter.  — 
Many  methods  have  been  proposed  for  the  determination 

1  Schreiner,  O.,  and  Brown,  B.  E.  Occurrence  and  Nature 
of  Carbonized  Material  in  Soils.  U.  S.  D.  A.,  Bur.  Soils, 
Bui.  90.     1912. 


142       SOILS:    PROPERTIES   AND  MANAGEMENT 

of  the  organic  matter  in  soils,  but  none  have  proved 
entirely  satisfactory,  since  the  composition  of  this  ma- 
terial is  so  complicated  and  so  likely  to  change  while 
under  investigation.  Other  soil  constituents  also  tend 
to  interfere  with  the  determination.  Two  general  methods 
seem  worthy  of  mention,  as  they  have  been  used  very 
widely  in  soil  analyses  and  at  least  give  comparative,  if 
not  absolutely  accurate,  results. 

Loss  on  ignition.1  —  This  is  a  simple  method  which 
designs  to  burn  off  the  organic  matter  and  determine 
its  loss  by  difference.  Five  grams  of  dry  soil  are 
placed  in  a  platinum  dish  and  ignited  at  a  low  red 
heat  until  the  organic  matter  is  all  oxidized.  The 
cold  mass  is  moistened  with  ammonium  carbonate 
and  heated  to  a  temperature  of  150°  C.  in  order  to 
expel  the  excess  of  ammonia.  The  loss  is  rated  as  or- 
ganic matter. 

This  method  is  open  to  the  objection  that,  besides  the 
loss  of  organic  matter,  a  certain  small  amount  of  water 
of  combination,  together  with  all  ammoniacal  compounds, 
nitrates,  all  carbon  dioxide,  and  some  alkali  chlorides  if 
the  temperature  is  carried  too  high,  is  driven  off.  The 
method  therefore  gives  high  results,  especially  in  the 
presence  of  large  amounts  of  hydrated  silicates.  An 
attempt  to  replace  the  carbon  dioxide  is  made  in  the 
treatment  of  the  cold  mass  wdth  ammonium  carbonate. 
Notwithstanding  these  objections,  this  method  is  one  of 
the  best  and  is  very  generally  used  all  over  the  world 
in  estimating  the  organic  matter  of  the  soil.     Very  often 

1  Houston,  H.  A.,  and  McBride,  F.  W.  A  Modification  of 
Grandeau's  Method  for  the  Determination  of  Humus.  U.  S. 
D.  A.,  Div.  Chem.,  Bui.  38  (edited  by  H.  W.  Wiley),  pp.  84-92. 
1893. 


THE  ORGANIC  MATTER    OF  THE  SOIL  143 

the  combustion  is  carried  on  in  a  current  of  oxygen  over 
hot  copper  oxide.  The  organic  carbon  may  thus  be 
determined  very  accurately,  and  the  organic  matter 
calculated  by  multiplying  the  carbon  found  by  the  factor 
1.724. 

Chromic  acid  method.1  —  This  method,  proposed  by 
Wolff,  has  been  modified  and  improved  by  various  chem- 
ists. Warington  and  Peake2  have  perhaps  done  more 
with  the  method  than  any  other  investigators.  In  the 
United  States  the  modification  <*f  Cameron  and  Brea- 
zeale 3  has  been  very  generally  accepted.  It  consists 
in  the  treatment  of  the  soil  sample  with  sulfuric  acid 
and  chromic  acid  or  potassium  bichromate.  The  organic 
matter,  in  the  presence  of  the  sulfuric  acid  and  an  oxidizing 
agent,  evolves  carbon  dioxide  until,  if  the  mixture  is 
boiled,  practically  all  of  the  carbon  is  thus  driven  off. 
This  gas  is  drawn  through  a  train  of  absorption  bulbs, 
caught  in  a  solution  of  potassium  hydrate,  and  thus 
weighed.  On  the  supposition  that  organic  matter  is 
58  per  cent  carbon,  it  is  very  easy  to  make  the  calcula- 
tion. The  carbon  found  may  be  multiplied  by  1.724,1 
or  the  carbon  dioxide  by  .471.  The  product  is  considered 
as  soil  organic  matter.  The  results  thus  obtained  are 
usually  lower  than  with  combustion  or  ignition  methods, 

1  For  comparison  of  methods,  see  Wiley,  H.  W.  Principles 
and  Practices  of  Agricultural  Analysis,  Vol.  I,  pp.  347-356. 
Easton,  Pa.     1906. 

2  Warington,  R.,  and  Peake,  W.  A.  On  the  Determina- 
tion of  Carbon  in  Soils.  Jour.  Chem.  Soc.  (London)  Trans., 
Vol.  37,  pp.  617-625.     1880. 

3  Briggs,  L.  J.,  and  others.  The  Centrifugal  Method  of 
Mechanical  Soil  Analysis.  U.  S.  D.  A.,  Bur.  Soils,  Bui.  24, 
pp.  33-38.  1904.  Also,  Cameron,  F.  K.,  and  Breazeale,  J.  F. 
The  Organic  Matter  in  Soils  and  Subsoils.  Jour.  Amer. 
Chem.  Soc,  Vol.  26,  pp.  29-45.     1904. 


144       SOILS:    PROPERTIES  AND  MANAGEMENT 

due  to  the  resistance  to  oxidation  1  by  the  carbonized 
matter,  already  discussed.  This  material,  while  it  suc- 
cumbs to  ignition,  resists  the  action  of  the  sulfuric  and 
chromic  acids  to  a  very  large  degree. 

97.  The  estimation  of  soil  humus.  —  The  common 
method  of  humus  estimation  is  that  proposed  by  Grand- 
eau.2  The  sample  of  soil  is  first  washed  with  acid  in  order 
to  remove  all  bases.  It  is  next  treated  with  ammonia, 
which  will  then  dissolve  out  the  humous  materials.  By 
catching  this  percolate,  evaporating  it  to  dryness,  and 
weighing  it,  the  percentage  of  humus  may  be  calculated. 
The  dark  humous  extract  obtained  with  the  ammonia 
is  called  the  Matiere  Noire. 

This  method  has  undergone  several  modifications,3  of 
which  that  of  Ililgard 4  and  that  of  Houston  and  McBride 5 
seem  the  most  promising.  The  method  of  the  latter 
chemists  has  been  adopted  by  the  Association  of  Official 
Agricultural  Chemists  and  is  considered  as  the  official 
method.  In  the  procedure  an  attempt  is  made  to  keep 
the  concentration  of  the  ammonia  in  contact  with  the 
soil  constant  during  the  extraction.  Consequently  the 
sample,  after  treatment  with  the  acid,  is  washed  into  a 

1  Schreiner,  O.,  and  Brown,  B.  E.  Occurrence  and  Nature 
of  Carbonized  Material  in  Soils.  U.  S.  D.  A.,  Bur.  Soils, 
Bui.  90,  pp.  19-21.     1912. 

2  Grandeau,  L.  Traiti  d'Analyse  de  Matieres  agricoles.  I, 
p.  151.     1897. 

3  A  comparison  of  the  various  methods  is  found  as  follows : 
Alway,  F.  J.,  and  others.  The  Determination  of  Humus. 
Nebr.  Agr.  Exp.  Sta.,  Bui.  115.     June,  1910. 

4  Hilgard,  E.  W.  Humus  Determination  in  Soils.  U.  S. 
D.  A.,  Div.  Chem.,  Bui.  38  (edited  by  H.  W.  Wiley),  p.  80. 
1893. 

5  Wiley,  H.  W.  Official  and  Provisional  Methods  of  Analy- 
sis.    U.  S.  D.  A.,  Bur.  Chem.,  Bui.  107,  p.  19.     1908. 


THE  ORGANIC  MATTER   OF  THE  SOIL  145 

500  cubic  centimeter  flask,  which"  is  filled  to  the  mark 
with  4  per  cent  ammonia.  Digestion  is  allowed  to  pro- 
ceed for  twenty-four  hours,  with  frequent  shakings,  and 
and  an  aliquot  portion  of  the  supernatent  liquid  is  taken 
for  analysis.  This  method  with  its  modifications  is 
practically  the  only  one  that  we  have  for  the  estimation 
of  soil  humus.  It  is  based  on  the  fact  that  when  a 
soil  is  lacking  in  active  basic  material,  the  humous 
matter  may  be  extracted  with  ammonia.  A  modification 
of  this  method  may  be  used  as  a  test  for  soil  acidity, 
as  any  soil  of  humid  regions  alldwing  the  extraction  of 
humus  by  ammonia  alone  must  lack  basic  materials. 

The  composition  of  the  ash  constituents  of  the  matiere 
noire  is  given  by  Snyder l  as  follows,  the  data  being  the 
average  of  eight  analyses :  — 

The  Ash  from  the  Humus  of  Minnesota  Prairie  Soils 

Percentage 

Insoluble 61.97 

Fe203        3.12 

A1203 3.48 

K20 7.50 

Na20 8.13 

CaO 09 

MgO 36 

P205     .     .     .     . 12.37 

S03 98 

C02 1.64 

The  relatively  high  percentage  of  phosphoric  acid  is 
immediately  noticeable  in  this  analysis.     This  indicates 

1  Snvder,  Harry.  Soils.  Minnesota  Agr.  Exp.  Sta.,  Bui.  41, 
p.  30.  "  1895. 

L 


146       SOILS:    PROPERTIES  AND  MANAGEMENT 

that  no  mean  portion  of  the  soil  phosphates  is  held  in 
organic  combination.  The  promotion  of  favorable  hu- 
mous decay  must  thus  liberate  a  considerable  amount  of 
phosphorus  for  plant  utilization. 

98.  Organic  content  of  representative  soils.  —  The 
organic  content  of  soils  varies  widely  according  to  climatic 
conditions.  The  following  average  data  show  the  limits 
of  variation  as  well  as  the  comparative  content  of  the 
important  soil  sections  of  the  United  States :  — 

Organic  Content  of  United  States  Soils 


Sandy  Soils 

Clay  Loams  and  Loams 

Soil 

Subsoil 

Soil 

Subsoil 

North  Central  States     . 
Northeastern  States 
South  Central  States     . 
Southeastern  States 
Semiarid  States    .     .     . 
Arid  States       .... 

1.84 

1.66 

1.16 

.93 

.99 

.89 

.76 
.60 
.55 
.41 
.62 
.64 

3.06 
3.73 
1.80 
1.53 
2.64 
1.05 

1.07 

1.35 

.65 

.73 

1.11 

.62 

It  is  at  once  apparent  that  the  subsoil  contains  con- 
siderably less  organic  matter  than  do  the  surface  layers. 
Also,  the  areas  of  the  United  States  that  have  been 
glaciated  are  noticeably  richer  in  organic  material  than 
the  residual,  coastal  plain,  and  arid  regions.  This  is 
largely  a  climatic  and  geochemical  relationship.  Some 
soils,  particularly  alluvial  soils,  very  often  run  higher 
than  the  average  data  given  above.  An  organic  content 
of  5  or  6  per  cent  is  not  an  uncommon  figure  with  such 
materials.  Muck  and  peat  soils  are  of  course  not  to  be 
classified  with  the  above,  as  their  organic  content  may 


3 


THE  ORGANIC  MATTER   OF  THE  SOIL 


147 


range  from  35  to  85  per  cent,  according  to  the  admixture 
of  mineral  matter  from  extraneous  sources. 

99.  The  humus  content  of  soils.  —  The  humus  content 
of  soils  is  of  course  lower  than  the  organic  matter  therein 
contained.  It  likewise  varies  according  to  climate  and 
region,  not  only  in  amount,  but  also  in  composition.  The 
following  data,  compiled  from  Hilgard,1  illustrate  this 
point :  — 

The  Humus  of  Arid  and  Humid  Soils 


41  Arid  uplands  soils     . 
15  Subirrigated  arid  soils 
24  Humid  soils     .     .     . 


Humus  in 
Soil 

(Percentage) 


.91 
1.06 

4.58 


Nitrogen  in 
Humus 

(Percentage) 


15.23 

8.38 
4.23 


Nitrogen  in 
Soil 

(Percentage) 


.135 
.099 
.166 


It  is  evident  that  humid  soils  not  only  contain  the, 
greatest  amounts  of  organic  matter,  but  also  excel  in 
humus.  The  humus  of  the  arid  regions,  however,  is 
richer  in  nitrogen,  due  to  the  peculiar  decomposition  going 
on.  As  a  consequence  the  nitrogen  in  the  soil  of  humid 
regions  is  not  greatly  in  excess  of  that  in  the  soils  of  drier 
climates. 

The  percentage  of  humus  not  only  decreases  as  the 
lower  depths  of  soil  are  examined,  but  also  changes  in 
composition,  becoming  poorer  in  nitrogen  the  deeper 
the  soil  is  penetrated.  The  following  data  on  a  Russian 
river  soil,  quoted  by  Hilgard,2  may  be  cited  as  an  in- 
stance :  — 


1  Hilgard,  E.  W.     Soils,  pp.  136-137.     New  York. 

2  Ibid.,  p.  139. 


1911. 


148       SOILS:    PROPERTIES   AND  MANAGEMENT 


The  Humus  of  a  Russian  Alluvial 

Soil 

Depth  in  Feet 

Percentage  or 
Humus 

Percentage  of 

Nitrogen  in 

Humus 

Percentage  op 

Humous  Nitrogen 

in  Soil 

1 

1.21 

5.30 

.064 

2 

1.16 

4.32 

.054 

3 

1.14 

3.87 

.044 

4 

1.17 

8.76 

.344 

5 

.74 

2.16 

.016 

6 

.60 

2.66 

.016 

7 

.47 

2.54 

.012 

8 

.78 

1.54 

.012 

9 

.54 

2.24 

.012 

'     10 

.52 

1.15 

.006 

11 

.53 

1.51 

.008 

12 

.44 

1.81 

.008 

Other  depth  relationships,  especially  regarding  the 
proportions  of  carbon,  humus,  and  nitrogen,  are  brought 
out  in  the  following  data,  obtained  by  Alway  and  Vail ! 
in  the  study  of  Nebraska  Soils :  — 

Composition    op   a   Nance   County,   Nebraska,    Soil   near 

Genoa 


Depth 

Percent- 
age op 
Nitrogen 

Percent- 
age op 
Carbon 

Percent- 
age op 
Humus 

Percent- 
age op 
Ash  IN 
Humus 

Ratio  op 

in  Feet 

C 

N 

H 

N 

c 

H 

1 

1 

3 
4 
5 
6 

.255 
.102 
.056 
.042 
.034 
.027 

2.61 
.85 
.31 
.24 
.17 
.14 

2.47 
1.00 
.40 
.30 
.19 
.16 

1.61 
.90 
.52 
.64 
.33 
.36 

10.2 
8.3 
5.5 
5.7 
5.0 
5.2 

9.7 
9.8 
7.1 
7.1 
5.6 
5.9 

1.0 
.9 

.8 
.8 
.9 
.9 

1  Alway,  F.  J.,  and  Vail,  C.  E.  The  Relative  Amounts 
of  Nitrogen,  Carbon,  and  Humus  in  Some  Nebraska  Soils. 
Nebraska  Agr.  Exp.  Sta.,  25th  Ann.  Rept.,  p.  155.     1912. 


THE  ORGANIC  MATTER    OF  THE  SOIL 


149 


100.  Influence  of  the  original  material  on  the  resultant 
humus.  —  It  is  evident  that  the  source  from  which  any 
humus  material  is  derived  will  exert  a  profound  influence 
on  its  composition,  especially  its  nitrogen  content. 
Snyder  *  has  investigated  this  by  mixing  certain  materials 
with  a  soil  poor  in  humus  and  allowing  the  process  of 
decay  to  proceed  for  a  year  under  favorable  conditions. 
At  the  end  of  the  period  the  humus  was  extracted  by  the 
Grandeau  method.     The  results  are  given  below  :  — 

The  Composition  of  Humus  produced  from    Various  Or- 
ganic Materials 


C 

H 

o 

N 

Sugar      

57.84 

3.04 

39.04 

.08 

Sawdust 

49.28 

3.33 

47.07 

.32 

Oats  straw 

54.30 

2.48 

40.72 

2.50 

Wheat  flour 

51.02 

3.82 

40.14 

5.02 

Cow  manure 

41.93 

6.26 

45.63 

6.16 

Green  clover 

54.22 

3.40 

34.14 

8.24 

Meat  scrap 

'48.77 

4.30 

35.97 

10.96 

Although  the  humification  may  not  have  reached 
completion  in  this  case,  the  great  variation  in  nitrogen 
is  striking.  Existing,  as  it  probably  does,  mostly  as 
acid  amides  and  monamino  acids,  it  will  change  readily 
to  ammonia  and  exert  a  marked  effect  on  plant  growth. 
Possibly  the  variation  of  the  nitrogen  in  soil  humus  is 
the  most  potent  factor  in  the  nutritive  functionings  of 
this  material.  The  variability  of  the  carbon,*  hydrogen, 
and  oxygen  of  the  soil  humus  is  not  such  an  important 
factor,  as  these  elements  can  easily  be  supplied  to  the 


1  Snyder,    Harry.     Production    of    Humus    from    Manures. 
Minnesota  Agr.  Exp.  Sta.,  Bui.  53,  p.  25.     1897. 


150       SOILS:    PROPERTIES  AND  MANAGEMENT 

soil  by  the  plowing-under  of  green  materials  or  of  barn- 
yard manures.  In  general,  the  percentage  content  of 
inorganic  matter  increases  as  the  organic  matter  decays. 

101.  Effects  of  organic  matter  on  soil.  —  The  effects 
of  the  organic  matter  on  soil  and  plant  conditions  are  as 
numerous  as  they  are  complex.  Some  of  the  influences 
are  direct,  others  are  indirect.  As  the  specific  gravity  of 
organic  matter  is  low,  the  first  effect  of  its  addition  would 
be  to  lower  the  absolute  and  the  apparent  specific  gravity 
of  the  soil.  As  the  water  capacity  of  humus  is  very  high, 
a  soil  rich  in  organic  constituents  usually  possesses  a 
high  water-holding  power.  This  makes  possible  greater 
volume  changes  both  on  drying  and  in  the  presence  of 
excessive  moisture.  The  granulating  effects  of  wetting 
and  drying  and  freezing  and  thawing  are  therefore  ac- 
celerated. The  organic  matter  tends  also  to  spread 
the  individual  particles  of  soil  farther  apart,  especially 
in  a  clay.  Its  loosening  effects  are  immediately  ap- 
parent in  such  soil.  On  the  other  hand,  because  organic 
matter  has  a  higher  cohesive  and  adhesive  power  than 
sand,  it  performs  the  function  of  a  binding  material 
with  the  latter  soil,  a  condition  much  to  be  desired  in  a 
material  possessing  such  textural  characteristics. 

The  better  tilth  induced  by  the  presence  of  organic 
matter  in  any  soil  tends  to  facilitate  ease  in  drainage 
and  to  encourage  good  aeration.  These  two  conditions 
are  of  course  necessary  for  the  promotion  of  soil  sanita- 
tion. Root  extension  and  bacterial  activity  are  thus 
increased.  It  is  of  especial  importance  that  the  splitting- 
up  of  the  organic  matter  shall  take  place  in  the  presence 
of  plenty  of  oxygen,  in  order  that  toxic  compounds  may 
not  be  generated  and  that  a  humus  highly  favorable  to 
plant  growth  shall  be  produced.     The  increased  water 


THE  ORGANIC  MATTER   OF  THE  SOIL  151 

capacity  of  the  soil  resulting  from  the  presence  of  organic 
materials  is  of  some  importance  in  drought  resistance, 
while  the  black  color  imparted  by  the  humus  tends  to 
raise  the  absorptive  power  of  the  soil  for  heat. 

The  soil  organic  matter,  however,  functions  in  other 
ways  than  those  strictly  physical.  The  humus  or  its 
degradation  products  may  serve  as  plant-food.  Bacteria 
and  other  soil  organisms  are  also  furnished  a  source 
of  energy  thereby,  and  the  production  of  carbon  dioxide 
is  much  increased.  This  carbon  dioxide,  as  well  as  the 
organic  acids  generated,  tends  to  raise  the  capacity  of 
the  soil  water  as  a  solvent  agent,  and  thus  the  amount  of 
mineral  plant  food  available  to  the  crop  is  greatly  in- 
creased. The  general  effect  of  organic  matter,  then,  is 
to  better  the  soil  as  a  foothold  for  plants,  and  to  increase 
either  directly  or  indirectly  the  available  food  supply  for 
the  crop. 

102.  Maintenance  of  soil  organic  matter.  —  The  main- 
tenance of  a  proper  supply  of  organic  matter  in  a  soil  is 
a  question  of  great  practical  importance,  as  productivity 
is  governed  very  largely  by  the  humus  content  of  the  soil. 
This  maintenance  of  the  soil  humus  depends  on  two 
factors  —  the  source  of  supply  and  methods  of  addition, 
and  the  promotion  of  proper  soil  conditions  in  order  that 
the  organic  matter  may  perform  its  legitimate  functions. 

The  organic  matter  of  the  soil  may  be  increased  in  a 
natural  way  by. the  plowing-under  of  green  crops.  This 
is  called  green-manuring  and  is  a  very  satisfactory  prac- 
tice. Such  crops  as  rye,  buckwheat,  clover,  peas,  beans, 
and  vetch  lend  themselves  to  this  method  of  soil  im- 
provement. Not  only  do  these  crops  increase  the  actual 
carbohydrate  content  of  a  soil,  but  in  the  case  of  legumes 
the  nitrogen   also  is  increased   in   amount,   due  to  the 


152       SOILS:    PROPERTIES  AND  MANAGEMENT 

symbiotic  action  of  the  nodule  bacteria.  Green-manure 
crops  may  also  protect  the  soil  from  loss  of  plant-food  by 
leaching.  The  addition  of  barnyard  manure  is  a  common 
method  of  raising  the  organic  content  from  external 
sources,  and  on  decaying  this  manure  performs  the  same 
function  as  natural  soil  humus.  Muck,  peat,  straw,  or 
leaves  may  be  used  in  a  similar  manner. 

Improper  soil  conditions  not  only  prevent  the  proper 
decay  of  organic  matter,  but  also  tend  to  encourage  the 
production  of  products  inimical  to  plant  growth.  There- 
fore, in  order  that  organic  materials  added  to  any  soil 
may  produce  the  proper  humous  constituents  and  per- 
form their  normal  functions,  soil  conditions  in  general 
must  be  of  the  best.  Tile  drainage  should  be  installed, 
if  necessary,  in  order  to  promote  aeration  and  granulation. 
Lime  should  be  added  if  basic  materials  are  lacking,  for 
it  promotes  bacterial  activity  as  well  as  plant  growth. 
The  addition  of  fertilizers  will  often  be  a  benefit,  as  will 
also  the  establishment  of  a  suitable  rotation.  The  rota- 
tion of  crops  not  only  prevents  the  accumulation  of 
toxic  materials,  but  also,  by  increasing  crop  growth, 
makes  possible  a  larger  addition  of  organic  matter  by 
green-manuring. 

Good  soil  management  seeks  to  adjust  the  addition 
of  organic  matter,  the  soil  conditions,  and  the  losses 
through  cropping  and  leaching,  in  such  a  way  that  paying 
crops  may  be  harvested  without  impairing  the  humus 
supply  of  the  soil.  Any  system  of  agriculture  that  tends 
to  permanently  lower  the  organic  matter  of  the  soil  is 
impractical,  and  improvident,  as  well  as  unscientific. 


CHAPTER  IX 
THE  COLLOIDAL  MATTER  OF  SOILS1 

It  has  already  been  noted,  in  the  discussion  of  the  clay 
separate  of  any  soil,  that  in  the  presence  of  water 
certain  of  the  very  finest  particles,  even  though  apparently 
dissolved,  assume  particular  ^ancLimportant  properties, 
such  as  high  adsorption,  nondiffusion  through  mem- 
branes, and  the  Brownian  movement.  Such  material 
has  been  designated  as  colloidal  in  nature.  It  must 
be    understood    from    the    beginning,     however,     that 

1  Some  of  the  following  general  references  may  prove  of  in- 
terest :  — 

Freundlich,  H.     Kapillarchemie.     Leipzig,  1909. 

Zsigmondy,  R.     Kolloidchemie.     Leipzig,  1912. 

Bancroft,  W.  D.  The  Theory  of  Colloid  Chemistry. 
Jour.  Phys.  Chem.,  Vol.  18,  No.  7,  pp.  549-558.     1914. 

Niklas,  H.  Die  Kolloidchemie  und  ihre  Bedeutung  fiir 
Bodenkunde,  Geologie,  und  Mineralogie.  Internat.  Mitt, 
fur  Bodenkunde,  Band  II,  Heft  5,  Seite  383-403.     1913. 

Ramann,  E.  Kolloidstudien  bei  Bodenkundlichen  Arbei- 
ten.  Kolloidchemische  Beihefte,  Band  II,  Heft  8/9,  Seite  285- 
303.     1911. 

Konig,  J.,  Hasenbaumer,  J.,  und  Hassler,  C.  Bestimmung 
der  Kolloide  im  Ackerboden.  Landw.  Ver.  Stat.,  Band  76, 
Heft  5/6,  Seite  377-441.     1912. 

Noyes,  A.  A.  The  Preparation  and  Properties  of  Colloidal 
Mixtures.     Jour.  Amer.  Chem.  Soc,  Vol.  27,  pp.  85-104.  *  1905. 

Thaer,  W.  Der  Einfluss  von  Kalk  und  Humus  auf  die 
Mechanische  und  Physikalische  Beschaffenheit  von  Ton-, 
Lehm-,  und  Sandboden.  Jour.  f.  Landw.,  Band  59,  Heft  1. 
Seite  9-57.  1911.  Also,  Kolloidchemische  Studien.  Jour, 
f.  Landw.,  Band  60,  Heft  1,  Seite  1-18.     1912. 

153 


154       SOILS:    PROPERTIES   AND  MANAGEMENT 

colloidal  material  does  not  differ  from  crystalloidal 
in  chemical  composition,  but  the  distinction  is  merely 
one  of  size  of  particles.  For  example,  if  large  particles 
are  suspended  in  water,  they  will  immediately  sink, 
since  their  weight  is  so  much  greater  than  the  sur- 
face that  is  exposed  for  buoyance.  When  these  particles 
are  decreased  in  size,  their  weight  decreases  much  faster 
than  the  surface  exposed.  It  is  therefore  evident  that 
a  point  will  at  last  be  reached  at  which  the  particles, 
because  of  their  minute  size,  will  form  a  homogeneous 
solution  The  upper  limit  of  the  colloidal  state  has  then 
been  entered. 

103.  The  colloidal  state.  —  The  colloidal  state  in  which 
these  particles  are  now  found  is  a  peculiar  one,  and  ex- 
hibits much  diversity,  not  only  in  properties,  but  also 
in  the  size  of  particle  in  which  the  material  exists.  The 
upper  limit  of  the  clay  group  as  designated  by  the  classi- 
fication of  the  United  States  Bureau  of  Soils  is  .005  milli- 
meter, while  the  upper  limit  of  the  particle  existing  in  a 
colloidal  state  is  estimated  to  be  below  .005  of  a  micron, 
or  .000005  millimeter.  Indeed,  so  small  are  the  colloidal 
particles  that  they  become  molecular  complexes,  that  is, 
a  few  molecules  may  go  to  make  up  a  particle. 
The  various  colloids,  or  the  same  colloid  under  different 
conditions,  may  exhibit  greatly  differing  sizes  of  particles. 
Some  colloidal  particles  are  very  large,  approaching  the 
upper  limit  already  set  for  material  in  such  a  state.  Other 
particles  are  finer.  It  is  evident  that  a  gradation  must 
exist  until  a  particle  is  reached  which  consists  of  only  one 
molecule.  The  solution  then  ceases  to  be  a  molecular 
complex  and  becomes  a  true  solution.  The  colloidal 
state  thus  grades  into  the  true  solution,  just  as  an 
ordinary    suspension    grades    into    a    true,    or    colloidal, 


THE   COLLOIDAL   MATTER    OF  SOILS  155 

suspension.  While  this  method  of  comparison  fails  to 
recognize  the  various  phases  that  colloidal  materials 
may  exhibit  and  is  therefore  faulty  in  this  regard,  it 
does  lay  emphasis  on  the  differences  as  to  size  of  par- 
ticle that  exist  between  colloidal  bodies  and  materials 
as  they  are  ordinarily  recognized.  ^This  relationship 
is  shown  by  the  following  diagram :  — 

ORDINARY  SUSPENSION   |    COLLOIDAL  STATE    I    TRUE  SOLUTION 

MOLECULAR   COMPLEX 

Fig.  22.  —  Diagram    showing    the    relationship    of    the   colloidal    state 
(molecular  complex)  to  ordinary  suspensions  and  true  solution^. 

Since  colloidal  particles  vary  in  size  from  .005  of  a 
micron  to  a  molecule,  the  range  must  be  very  great. 
Just  how  great  cannot  be  very  accurately  stated.  It 
is  interesting  to  note,  however,  that  this  range  is  much 
greater  in  proportion  than  is  exhibited  between  the  fine 
gravel  and  the  ordinary  clay  particles  found  in  soil.  With 
this  possible  difference,  it  is  no  great  wonder  that  the 
various  colloids  exhibit  with  different  intensities  the 
characteristics  so  peculiar  to  them  and  of  such  great  im- 
portance in  everyday  life.  The  particles  in  the  upper 
range  of  the  colloidal  field  can  be  seen  with  the  ordinary 
microscope.  As  such  particles  become  smaller  they  cease 
to  be  visible  under  the  ordinary  microscope  and  can  be 
detected  only  by  the  ultramicroscope.  It  is  probably 
true  that  by  far  the  greater  proportion  of  the  particles 
of  material  in  a  colloidal  state  cannot  be  detected  by 
microscopic  means.  This  gradation  of  colloidal  materials 
and  the  extreme  fineness  of  the  particles  is  well  illus- 
trated by  the  following  diagram  (Fig.  23),  although  it 
fails  to  convey  any  idea  regarding  the  various  phases 
that  colloids  may  occupy. 


156       SOILS:    PROPERTIES  AND  MANAGEMENT 


*SU<5PE/yS/Ort3 


ySOt-UT/Ort$ 


f%%VER.SIBLC 


■/0./ZC  VE/Z  -SfBt-C  ■ 


Fig.  23.  —  Diagram  showing  the  possible  range  in  the  size  of  colloidal 

particles. 

104.  The  properties  of  colloids.  —  In  general  there  are 
certain  properties  which  materials  in  a  colloidal  state 
exhibit  and  by  which  they  are  distinguished  from  true 
solutions.  In  the  first  place,  since  they  are  not  in 
true  solution  they  exert  little  effect  on  the  freezing 
point,  on  vapor  tension,  and  on  vapor  pressure.  Some 
colloids  have  absolutely  no  effect  on  these  conditions, 
while  others,  as  they  allow  a  certain  small  amount 
of    true     solution    to    take    place,     do     possess     such 


THE   COLLOIDAL   MATTER   OF  SOILS  157 

influences  to  a  slight  degree.  Secondly,  colloids  do  not 
pass  readily  through  semipermeable  membranes,  as 
parchment  paper,  while  crystalloids  do.  This  serves  as 
a  very  easy  way  of  separating  colloidal  and  crystalloidal 
material.  As  a  matter  of  fact,  the  membrane  is  itself  a 
colloid.  Thirdly,  heat  and  the  addition  of  electrolytes 
will  serve  to  coagulate  or  precipitate  certain  colloids,  a 
property  which  again  serves  to  distinguish  them  sharply 
from  a  true  solution.  Fourthly,  colloidal  material  has  great 
adsorptive  power,  not  only  for  water,  but  also  for  materials 
in  solution,  a  quality  of  extreme. importance  in  soil  studies. 
It  has  been  shown  that  a  colloid  is  a  material  in  a 
certain  state  of  division,  in  which  it  exhibits  properties 
not  possessed  by  an  ordinary  suspension  or  by  a  true 
solution.  It  is  therefore  proper  to  speak  of  matter  so  di- 
vided as  being  in  the  colloidal  state,  or  colloidal  condition. 
It  is  not  to  be  inferred,  because  the  colloidal  phase  is 
•  contrasted  with  the  crystalloidal,  that  colloids  are  amor- 
phous. They  may  or  may  not  be  in  such  a  condition. 
Moreover,  the  same  material  may  exist  without  chemical 
change  either  in  the  colloidal  or  non-colloidal  state.  For 
example,  silicic  acid,  ferric  hydrate,  gold,  carbon  black, 
and  other  materials,  may  or  may  not  be  colloidal,  ac- 
cording to  circumstances.  The  fineness  of  division  is 
the  explanation  of  colloidal  properties.  In  order  to 
place  such  a  discussion  on  a  more  understandable  basis, 
a  few  illustrations  of  the  colloidal  state  will  not  be  amiss. 
The  following  materials,  which  may  exist  as  colloids,  may 
be  for  convenience  grouped  under  two  general  heads, 
organic  and  inorganic :  — 

Organic:     Gelatin,  agar,  caramel,  albumin,  starch,  jelly, 
humus,  carbon  black,  tannic  acid,  etc. 


158       SOILS:    PROPERTIES  AND  MANAGEMENT 

Inorganic:  Gold,  silver,  iron,  ferric  hydrate,  arsenious 
oxide,  zinc  oxide,  silver  iodide,  Prussian 
blue,  etc. 

105.  Colloidal  phases.  —  In  general,  two  conditions 
are  necessary  for  the  colloidal  state  —  a  dispersive  medium, 
and  a  material  that  will  disperse,  the  latter  being  usually 
designated  as  the  disperse  phase.  Three  materials  may 
function  as  a  dispersive  medium  —  a  liquid,  a  solid,  or  a 
gas.  In  the  same  way,  witli  each  dispersive  medium  there 
may  be  three  disperse  phases  —  a  liquid,  a  solid,  or  a  gas. 
This  gives  nine  general  phases  to  be  considered  in  colloidal 
chemistry.  From  the  soil  standpoint,  the  liquid-solid 
and  the  liquid-liquid  phases  are  by  far  the  most  important 
and  will  be  the  only  ones  to  receive  detailed  attention 
here.  In  the  liquid-solid  phase,  as  with  colloidal  gold 
or  ferric  hydrate,  the  particles  are  suspended  in  water 
as  the  dispersive  medium.  In  the  case  of  gelatin,  another, 
liquid-solid  example,  the  jelly  surrounds  the  dispersive 
medium,  or  liquid.  An  emulsion  may  exhibit  the  liquid- 
liquid  phase,  and  possibly  exists  in  soils  rich  in  humus. 

In  these  colloidal  phases  under  discussion  and  of  such 
particular  interest  in  soil  study,  two  general  classes  of 
materials  are  found,  which  seem  to  differ  radically  from 
each  other  and  yet  are  likely  to  lead  to  considerable  con- 
fusion unless  special  pains  are  taken  to  distinguish  between 
them.  As  types,  gelatin  and  a  colloidal  suspension  of 
ferric  hydrate  may  be  cited.  The  gelatin  is  considerably 
more  viscous  than  water,  while  the  ferric  hydrate  does 
not  differ  from  water  in  this  respect.  The  former  gelatin- 
izes on  cooling  or  on  loss  of  moisture,  but  will  become 
dispersed  again  on  the  addition  or  presence  of  water.  In 
other  words,  it  will  pass  again  and  again,  back  and  forth, 


THE  COLLOIDAL   MATTER    OF  SOILS  159 

from  a  sol  to  a  gel.  It  is  what  might  be  called  a  reversible 
colloid.  Moreover,  it  is  not  coagulated  by  ordinary  addi- 
tions of  salt  or  by  heating.  The  ferric  hydrate  colloid, 
on  the  other  hand,  when  precipitated  or  agglutinated  by 
any  means  may  not  easily  be  brought  back  again  to  the 
sol  state.  It  is  a  so-called  irreversible  colloid.  Moreover, 
it  is  thrown  down  by  the  addition  of  electrolytes.  There 
exist,  then,  the  viscous,  gelatinizing,  reversible  colloids,  and 
the  non-viscous,  non-gelatinizing,  easily  coagulable,  and 
non-reversible  colloids,  besides  all  gradations  and  variations 
between  the  two.  In  the  ordinary  clay  soil,  both  types 
of  these  materials  probably  exist  and  play  important  parts 
in  the  physical  and  chemical  characteristics  exhibited. 

106.  Flocculation.  —  While  the  gelatinous  colloids  of 
the  soil,  such  as  some  of  the  humic  materials,  are  not 
agglutinated  by  the  addition  of  electrolytes,  most  of  the 
colloids  of  a  nature  similar  to  colloidal  silicic  acid  and 
ferric  hydrate  are  thrown  down  by  this  treatment.  This 
phenomenon  is  often  spoken  of  as  flocculation.  A  very 
good  example  is  afforded  by  treating  a  clay  suspension 
with  a  little  caustic  lime.  The  tiny  particles  almost 
immediately  coalesce  into  floccules,  and,  because  of  their 
combined  weight,  sink  to  the  bottom  of  the  con- 
taining vessel,  leaving  the  supernatant  liquid  clear. 
The  same  action  will  take  place  in  the  soil  itself,  but  of 
course  with  less  rapidity  and  under  conditions  less  notice- 
able to  the  eye.  The  colloids  thus  thrown  down,  being 
largely  irreversible,  cannot  again  assume  their  former 
attributes  and  thus  lose  their  distinguishing  character- 
istics. In  general,  acids  bring  about  flocculation  while 
alkalies  do  not,  calcium  oxide  and  calcium  hydrate  being 
the  best-known  exceptions  to  the  latter.  Ammonia  is 
an  intense  deflocculator. 


160       SOILS:    PROPERTIES  AND  MANAGEMENT 

Just  how  this  phenomenon  of  flocculation  or  agglutina- 
tion may  be  accounted  for  theoretically  it  is  rather  difficult 
to  state.  The  general  theory  is  one  of  electrification.  It 
is  found  that  certain  colloids,  when  subjected  to  the 
proper  electric  current,  will  migrate  to  either  the  positive 
(anode)  or  the  negative  (cathode)  pole.  These  particles 
evidently  carry  a  charge  of  electricity.  Ferric  hydrate, 
aluminium  hydrate,  and  basic  dyes,  for"  example,  move 
toward  the  cathode  and  carry  a  positive  charge;  while 
arsenious  sulfide,  silicic  acid,  gold,  silver,  and  acid  dyes 
move  toward  the  anode  and  are  negative.  It  is  assumed 
that  as  long  as  the  colloidal  particles  remain  charged 
they  repel  each  other  and  the  colloidal  state  persists. 
When  an  electrolyte  is  added  the  ionization  is  supposed 
to  cause  a  discharge  of  the  repellent  electricity  carried 
by  the  colloidal  particles,  and  flocculation  or  agglutina- 
tion immediately  takes  place. 

Certain  colloids  may  flocculate  certain  others,  as  the 
gelatinization  of  silicic  acid  by  ferric  hydrate.  At  times 
one  colloid  may  protect  another,  probably  by  surrounding 
it  with  a  protective  film.  Such  a  case  may  be  shown  by 
adding  gelatin  to  a  clay  suspension.  When  a  colloid  such 
as  ferric  hydrate  is  flocculated,  it  loses  to  a  certain  extent 
its  peculiar  properties,  and  assumes  the  characteristics  of 
ordinary  materials.  It  is  evident,  therefore,  that  if  the 
properties  exhibited  by  colloidal  materials  become  either 
directly  or  indirectly  detrimental  to  plants,  their  floccula- 
tion would  be  beneficial.  In  field  practice  this  is  usually 
accomplished  by  the  addition  of  lime.  The  colloidal 
material  existing  in  a  normal  soil  and  possessing  a  gelat- 
inous nature,  similar  in  general  to  gelatin,  is  probably 
not  all  flocculated  by  the  addition  of  ordinary  amounts 
of    electrolytes.     This   material   may    be   influenced    by 


THE  COLLOIDAL   MATTER   OF  SOILS  161 

drying,  whereby  it  slowly  gives  off  water,  becomes  more 
and  more  viscous,  and  at  last  may  lose  its  gel  qualities 
and  become  hard  and  irreversible.  It  is  evident,  there- 
fore, that  wetting  and  drying,  frost,  and  the  like,  become 
factors  in  dealing  with  this  form  of  colloidal  matter. 

107.  Common  soil  colloids  and  thSir  generation.1  — 
The  common  soil  colloids  may,  for  convenience,  be  dis- 
cussed under  two  heads,  organic  and  inorganic.  Of  the 
former,  .the  so-called  humic  acid  stands  as  the  example; 
of  the  latter,  silicic  acid,  ferric  hydrate,  and  amorphous 
zeolitic  silicates  are  the  commonest. 

Organic  colloids.  —  The  humic  colloids  in  a  normal 
fertile  soil  probably  make  up  the  bulk  of  the  colloidal 
matter.  Such  material  is  very  heterogeneous,  very 
complex,  and  constantly  changing.  As  yet  very  little 
study  of  the  organic  soil  colloids  has  been  made  because 
of  the  difficulties  presented  by  the  problem.  Humic 
colloids  may  be  viscous  or  non-viscous,  as  the  case  may 
be,  and  may  or  may  not  be  thrown  down  by  lime.  The 
adsorptive  power  of  these  colloids  for  water,  gases,  and 
such  materials  as  calcium,  magnesium,   and  potash,   is 


1  Van  Bemmelen,  J.  M.  Die  Absorption.  Seite  114-115. 
Dresden,  1910.  Also,  Die  Absorptionsverbindungen  und  das 
Absorptionsvermogen  der  Ackererde.  Landw.  Ver.  Stat., 
Band  35,   Seite  69-136.  .   1888. 

Way,  J.  T.  On  Deposits  of  Soluble  or  Gelatinous  Silica 
in  the  Lower  Beds  of  the  Chalk  Formation.  Jour.  Chem.  Soc., 
Vol.  6,  pp.   102-106.     1854. 

Warington,  R.  On  the  Part  Taken  by  Oxide  of  Iron  and 
Alumina  in  the  Adsorptive  Action  of  Soils.  Jour.  Chem.  Soc, 
2d  ser.,  Vol.  6,  pp.  1-19.     1868. 

Cushman,  A.  S.  The  Colloid  Theory  of  Plasticity.  Trans. 
Amer.  Cer.  Soc,  Vol.  6,  pp.  65-78.     1904. 

Ashley,  H.  E.  The  Colloid  Matter  of  Clay  and  its  Meas- 
urements.    U.  S.  Geol.  Sur.,  Bui.  388.     1909. 


162       SOILS:    PROPERTIES  AND  MANAGEMENT 

very  highly  developed  —  more  so,  probably,  than  that 
of  the  inorganic  colloids.  These  organic  colloids  are 
formed  during  the  tearing-down  and  splitting-off  processes 
of  bacterial  activity.  Some  of  the  humic  materials  are 
thrown  off  in  a  sufficiently  fine  state  of  division  to  assume 
the  condition  that  has  been  designated  as  colloidal.  Of 
course  the  chemical  forces  of  weathering  are  also  operative 
in  this  process  of  organic  colloidal  production. 

Mineral  colloids.  —  The  inorganic  soil  colloids,  especially 
ferric  oxide  and  silicic  acid,  are  less  complex  than  the 
organic  and  have  been  more  thoroughly  studied.  Such 
colloids  are  generated  during  the  operation  of  the  ordinary 
forces  of  weathering,  especially  the  chemical  phase.  For 
example,  when  a  feldspar  undergoes  decomposition,  the 
following  reaction  may  be  used  to  illustrate  the  possible 
change  that  takes  place :  — - 

2  KAlSi308  +  2  H20  +  C02  =  H4Al2Si209  +  4  Si02  + 

K2C03 

Kaolin  practically  always  has  its  origin  in  this  way, 
together  with  an  alkali  carbonate  and  silica.  The  process 
is  essentially  one  of  hydration  and  carbonation ;  the  C02 
by  reacting  with  the  alkali  permits  the  process  to  go  on. 
The  silica  may  go  in  three  directions,  according  to  con- 
ditions —  to  free  quartz,  to  hydrated  silicates,  and  to 
colloidal  silica.  Similar  reactions  may  be  written  for 
iron  and  aluminium,  but  they  can  only  show,  as  does 
the  above,  the  general  trend  of  the  change.  In  general  it 
can  be  concluded  that  most  inorganic  colloids  arise  from 
ordinary  chemical  weathering,  together  with  secondary 
minerals  of  various  kinds.  Such  colloids  must  be  very 
dilute  and  are  difficult  to  study  because  of  their  reaction 
among  themselves. 


THE  COLLOIDAL   MATTER   OF  SOILS  163 

108.  Preparation  of  colloids.  —  There  are  a  number  of 
methods  that  may  be  used  in  the  preparation  of  artificial 
colloidal  solutions,  but  the  description  of  only  one  will 
suffice  in  the  present  discussion.  This  is  the  use  of  a 
semipermeable  membrane.  It  has  already  been  men- 
tioned that  crystalloids  pass  with  ease  through  a  membrane 
such  as  parchment  paper,  while  colloids  do  not.  It  is 
reasonable  to  expect,  then,  that  these  materials  may  be 
thus  separated  by  proper  adjustments.  As  a  matter  of 
fact,  such  a  procedure  is  employed  in  many  cases.  The 
operation  is  called  dialysis,  and  the  membrane,  itself  a 
colloid,  is  designated  as  the  dialyzing  membrane. 

For  example,  if  a  solution  of  ferric  chloride  to  which 
some  ammonium  carbonate  has  been  added  is  placed  in  a 
dialyzer  with  pure  water  on  the  outside,  the  hydrochloric 
acid  and  other  impurities  gradually  pass  through  the 
membrane  and  a  more  or  less  pure  colloidal  solution  of 
ferric  hydrate  is  left  behind.  The  objection  to  this 
method  lies  in  its  extreme  slowness.  Nevertheless,  since 
the  cells  of  plants  present  a  semipermeable  membrane, 
this  method  of  preparation  serves  to  explain  many  actions 
that  go  on  between  soil  and  plant  during  the  processes 
of  nutrition.  In  the  soil  the  formation  of  colloidal  ma- 
terial is  entirely  a  natural  exertion  of  chemical  and  bio- 
logical forces  under  such  conditions  that  the  particles 
split  off  are  in  that  state  of  division  which  has  been  desig- 
nated as  colloidal. 

It  may  be  inferred  that  the  quantity  of  colloidal  matter 
in  an  average  soil  is  large,  but  as  a  matter  of  fact  this  is 
not  the  case.  The  proportion  of  the  soil  in  a  colloidal 
state  at  any  one  time  is  very  small.  It  must  be  remem- 
bered, however,  that  material  is  continually  being  thrown 
out  of  the  colloidal  condition  and  at  the  same  time  more 


164      SOILS:    PROPERTIES  AND  MANAGEMENT 

is  generated;  thus  the  effects  may  be  marked,  although 
the  amount  present  at  any  one  time  is  extremely  minute. 

109.  Colloids  and  soil  properties.  —  As  may  naturally 
be  inferred,  the  influence  of  the  colloidal  matter  on  soil 
conditions,  especially  as  related  to  plants,  is  extremely 
important.  This  influence  is  exerted  in  two  ways.  First, 
on  cohesion  and  plasticity;  and,  secondly,  on  the  adsorp- 
tlVe  power  of  the  soil.  Both  these  qualities  must  be  con- 
sidered, not  only  in  the  physical,  but  also  in  the  chemical 
and  the  biological,  study  of  the  soil  as  a  medium  for  crop 
production. 

In  general  it  is  found  that,  other  conditions  being  equal, 
an  Jm^T^^lJ^lj^P^^^  matter  increases  plasticity ;  in 
other  words,  the  ease  with  which  a  soil  may  be  worked 
into  a  puddled  condition  becomes  greater.  This  is  a  rather 
undesirable  quality  when  too  pronounced,  and  in  clays 
in  which  it  is  most  likely  to  be  developed  some  means 
of  decreasing  the  colloidal  influence  is  advisable.  This 
great  plasticity  is  developed  because  the  colloids,  espe- 
cially those  of  a  gelatinous  or  viscous  nature,  facilitate 
the  ease  with  which  the  particles  may  move  over  one  an- 
other and  yet  cohere  sufficiently  to  prevent  disruption 
of  the  mass.  In  general,  also,  the  greater  the  plasticity 
of  a  soil,  the  greater  is  the  cohesion  when  dry.  In  soils, 
then,  in  which  the  colloidal  material  is  very  high,  clodding 
may  occur  if  the  soil  is  tilled  too  dry  because  of  the  great 
tendency  of  the  particles  to  cohere.  This  cohesion  and 
plasticity,  as  factors  in  soil  structure,  soil  granulation, 
and  tilth,  will  be  discussed  in  the  succeeding  chapter. 
It  is  sufficient  at  this  point  only  to  observe  the  relation- 
ship of  colloidal  materials  to  the  development  of  such 
qualities. 

The  second  important  attribute  imparted  to  soil  by 


THE  COLLOIDAL  MATTER   OF  SOILS  165 

colloid  development  is  high  adsorptive  power.  This 
power  extends  not  only  to  condensation  of  gases,  but  also 
to  water  and  to  materials  in  solution.  The  water  of 
condensation  on  dry  soil  particles  when  exposed  to  a 
saturated  atmosphere  is  largely  determined  by  the  col- 
loidal content.  In  other  words,  the  surface  exposure  of 
colloidal  matter  is  so  preponderant  in  water  condensation 
as  in  a  general  way  to  allow  the  one  to  be  a  relative  meas- 
ure of  the  other.  Again,  colloids  exert  adsorptive  power 
for  material  existing  in  the  soil  water,  and  to  a  limited 
extent  compete  with  the  plant  for  food.  Until  the  col- 
loids are  satisfied  the  soil  solution  may  not  reach  its 
maximum  concentration  for  crop  growth.  This  adsorp- 
tive power  is  exerted  especially  on  the  basic  materials, 
such  as  calcium,  and  unless  the  existing  colloids  are  fully 
satisfied  the  soil  tends  to  become  lacking  in  available 
bases.  This  condition  is  generally  termed  soil  acidity. 
It  may  readily  be  seen  that  the  concentration  of  the  soil 
solution  is  governed  to  a  considerable  extent  by  the  col- 
loidal content  of  the  soil,  and  that  the  adjustments  in 
concentration  are  always  toward  an  equilibrium  between 
the  two.  Colloidal  matter  does  not  exert  the  same  adsorp- 
tive power  for  all  materials,  but  is  capable  of  what  might 
be  called  selective  adsorption.  For  example,  if  ammonium 
sulfate  is  added  to  a  soil,  the  ammonia  is  strongly  taken 
up,  which  tends  to  release  the  sulfate.  The  continuous 
use  of  such  a  fertilizer  on  a  soil  poor  in  lime  will  ultimately 
result  in  the  presence  of  free  sulfuric  acid.  This  example 
is  sufficient  to  emphasize  the  relationship  of  adsorptive 
powers  to  fertilizer  practice. 

110.  Factors  affecting  colloids.  —  It  must  not  be  in- 
ferred from  the  preceding  discussion  that  the  generation 
of  colloids  is  detrimental  to  soil   conditions.     In  light 


166       SOILS:    PROPERTIES  AND   MANAGEMENT 

soils  the  presence  of  such  material  is  extremely  necessary, 
as  it  tends  to  bind  the  soil  together,  facilitates  granula- 
,tion,  and  prevents  loss  of  plant  food  by  leaching.  It  is 
only  in  heavy  soils  in  which  such  material  is  excessive 
that  a  detrimental  condition  is  likely  to  exist.  This  occurs 
because  of  a  high  cohesion  and  plasticity,  because  of  the 
competition  for  food  that  is  likely  to  arise  with  the  crop, 
and  because  of  the  tendencies  toward  acidity.  Where 
lime  is  low  or  lacking,  the  situation  has  a  tendency  to 
become  still  more  aggravated  by  further  colloidal  devel- 
opment. 

In  general,  the  practice  of  underdrainage  by  allowing 
the  wetting  and  drying  of  the  soil  to  proceed,  is  the  first 
step  not  only  for  the  curbing  of  excessive  and  improper 
colloidal  influence,  but  also  for  the  encouragement  of 
just  the  right  development  thereof.  The  freezing  of 
winter,  tillage  at  proper  times,  the  addition  of  humus, 
and  the  application  of  lime  are  all  practices  that  aid 
in  th^-cofitcQl  of  colloidal  conditions.  Since  this  control 
and  utilization  of  colloid3~lTrfktences  is  only  a  phase  of 
soil  structure  as  related  to  tilth  and  granulation,  a  further 
discussion  of  the  subject  will  be  reserved  for  later  consid- 
eration. 

111.  Estimation  of  colloidal  content.  —  The  colloids 
in  the  soil  are  so  complex,  so  numerous,  so  variable  in 
function,  and  so  susceptible  to  change,  that  an  exact 
determination  of  their  amount  is  impossible.  The  knowl- 
edge of  colloidal  material  in  general  is  so  meager  that  it 
is  not  surprising  that  such  slight  advances  have  been 
made  in  fully  and  clearly  determining  their  character  in 
a  complicated  material,  as  the  soil  undoubtedly  is.  The 
important  methods  of  estimating  the  colloid  content  of 
the  soil  depend  for  their  expression  on  the  intensity  of 


THE  COLLOIDAL   MATTER    OF  SOILS  167 

certain  qualities,  supposed  to  be  developed  largely  by 
colloid  content.  This  indicates  that  the  methods  are 
largely  comparative,  rather  than  exact  or  strictly  analyti- 
cal in  nature.  These  important  methods  l  are  three  in 
number:    Van  Bemmelen's,  Ashley's,  and  Mitscherlich's. 

Van  Bemmelen.  —  The  first  investigator  to  advance 
a  method  for  colloid  estimation  was  Van  Bemmelen,2 
who  considered  that  the  amount  of  silica  dissolved  from 
a  soil  by  digestion  with  hydrochloric  or  sulfuric  acids 
was  a  measure  of  its  colloidal  content.  It  is  now  known 
that  some  materials,  such  as  crushed  rock,  may  yield 
as  much  silica  with  this  treatment  as  a  highly  colloidal 
clay.  This  method  is  not  of  great  importance  at  the 
present  time,  except  as  to  the  information  that  it  gives 
regarding  the  evolution  of  colloidal  soil  study. 

Ashley.  —  A  second  method,  and  one  of  much  more 
value,  has  been  evolved  by  Ashley.3  He  found  that  the 
adsorption  of  certain  dyes  by  soils  afforded  a  very  good 
index  to  colloidal  content.  The  difficulty  in  this  method, 
however,  lies  in  choosing  the  most  effective  dye  and  regu- 
lating its  concentration.  Moreover,  different  colloids 
vary  so  much  in  adsorptive  capacity  for  the  same  dye, 
that  only  roughly  comparative  results  have  thus  far  been 
possible. 

Mitscherlich.  —  The  third,  and  as  yet  the  most  valu- 

1 A  comparison  of  these  methods  is  found  as  follows : 
Stremme,  H.,  and  Aarnio,  B.  Die  Bestimmung  des  Gehaltes 
anorganischer  Kolloide  in  Zersetzten  Gesteinen  und  deren 
tonigen  Unlagerungsprodukten.  Zeitsch.  f.  Prak.  Geol., 
Band  19,   Seite  329-349.     1911. 

2  Van  Bemmelen,  J.  M.  Die  Adsorptionsverbindungen 
und  das  Absorptionsvermogen  der  Ackererde.  Landw.  Ver. 
Stat.,  Band  35,  Seite  69-136.     1888. 

3  Ashley,  H.  E.  The  Colloid  Matter  of  Clay  and  Its 
Measurement.     U.  S.  Geol.  Sur.,  Bui.  388.     1909. 


168       SOILS:    PROPERTIES  AND  MANAGEMENT 

able,  mode  of  colloidal  estimation  is  that  of  Mitscherlich,1 
in  which  the  adsorptive  capacity  of  the  soil  is  again  made 
the  comparative  index.  Water  instead  of  dye  is  used  as 
the  adsorbed  material.  In  this  method  the  air-dry  soil 
in  a  thin  layer  is  brought  to  absolute  dryness  over  phos- 
phorus pentoxide.  It  is  then  placed  in  a  desiccator  over 
a  10  per  cent  solution  of  sulfuric  acid  and  the  condensa- 
tion is  hastened  by  a  partial  vacuum.  The  sulfuric  acid 
is  used  in  order  to  prevent  the  deposition  of  dew  on  the 
soil.  After  exposure  for  at  least  twenty -four  hours  the 
soils  are  found  to  have  taken  up  their  maximum  moisture 
of  condensation,  which  is  called  the  hygroscopic  water. 
The  soil  is  then  weighed,  and  the  increase,  figured  to  a 
percentage  basis,  is  taken  as  a  measure  of  colloidal  con- 
tent. The  reverse  process  may  also  be  followed,  by 
exposing  air  dry  soil  in  a  saturated  atmosphere  and 
afterwards  drying  over  phosphorus  pentoxide.  The 
hygroscopicity  of  the  soil,  or  its  hygroscopic  coefficient, 
is  thus  the  basis  for  colloidal  comparison.  It  is  now 
clear  why  the  term  colloidal  estimation  is  employed  in 
this  discussion,  rather  than  colloidal  determination. 

An  objection  to  the  Mitscherlich  method  is  advanced 
by  Ehrenfcerg  and  Pick,2  who  claim  that  the  drying  over 

1  Rodewald,  IT.,  und  Mitscherlich,  A.  E.  Die  Bestimmung 
der  Hygroskopizitat.  Landw.  Ver.  Stat.,  Band  59,  Seite 
433-441.  1903.  Also,  Mitscherlich,  E.  A.,  und  Floess,  R. 
Ein  Beitrage  zur  Bestimmung  der  Hygroskopizitat  und  zur  Bewer- 
tung  der  physikolischen  Bodenanalyse.  Internat.  Mitt.  f.  Boden- 
kunde,  Band  I,  Heft  5,  Seite  463-480.     1912. 

2  Ehrenberg,  P.,  und  Pick,  II.  Beitrage  zur  physikalischen 
Bodenuntersuchung.  Zeit  f.  Forst-  und  Jagdwesen,  Band 
43,  Seite  35-47.  1911.  Also,  Vageler,  P.  Die  Rodewald- 
Mitscherlichsche  Theorie  der  Hygroskopizitat  vom  Standpunkte 
der  Colloidchemie  und  ihr  Wert  zur  Beurteilung  der  B6den. 
Fuhling's  Landw.  Zeit.,  Band  61,  Heft  3,  Seite  73-83.     1912. 


THE  COLLOIDAL   MATTER   OF  SOILS  169 

phosphorus  pentoxide  will  coagulate  certain  colloids  and 
lower  their  adsorptive  power,  thus  causing  the  hygro- 
scopic coefficient  to  become  an  unreliable  comparative 
figure.  They  suggest  first  the  exposure  of  the  field 
soil  over  the  water  and  sulfuric  acid,  and  then  the  ex- 
traction of  the  hygroscopic  water  over  phosphorus  pent- 
oxide.  Since  this  modification  is  very  slow,  the  original 
Mitscherlich  method,  in  spite  of  its  faults,  remains  the 
most  valuable  up  to  the  present  time. 


CHAPTER  X 
SOIL  STRUCTURE 

While  texture  is  the  term  used  in  reference  to  the  size 
of  the  particles  in  a  soil  mass,  the_word  structure  is  em- 
ployed in  reference  to  the  arrangement  of  the  grains. 
The  structural  condition  of  the  soil  is  very  important  to 
plant  growth,  since  the  circulation  of  air  and  water  are 
so  necessary  to  normal  development.  The  structural 
condition  may  be  loose  or  compact,  hard  or  friable,  granu- 
lated or  non-granulated,  as  the  case  may  be.  Of  these 
conditions,  granulation,  especially  in  heavy  soils,  is  of 
vital  importance,  since  it  is  really  a  summation  of  all 
favorable  structural  conditions.  By  granulation  is  meant 
the  drawing  together  of  the  small  particles  around  a 
suitable  nucleus,  so  that  a  crumb  structure  is  produced. 
The  grains  thus  cease  to  function  singly.  The  impor- 
tance of  such  a  structural  condition  on  a  heavy  soil  is 
very  obvious.  The  soil  becomes  loose  because  of  the 
larger  units,  air  moves  more  freely,  and  water  not  only 
drains  away  readily  when  in  excess,  but  responds  with 
celerity  to  the  capillary  pull  of  the  plant  Before  the 
promotion  of  granulation  and  the  factors  that  function 
therein  may  be  clearly  discussed,  however,  two  properties 
of  particular  importance,  especially  in  soils  of  fine  tex- 
ture, must  be  considered.  These  properties  are  plasticity 
and  cohesion. 

112.  Plasticity.  —  Any  material  wThich  allows  a  change 
of  form  without  rupture,  and  which  will  retain  this  form 

170 


SOIL   STRUCTURE  171 

not  only  when  the  pressure  is  removed,  but  also  when  dry, 
is  said  to  be  plastic.  Putty  with  a  proper  admixture  of 
oil  is  a  very  good  example  of  a  plastic  body.  As  is  well 
known,  the  various  plastic  materials  differ  in  their  plastic- 
ity. Not  only  this,  but  such  substances  as  clay  or  some 
other  soils  vary  in  plasticity  with  their  moisture  content, 
their  granulation,  and  their  texture.  The  great  diffi- 
culty in  the  study  of  plasticity  has  been  in  finding  a 
means  of  estimation  allowing  an  exact  numerical  expres- 
sion. The  amount  of  hygroscopic  water  that  a  soil  will 
hold  has  been  used  as  an  expression  of  plastic  qualities, 
as  well  as  shrinkage  on  drying,  the  ability  to  adsorb  dyes, 
tensity,  and  other  characteristics.  None  of  these  has 
proved  satisfactory,  since  one  quality  of  a  clay  or  other 
soil  is  used  as  a  measure  of  another  quality. 

Atterberg  1  has  suggested  that  the  difference  in  mois- 
ture content  of  a  clay  at  the  point  at  which  it  ceases  to 
be  plastic,  as  compared  with  the  moisture  content  at 
which  it  becomes  viscous,  might  be  used  as  an  expres- 
sion of  plasticity.  He  has  called  this  figure  the  plasticity 
coefficient.  Thus,  a  soil  may  cease  to  be  plastic  at  20 
per  cent  of  moisture  and  may  flow  at  40  per  cent.  The 
plasticity  coefficient  would  then  be  20.  While  this  is  one 
of  the  latest  methods,  it  is  open  to  the  objection  already 
stated  —  that  one  quality  of  a  soil  is  used  as  a  measure 
of  another.  Two  soils  showing  the  same  plasticity  coeffi- 
cient by  this  method  may  exhibit  undoubted  differences 
in  actual  plasticity.  Kinnison,2  in  testing  several  methods 
of  expression,  found  Atterberg's  no  better  than  others 

1  Atterberg,  A.  Die  Plastizitat  der  Ton.  Internat. 
Mitt.  f.  Bodenkunde,  Band  I,  Heft  1,  Seite  10-43.     1911. 

2  Kinnison,  C.  S.  A  Study  of  the  Atterberg  Plasticity 
Method.     Trans.  Amer.  Cer.  Soc.,  Vol.  16,  pp.  472-484.     1914. 


172       SOILS:    PROPERTIES  AND  MANAGEMENT 

already  in  use.     For  all  practical  purposes  in  soil  dis- 
cussions, general  descriptive  terms  may  be  employed. 

113.  The  cause  of  plasticity.  —  Exactly  what  may  be 
the  cause  of  plasticity  has  long  been  under  discussion. 
The  various  theories  advanced  may  be  grouped  under  the 
following  heads : 1  — 

A.  Structure  of  clay  particles 

1.  Fineness  of  grains 

2.  Plate  structure 

3.  Interlocking  particles 

4.  Sponge  structure 

B.  Presence  of  hydrous  aluminium  silicates 

C.  Molecular  attraction  between  particles 

D.  Presence  of  colloidal  matter 

Of  these  theories  accounting  for  the  plasticity  of  cer- 
tain bodies,  that  of  colloid  content  seems  the  most  rea- 
sonable.2 The  presence  of  gelatinous  colloidal  matter, 
with  a  certain  optimum  amount  of  water,  seems  to  facili- 
tate the  ready  movement  of  the  particles  while  at  the 
same  time  exerting  sufficient  force  to  prevent  the  body 
from  splitting  apart  at  the  time  of  movement,  or  when 
the  pressure  is  removed  or  the  material  dried.  Thus, 
in  general,  other  conditions  remaining  equal,  materials 
become  more  plastic  the  greater  the  content  of  colloidal 
matter.  In  general  the  colloids  function  as  a  measure 
of  plasticity.     The   consideration    of   shrinkage,    hygro- 

1  Davis,  N.  B.,  The  Plasticity  of  Clay.  Trans.  Amer. 
Cer.  Soc,  Vol.  16,  pp.  65-79.     1914. 

2Cushman,  A.  S.  The  CoUoid  Theory  of  Plasticity. 
Trans.  Amer.  Cer.  Soc,  Vol.  6,  pp.  65-78.  1904.  Also,  Ashley, 
H.  E.  The  Colloid  Matter  of  Clay  and  Its  Measurement. 
U.  S.  Geol.  Survey,  Bui.  388.     1909. 


SOIL   STRUCTURE  173 

scopic  water,  and  dye  adsorption  as   an  expression   of 
plasticity  become  logical  on  this  basis. 

114.  The  importance  of  plasticity.  —  Plasticity  assumes 
considerable  importance  in  a  soil  when  it  becomes  highly 
developed,  since  it  promotes  ease  in  puddling.  The 
more  plastic  a  soil  is,  the  more  likely  it  is  to  become  pud- 
dled by  tillage,  especially  if  it  has  a  high  moisture  con- 
tent. Thus  a  clay  cannot  be  plowed  wet,  since  this 
would  allow  its  particles  to  be  worked  into  that  very  intri- 
cate condition  so  detrimental  to  plant  growth.  A  sand, 
on  the  contrary,  may  be  stirred  even  when  saturated, 
and  still  its  structural  condition  will  not  be  impaired 
since  its  plasticity  is  low  or  nihil.  A  very  plastic  soil 
is  also. likely  to  become  exceedingly  hard  when  dry  unless 
it  is  well  granulated,  which  shows  the  great  care  demanded 
by  soils  having  high  plasticity  coefficients. 

The  three  factors  that  affect  plasticity  to  the  greatest 
extent  are  texture,  granulation,  and  moisture.  In  gen-' 
eral,  the  finer  the  texture  of  the  soil,  the  higher  is  the 
maximum  plasticity  thereof.  The  more  granular  a  soil, 
the  lower  is  the  plasticity  or  the  tendency  to  puddle 
when  plowed.  The  amount  of  water  is  the  third  vital 
factor.  A  soil  will  exhibit  its  maximum  plasticity  at  a 
definite  moisture  content.  This  point  will  lie  somewhere 
between  the  flowing,  or  viscous,  condition  and  the  point 
at  which  a  soil  refuses  to  mold,  or,  in  other  words,  to  be- 
come crumbly.  With  a  soil  such  as  a  clay,  in  which  the 
plasticity  is  high,  plowing  should  be  done  when  the 
moisture  condition  is  such  that  there  is  no  likelihood  of 
puddling,  and  yet  the  soil  will  turn  over  with  a  maximum 
granulating  effect. 

115.  Cohesion.  —  Very  closely  correlated  with  plas- 
ticity, but  not  in  exact  similarity,  is  cohesion.     By  the 


174^    SOILS:    PROPERTIES  AND  MANAGEMENT 

cohesion  of  a  soil  is  meant  the  tendency  that  its  particles 
exhibit  in  sticking  together  and  in  conserving  the  mass 
intact.  In  general,  the  greater  the  plasticity  of  a  soil, 
the  higher  is  its  cohesion,  especially  when  it  is  dry  or 
only  slightly  moist.  For  that  reason,  cohesion  might 
be  made  a  rough  measure  of  plasticity.  Cohesion  of  a 
soil  occurs  under  two  general  conditions,  the  wet  and  the 
dry.  When  a  soil  is  moist  its  cohesion  is  developed  by  the 
moisture  films  and  the  colloidal  materials  that  may  be 
present.  This  form  of  cohesion  is  often  spoken  of  as 
tenacity.  When  a  soil  is  dry  its  cohesion  is  developed  to 
some  extent  by  the  interlocking  of  its  grains  and  the  dep- 
osition of  cementing  salts.  The  greatest  force  is  devel- 
oped, however,  by  the  drying  and  shrinking  .of  the 
gelatinous  colloidal  matter.  As  a  general  rule,  the  greater 
the  amount  of  colloidal  material,  the  more  firmly  the  soil 
is  bound  together  when  dry,  or,  in  other  words,  the  greater 
is  its  cohesion. 

Cohesion  is  important  in  tillage  operations,  in  that 
soils  having  a  high  coefficient  of  cohesion  tend  to  become 
cloddy  when  plowed  and  may  thus  be  rendered  poor  in 
physical  condition.  This  may  be  avoided  by  timing  the 
operation  so  that  the  moisture  content  is  somewhere 
above  the  point  at  which  excessive  cohesion  is  exerted. 
As  cohesion  is  not  greatly  developed,  except  in  a  heavy 
soil,  it  is  only  where  fine  texture  is  found  that  such  a 
danger  exists.  As  already  shown,  the, danger  is  a  double 
one,  for,  since  high  plasticity  and  high  cohesion  go  to- 
gether, a  soil  plowed  too  wet  may  puddle  while  one 
plowed  too  dry  may  clod. 

116.  Methods  of  determining  cohesion.  —  A  number 
of  methods  have  been  devised  for  determining  the  co- 
hesion of  clays  and  other  soils.     One  of  the  earliest  was 


SOIL    STRUCTURE  175 

Schiibler's,1  in  which  was  tested  the  resistance  of  rec- 
tangular prisms  of  dry  soils  to  penetration  by  a  steel 
blade.  The  apparatus  consisted  of  a  beam  supported 
on  a  fulcrum  more  than  one  third  of  the  distance 
from  the  end.  A  pan  for  holding  the  weights  added 
for  causing  the  crushing  was  hung  at  the  end  of  the 
long  arm,  while  a  counterpoise  on  the  short  end  of  the 
beam  acted  as  a  balance.  The  steel  knife  was  placed 
on  the  long  end  of  the  arm  near  the  fulcrum.  The  dry 
soil  prism  was  placed  under  the  knife  and  weights  were 
added  to  the  pan  until  crushing  occurred.  The  weight 
necessary  was  designated  as  the  cohesion  coefficient  of 
that  soil. 

Haberlandt 2  measured  cohesion  by  crushing  soil 
cylinders  of  a  definite  size.  A  glass  vessel  was  placed 
on  the  top  of  the  column  and  water  was  added  until  the 
column  gave  way.  The  weight  necessary  to  bring  this 
about  was  designated  as  the  absolute  cohesion  of  the 
sample.  Haberlandt  also  measured  cohesion  of  soil 
cylinders  of  a  definite  size  by  determining  the  resistance 
to  breaking  under  a  transverse  load,  the  soil  column  being 
placed  across  supports  six  centimeters  apart.  On  the 
column  midway  between  the  ends  a  scale  pan  was  sup- 
ported, into  which  weights  were  put  until  breaking  oc- 
curred. The  figure  thus  obtained  was  called  the  relative 
cohesion. 

1  A  good  description  of  Schiibler's  apparatus  is  found  on  page 
104  of  Bodenkunde,  by  E.  A.  Mitschertich,  published  by  Paul 
Parey,  Berlin,  in  1905. 

2  Haberlandt,  H.  Ueber  die  Koharescenz  Verhaltnisse 
verschiedener  Bodenarten.  Forsch.  a.  d.  Gebiete  d.  Agri.- 
Physik.,  Band  I,  Seite  118-157.  1878.  Also,  Wissenschaftlich 
praktische  Untersuchungen  auf  deni  Gebiete  des  Pflanzenbaues, 
Band  I,  Seite  22.     1875. 


176       SOILS:    PROPERTIES  AND  MANAGEMENT 

Puchner *  used  a  penetration  apparatus,  consisting 
of  a  vertical  shaft  held  by  metal  guides  and  counterpoised 
by  a  weight  hung  over  a  pulley.  The  shaft  was  armed 
at  the  lower  end  with  a  cutting  blade,  while  the  upper 
end  carried  a  scale  pan  for  holding  the  necessary  weights 
for  penetration.  The  cohesion  coefficient  was  the  weight 
necessary  to  force  the  blade  a  certain  distance  into  the 
soil.  All  important  apparatus  have  been  modeled  after 
either  Puchner's  or  Schiibler's.  That  of  Atterberg  2  (see 
Fig.  24)  follows  the  former,  while  that  of  the  Bureau  of 
Soils  3  (see  Fig.  25)  resembles  the  latter. 


Fig.  24.  —  Atterberg's  apparatus  for  determining  the  cohesion  of  soil 
prisms.  The  soil  prism  is  placed  between  the  cutting  edge  (K)  and 
the  sharp  plunger  (P).  Weights  are  added  at  (W).  (C)  is  a  coun- 
terpoise. 


. !  Puchner,  H.  Untersuchungen  iiber  die  Kohareszenz  der 
Bodenarten.  Forsch.  a-  d.  Gebiete  d.  Agri.-Physik.,  Band  12, 
Seite  195-241.     1889. 

2  Atterberg,  A.  Die  Konsistenz  und  die  Bindigkeit  der  Boden. 
Internat.  Mitt.  f.  Bodenkunde,  Band  II,  Heft  2-3,  Seite  149- 
189.     1912. 

3  Cameron,  F.  K.,  and  Gallagher,  F.  E.  Moisture  Content 
and  Physical  Condition  of  Soils.  U.  S.  D.  A.,  Bureau  of  Soils, 
Bui.  50.     1908. 


SOIL    STRUCTURE 


177 


Recently  Puchner  '  has  found  a  machine  for  measuring 
the  crushing  strength  of  dry  soil  cylinders  to  be  of  value 
in  determining  the  absolute,  or  maximum,  cohesion  of 
soils.  The  results,  as  he  has  already  demonstrated  in 
his  previous  work,  are  comparable,  in  relative  value  at 
least,  to  those  obtained  by  simpler  methods. 

The  great  difficulty  encountered  in  measuring  the 
natural  cohesion  of  a  soil,  either  wet  or  dry,  is  not  so 


Fig.  25.  —  The  apparatus  used  by  the  United  States  Bureau  of  Soils 
for  determining  the  penetration  of  soils.  (C)  is  mechanically  packed 
soil ;  (A),  steel  point;  (P),  pail  for  receiving  sand  from  funnel  (F). 
(W)  is  a  counterpoise. 

much  in  the  accuracy  of  the  determination  as  in  controlling 
physical  conditions.  The  cohesion  of  a  soil  depends 
very  much  on  the  handling  it  has  received  in  the  prepara- 
tion, in  the  amount  of  water  that  is  added,  and  in  the 
amount  and  length  of  time  of  drying.  Natural  granu- 
lation cannot  be  secured.  The  Bureau  of  Soils  attempted 
to  obviate  these  difficulties  by  mechanical  sifting  and 
packing,  but  the  results  were  abnormal,  due  to  the  sift- 
ing process.     As  a   consequence,   most   cohesion  results 

1  Puchner,  H.  Vergleichende  Untersuchungen  fiber  die 
Kokareszenz  verschiedener  Bodenarten.  Internat.  Mitt,  fur 
Bodenkunde,  Band  III,  Heft  2-3,  Seite  141-239.     1913. 

N 


178       SOILS:    PROPERTIES  AND   MANAGEMENT 

have  been  determined  either  on  samples  that  have  been 
worked  to  a  maximum  plasticity  and  then  brought  to 
the  required  moisture  content,  or  on  uniformly  compacted 
samples  that  have  been  allowed  to  take  up  water  by  capil- 
larity. In  general  the  curve  that  would  occur  with  normal 
grani^lation,  while  lower,  will  follow  the  direction  of  a 
maximum  plasticity  curve. 

117.  Factors  affecting  cohesion.  —  It  is  obvious,  from 
what  has  already  been  said  regarding  the  general  char- 
acteristics of  soils,  that  texture  must  play  an  important 
role  in  the  determination  of  the  cohesion  factor.  In 
general,  the  finer  the  texture,  the  greater  is  the  cohesion, 
since,  whether  the  soil  is  wet  or  dry,  the  forces  that  tend 
to  hold  the  particles  together  are  stronger  than  in  a  coarser 
soil.  The  drier  the  soil,  however,  the  greater  is  the  tex- 
tural  influence  in  this  regard,  due  to  the  very  great  in- 
crease in  the  binding  capacity  of  the  colloidal  matter 
on  drying.  In  a  coarse  soil  this  binding  effect  is  small  or 
entirely  absent. 

Another  factor  is  the  granular  condition  of  the  samples. 
In  general  granulation  may  be  said  to  be  due  to  an  exer- 
tion of  cohesion  between  a  limited  number  of  particles, 
resulting  in  a  crumb,  or  granular,  structure.  This  granu- 
lation, by  loosening  the  soil  mass,  lowers  not  only  plastic- 
ity but  cohesion  also.  The  addition  of  organic  matter 
to  a  soil,  by  hastening  and  increasing  granulation,  will 
tend  to  lower  cohesion  at  every  moisture  content  ranging 
from  a  dry  to  a  saturated  condition.  The  following 
data,  taken  from  Puchner,1  bring  out  the  points  just 
discussed :  — 

1  Puchner,  H.  Untersuchungen  iiber  die  Kohareszenz  der 
Bodenarten.  Forsch.  a.  d.  Gebiete  d.  Agri.-Physik.,  Band  12, 
Seite  195-241.     1889. 


SOIL    STRUCTURE 


179 


Effects  of  Texture,  Granulation,  Humus,  and  Moisture 
on  Cohesion  of  Soils.     (Puchner.) 


Soil 


Penetration  in  Grams  at  Various 
Moisture  Contents, 


100 
per 

cent 


80 
per 
cent 


60 
per 


40    * 
per  cent 


20 

per  cent 


0 

per  cent 


Clay 

2  clay  +  1  quartz 
1  clay  +  2  quartz 
Quartz  .... 
2 'clay  +  1  humus 
1  clay  +  2  humus 
Humus  .... 
Pulverized  loam  . 
Granulated  loam 

(granules  .5-9  mm 
diam.)      .     .     .  " 


114 
30 
85 

167 
44 
59 

115 
35 


58 


2,404 

437 

1,887 

2,937 

754 

1,010 

1,414 

272 


85 


9,537 
6,304 
3,803 
4,237 
4,704 
1,704 
1,904 
775 


115 


11,870 
12,370 
10,003 
5,137 
7,204 
4,204 
1,804 
1,408 


492 


15,037 

13,204 

13,703 

8,370 

9,537 

5,070 

870 

8,125 


808 


20,037 

15,704 
6,057 
2,370 

12,037 

1,637 

487 

12,358 


1,342 


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(4-     - 

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tZ      ' 

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HI/MUX 

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Fig.  26 


so 


The  effects  of  texture,  humus,  and  moisture  on  the  cohesion 
of  soils. 


180       SOILS:    PROPERTIES  AND  MANAGEMENT 


The  relationships  already  spoken  of  are  especially  well 
shown  by  the  curves  (see  Fig.  26),  particularly  the  effect 
of  the  moisture  content  on  cohesion.  In  a  heavy  soil  the 
cohesion  increases  steadily  from  a  saturated  condition  until 
dryness  is  reached,  the  increase  becoming  accelerated  as 
the  percentage  of  moisture  decreases.     This  is  because 


* 

fn 

SO 

JO 
20 

c/.jy 

c 

/( 

0                        £ 

7                       J 

0                       4- 

o  <?e> 

Fig.  27.  —  The  cohesion  curves  of  clay  and  fine  sandy  loam  at  various 
moisture  contents.     (C),  point  at  which  soil  changes  in  color. 


SOIL   STRUCTURE  181 

the  binding  power  of  the  colloidal  material  is  greatly 
augmented  by  desiccation.  In  a  coarse  soil  such  as 
quartz,  in  which  there  is  very  little  colloidal  matter,  the 
cohesion  is  developed  principally  by  the  water  film.  As 
this  thins,  its  pulling  power  increases  £nd  the  curve 
ascends;  but  when  the  soil  dries  this  film  disrupts  and 
the  curve  drops  again,  having  no  colloidal  binding  ma- 
terial. The  same  relationships  are  shown  by  curves  (see 
Fig.  27)  adopted  from  recent  determinations  by  Atter- 
berg.1 

118.  Moisture  limits  for  successful  tillage.  —  In  heavy 
soils,  in  which  the  colloidal  content  is  usually  high,  plas- 
ticity and  cohesion  also  are  high.  This  means  that  the 
soil  when  too  moist  will  be  puddled  by  tillage  implements, 
especially  such  as  the  plow,  and  when  too  dry  clodding 
will  occur  because  of  very  high  cohesion.  A  moisture 
limit  must  therefore  exist  on  a  heavy  soil,  within  which 
successful  plowing  may  be  done  and  maximum  granu- 
lation results  may  be  secured.  That  this  moisture  limit 
is  narrow  is  obvious,  since  high  cohesion  and  high  plas- 
ticity bound  it  so  closely  on  either  hand.  Such  a  rela- 
tionship must  be  kept  in  mind  not  only  by  the  farmer 
but  by  the  technical  man  as  well,  since  so  much  depends 
in  any  work  upon  good  soil  tilth.  The  relationship  is 
clearly  shown  by  the  following  curves  (Fig.  28)  partially 
adopted  from  Atterberg.1  The  cohesion  and  plasticity 
curves  are  seen  to  cross  near  the  center  of  the  diagram 
and  indicate  the  existence  of  a  zone  where  neither  is 
exceedingly  high  or  low. 

In  a  clay  soil  a  study  of  the  optimum  moisture  condi- 

1  Atterberg,  A.  Die  Konsistenz  Kurven  der  Mineralboden. 
Internat.  Mitt.  f.  Bodenkunde,  Band  IV,  Heft  4-5,  Seite  418- 
431.     1914. 


182       SOILS:    PROPERTIES  AND  MANAGEMENT 


\COHES/Ott 

PLASTICITY 

\C.                            &/ 

Fig.  28.  —  Diagram  showing  the  moisture  limits  for  successful  plowing 
in  soils  of  different  class,  cc  and  hh'  represent  the  moisture  limits 
within  which  successful  plowing  may  be  accomplished  on  a  clay  and 
humus  clay  respectively. 


tions  is  necessary  in  order  to  determine  what  is  just  the 
right  moisture  content  for  good  plowing.  That  this 
condition  must  be  carefully  gauged  and  immediate  use 
made  of  the  advantages  it  offers  is  shown  by  its  narrow 
limit.  A  few  days  may  suffice  for  the  moisture  to  drop 
through  such  a  narrow  area  of  fluctuation.  A  clay  soil  is 
so  difficult  to  handle  at  best  that  no  opportunities  such  as 
are  offered  by  optimum  moisture  conditions  should  be  lost. 
Moreover,  a  heavy  soil  plowed  too  dry  or  too  wet  does  not 
regain  its  normal  granular  condition  for  several  seasons. 
In  a  sandy  soil  no  such  difficulties  are  encountered. 


SOIL   STRUCTURE  183 

Since  the  soil  has  low  cohesion,  plowing  when  it  is  too 
dry  will  not  clod,  while,  because  of  low  plasticity,  little 
puddling  will  occur  if  tillage  is  in  progress  when  the  soil 
is  wet.  Being  always  rather  loose,  sucfy  a  soil  is  often 
benefited  by  plowing  when  it  is  slightly  wet,  the  parti- 
cles being  brought  into  closer  contact  and  excessive  per- 
colation being  stopped  while  at  the  same  time  the  water 
capacity  is  raised. 

119.  Control  of  cohesion  and  plasticity.  — It  is  evi- 
dent not  only  that  cohesion  and  plasticity  control  the 
successful  tillage  of  the  land,  especially  where  the  soil 
texture  is  fine,  but  also  that  these  same  factors  vary  with 
the  moisture  and  the  granular  structure  of  the  soil.  It 
has  been  shown  that  there  is  a  moisture  zone  in  all  soils  — 
this  being  narrower  the  finer  the  soil  texture  —  at  which 
neither  cohesion  nor  plasticity  is  excessive.  In  this  zone 
a  heavy  soil  may  be  successfully  plowed,  with  results 
favorable  to  the  structural  condition  of  the  soil.  Since 
the  processes  of  granulation  have  already  been  shown  to 
lower  cohesion  and  plasticity,  it  is  evident  that  as  a  crumb 
structure  is  developed  the  moisture  zone  for  proper  plow- 
ing will  be  widened,  especially  in  a  heavy  soil.  This  is  a 
very  important  point  in  the  handling  of  clays  and  clay 
loams,  since  it  not  only  opens  a  way  for  the  elimination 
of  the  dangers  of  bad  structural  relationships,  but  also 
provides  for  putting  the  soil  in  a  condition  for  easier  and 
more  convenient  tillage.  In  a  sandy  soil,  particularly 
where  humus  is  the  granulating  agent,  a  soil  with 
more  cohesion,  more  plasticity,  and  a  greater  water- 
holding  power  is  developed,  all  of  which  tend  toward  a 
better  medium  for  plant  growth.  Methods  of  develop- 
ing this  granulation  thus  become  the  logical  topic  for 
further  discussion. 


184       SOILS:    PROPERTIES  AND  MANAGEMENT 

120.  Soil  tilth.  —  The  previous  data  and  discussion 
have  clearly  shown  the  very  great  importance  of  a  crumb 
structure  in  the  working  of  the  soil  in  the  field.  Since 
good  physical  condition  will  reflect  itself  on  crop  yield, 
it  is  evident  that  structure  must  ultimately  be  considered 
in  relation  to  all  plant  growth.  This  relationship  is 
usually  expressed  by  the  term  tilth.  While  structure 
refers  to  the  arrangement  of  the  particles  in  general,  and 
granulation  to  a  particular  aggregate  condition,  tilth 
goes  one  step  further  and  includes  the  plant.  Tilth,  then, 
refers  to  the  physical  condition  of  the  soil  as  related  to 
crop  growth.  It  may  be  poor,  medium,  good,  or  excel- 
lent, according  to  circumstances.  Good  tilth  may  de- 
mand in  some  soils  maximum  granulation,  in  others 
only  a  medium  development.  Maximum  tilth  always 
implies  the  presence  of  water,  since  the  best  physical 
relationships  cannot  be  developed  without  optimum 
moisture  conditions. 

From  the  curves  already  presented,  it  is  evident  that 
an  optimum  moisture  condition  exists  for  the  proper 
tillage  of  a  soil,  especially  one  of  a  heavy  character.  Also, 
an  optimum  moisture  condition  must  exist  for  proper 
tilth,  and  therefore  for  proper  plant  development,  since 
adequate  tilth  is  the  best  physical  condition  for  crop 
growth.  Practical  experience  and  theoretical  evidence  x 
have  shown  that  these  two  optimum  conditions  are 
identical  in  nearly  all  cases,  a  happy  coincidence  in  the 
practical  management  of  a  soil.  The  optimum  moisture 
condition  for  plant  growth,  then,  is  the  proper  moisture 
condition  for  effective  plowing.     The  optimum  condition 

1  Cameron,  F.  K.,  and  Gallagher,  F.  E.  Moisture  Content 
and  Physical  Condition  of  Soils.  U.  S.  D.  A.,  Bur.  Soils,  BuL 
50,  p.  8.     1908. 


SOIL   STRUCTURE 


185 


for  developing  a  favorable  crumb  structure  is  obviously 
the  optimum  moisture  content  for  the  development  of 
the  highest  tilth.  In  fact,  it  can  be  stated  with  certainty 
that  the  optimum  moisture  condition  for  plant  growth 
is  the  optimum  for  all  favorable  soil  activities,  whether 
physical,  chemical,  or  biological.  Granulation,  then, 
becomes  the  vital  factor  in  placing  the  soil  in  a  physical 
condition  such  that  the  highest  tilth,  that  physical  criterion 
which  every  farmer  should  strive  for,  may  be  developed 
in  any  soil.  Until  proper  granulation  is  reached,  no  soil 
can  be  expected  to  yield  maximum  paying  returns. 

121.  Granulation.  —  While  it  is  possible  to  list  the 
factors  that  bring  about  granulation  in  a  soil,  it  is  diffi- 
cult to  state  specifically  just  why  this  phenomenon  takes 
place.  It  has  been  suggested  that  much  of  the  granule 
formation  in  the  soil  is  due  to  the  contraction  of  the 
moisture  film  around  the  particles  when,  for  any  reason, 
the  moisture  content  is  reduced  (see  Fig.  29).     It  is  known 


Fig.  29.  —  A  puddled  and  a  well-granulated  soil. 


that  the  soil  particles  tend  to  be  drawn  together  by  this  re- 
duction in  the  soil  moisture,  due  to  the  pulling  power  of  the 
thinned  film.  If  to  this  condition  is  added  a  material  which 
tends  to  exert  not  only  a  drawing  power  on  loss  of  mois- 
ture, but  also  a  binding  and  cementing  power  when  dry,  all 


186       SOILS:    PROPERTIES  AND  MANAGEMENT 

the  essentials  for  successful  granulation  are  present.  This 
second  force  is  found  in  the  colloidal  material  present  in 
considerable  quantities  in  heavy  soils.  These  are  the  same 
forces  that  have  already  been  shown  to  determine  the 
cohesion  and  plasticity  of  the  soil,  except  that  in  granu- 
lating operations  they  are  localized  at  numberless  foci, 
and  clodding  or  puddling  is  thereby  prevented.  It  is 
evident  that  if  cohesion  and  plasticity  forces  are  to  func- 
tion for  granulation  —  or,  in  other  words,  locally  in  the 
soil  instead  of  generally  and  uniformly  as  when  clodding 
or  puddling  occurs  —  a  certain  moisture  content  must 
be  maintained.  From  what  has  already  been  shown, 
it  is  hardly  necessary  to  restate  that  this  moisture  condi- 
tion is  near  the  optimum  moisture  content  for  plant 
growth. 

Warington  !  attributes  granulation  to  unequal  expan- 
sion and  contraction  of  the  soil  mass,  due  to  the  imbibi- 
tion and  loss  of  water.  In  a  soil  subject  to  such  a  condi- 
tion, the  cohesive  forces  being  localized,  the  internal 
strains  and  pressures  are  unequal  and  a  tendency  arises 
for  the  mass  to  divide  along  lines  of  weakness  into  groups 
of  particles.  The  binding  capacity  of  colloidal  material, 
as  well  as  of  salts  deposited  from  the  soil  solution,  tends 
to  make  such  a  crumb  structure  more  or  less  permanent. 
Tillage  operations,  development  of  roots,  burrowing 
of  animals  and  insects,  the  presence  of  humus,  and  the 
formation  of  frost  crystals,  may  assist  in  further  develop- 
ing these  lines  of  weakness  in  the  soil  mass,  on  which  the 
tension  of  the  moisture  films  around  the  soil  particles  is 
brought  to  bear.  The  flocculation  of  soil  particles  may 
also  develop  lines  of  cleavage  by  their  aggregation  around 

1  Warington,  R.  Physical  Properties  of  Soils,  pp.  36-41. 
Oxford.     1900. 


SOIL   STRUCTURE  187 

certain  centers.  This  movement  of  the  soil  particles  is 
in  every  case  facilitated  by  the  presence  of  a  moderate 
amount  of  moisture. 

122.  Forces  facilitating  granulation.  —  Granulation  is 
nothing  more  or  less  than  a  condition  brought  about  by 
the  force  exerted  by  a  variable  water  film  and  the  pull- 
ing and  binding  capacities  of  colloidal  material,  operating 
at  numberless  localized  foci.  It  is  evident  that  any  influ- 
ence or  change  in  the  soil  which  will  cause  a  greater  locali- 
zation of  these  operative  forces  will  promote  increased 
granulation.  The  addition  of  materials  from  extraneous 
sources  is  also  a  practice  that  may  tend  to  develop  lines  of 
weakness  and  thus  cause  a  more  intense  localization  of 
the  forces  at  work. 

The  conditions,  additions,  and  practices  tending  to 
develop  or  facilitate  a  granular  structure  in  soils  may  be 
listed  under  six  heads :  (1)  wetting  and  drying  of  the  soil, 
(2)  freezing  and  thawing,  (3)  addition  of  organic  matter, 
(4)  action  of  plant  roots  and  animals,  (5)  addition  of 
lime,  and  (6)  tillage. 

123.  Wetting  and  drying.  —  The  drying  of  a  soil  has 
been  shown  to  result  in  a  drawing  together  of  the  particles 
into  aggregates.  When  this  process  is  repeated  again 
and  again  by  alternate  wetting  and  drying,  the  influence 
on  granulation  becomes  marked. 

In  drying,  the  small  particles  are  moved  into  the  spaces 
between  the  larger  ones,  thereby  reducing  the  volume 
as  is  shown  by  the  checks  produced.  These  checks  that 
result  from  shrinkage  are  due  to  the  unequal  contraction. 
There  comes  a  time  when  the  general  film  around  the 
whole  mass  must  rupture,  and  it  breaks  along  the  lines 
of  least  resistance.  If  the  soil  mass  is  very  uniform,  there 
will  be  few  breaks  and  the  shrinkage  will  be  mainly  around 


/ 


188       SOILS:    PROPERTIES  AND  MANAGEMENT 

a  relatively  few  centers.  This  process  produces  clods, 
or  "  overgrown  "  granules.  If  there  are  numerous  lines 
of  weakness,  however,  there  will  be  many  centers  of  con- 
traction, and  consequently  a  larger  number  of  small 
clods,  or  granules,  will  be  formed.  This  is  the  desirable 
condition  and  constitutes  good  tilth  —  that  is,  the  most 
favorable  physical  condition  for  plant  growth. 

Just  what  may  be  the  effects  of  wetting  and  drying  on 
the  colloidal  matter  of  soil  is  a  question.  In  general, 
desiccation  tends  to  flocculate  colloids  and  in  many  cases 
their  binding  power  becomes  highly  developed  thereby. 
If  such  colloids  are  irreversible,  as  many  in  the  soil  un- 
doubtedly are,  this  binding  becomes  more  or  less  per- 
manent, which  explains  the  tendency  for  a  crumb  struc- 
ture to  persist.  Wetting,  on  the  other  hand,  tends  to 
develop  colloidal  matter  which  will  become  binding  mate- 
rial on  the  next  drying.  The  desiccation  and  throwing 
down  of  colloids,  as  well  as  their  generation,  thus  be- 
comes a  very  important  factor  in  the  wetting  and  drying 
as  related  to  granulation . 

The  following  figures l  represent  the  relative  force 
necessary  to  penetrate  puddled  clay  dried  once,  as  com- 
pared with  the  same  puddled  soil  wet  and  dried  twenty 
times.     The  relative  hardness  may  be  taken  as  a  rough 

measure  of  granulation  :  — 

Percentage  of  pene- 
tration 

1.  Puddled  clay  dried  once  ....  100.0 

2.  Puddled  clay  dried  twenty  times  .    31.4 

3.  Puddled  clay  dried  twenty  times  .    30.6 

4.  Puddled  clay  dried  twenty  times  .    32.0 

Pippin,  E.  O.  Some  Causes  of  Soil  Granulation.  Trans. 
Amer.  Soc.  Agron.,  Vol.  2,  pp.  106-121.     1910. 


SOIL    STRUCTURE  189 

The  fact  illustrated  above  has  many  practical  appli- 
cations. It  should  be  observed  that  the  change  in  struc- 
ture is  not  associated  with  continual  wetness,  nor  is  it 
identified  with  a  continued  dry  state.  In  neither  condi- 
tion is  any  force  brought  to  bear  on  the  particles.  The 
force  is  exerted  only  during  the  drying  process  and  the 
wetting  process.  It  is  a  well-known  fact  that  soils  which 
are  continually  wet  are  usually  in  bad  physical  condition. 
In  the  drainage  of  wet  land,  it  is  found  that  the  soil  is  at 
first  very  refractory;  but  when  good  drainage  is  estab- 
lished there  is  a  gradual  amelioration  of  the  physical 
condition,  which  is  primarily  a  change  in  structure.  On 
the  other  hand,  in  a  soil  continually  in  a  dry  state  there 
is  no  change  in  granulation.  The  improvement  of  soil 
structure,  as  a  result  of  changes  in  the  moisture  content, 
is  dependent  largely  on  lines  of  weakness  in  the  soil  mass. 
Some  of  these  are  produced  in  the  process  of  drying,  and 
others  in  the  ways  already  listed. 

124.  Freezing  and  thawing.  —  As  will  be  seen  in 
the  consideration  of  soil  moisture,  the  water  is  distributed 
in  the  fine  pores  of  the  soil.  When  it  freezes  it  forms  long, 
needle-like  crystals.  This  crystallizing  force  is  very 
great,  amounting  to  about  150  tons  when  a  cubic  foot  of 
water  changes  to  ice.  In  freezing,  the  crystals  gradually 
grow  first  in  the  larger  spaces.  During  this  process  there 
is  a  marked  withdrawal  of  moisture  from  the  smallest 
spaces,  so  that  the  ice  crystals  in  the  large  spaces  may  be 
built  up.  The  soil  mass  is  separated  by  the  crystals,  and 
as  the  result  of  even  a  single  hard  freeze  a  wet,  puddled  soil 
is  shattered  into  pieces.  The  repetition  of  this  process  by 
subsequent  freezing  and  thawing  will  further  break  up  the 
soil  by  creating  new  lines  of  weakness.  The  granulating 
power  of  freezing  and  thawing  is  shown  in  the  following 


190       SOILS:    PROPERTIES  AND  MANAGEMENT 

figures,1  expressed   as    the    relative    force    necessary    to 
penetrate  a  puddled  clay  treated  in  various  ways :  — 

Percentage 
penetration 

1.  Puddled  clay  dried  once 1()0.0 

2.  Puddled  clay  frozen  once  and  dried  once      .     .     30.3 

3.  Puddled  clay  frozen  three  times  and  dried  once    27.3 

4.  Puddled  clay  frozen  five  times  and  dried  once  .     21.8  • 

Freezing  probably  affects  the  colloidal  material  in 
the  same  general  way  as  does  drying.  This  has  been 
indicated  by  the  work  of  certain  investigators,2  in  which 
it  was  found  that  lowering  the  temperature  of  a  soil  below 
freezing  lowered  the  hygroscopic  coefficient. 

125.  Addition  of  organic  matter.  —  Soils  rich  in  humus 
or  decomposed  organic  matter  are  generally  in  better 
physical  condition  than  soils  low  in  organic  content. 
The  marked  effect  of  the  absence  of  this  material  in  many 
long-cultivated  soils  is  well  known.  For  example,  in 
much  of  the  southern  New  York  hill  regions,  the  soils 
are  now  recognized  to  have  a  very  different  relation  to 
crop  growth  from  what  they  had  for  a  few  years  after 
they  were  cleared.  Their  color  has  become  lighter,  and 
with  the  decay  of  the  humus  a  decided  physical  change 
has  taken  place  in  the  soil,  which  is  to  some  extent  cor- 
rected by  the  restoration  of  the  organic  content.  In 
certain  prairie  soils  the  effect  of  humus  depletion  on  struc- 

1  Fippin,  E.  O.  Some  Causes  of  Soil  Granulation.  Trans. 
Amer.  Soc.  Agron.,  Vol.  II,  pp.  106-121.     1910. 

2  Czermak,  W.  Ein  Beitrag  zur  Erkenntnis  der  Veran- 
derungen  der  Sog.  physikalischen  Bodeneigenshaften  durch 
Frost,  Hitze,  und  die  Beigabe  einiger  Salze.  Landw.  Ver. 
Stat.,  Band  76,  Heft  1-2,  Seite  73-116.  1912.  Also,  Ehrenberg, 
P.,  und  Romberg,  G.  F.  von.  Zur  Frostwirkung  auf  den  Erd- 
boden.     Jour.  f.  Landw.,  Band  61,  Heft  1,  Seite  73-86.     1913. 


SOIL   STRUCTURE  191 

ture  is  even  more  marked.  While  the  actions  of  humus 
are  many,  as  has  already  been  shown,  its  relationship  to 
physical  condition  is  always  particularly  emphasized. 

Humus  contains  much  colloidal  material  and  in  this 
way  possesses  a  certain  degree  of  plasticity.  It  is,  how- 
ever, of  a  very  loose  structure  and  the  large  spaces  con- 
stitute lines  of  weakness.  Another  property  of  humus  is 
that  it  undergoes  great  change  in  volume  when  dried 
out  —  a  property  akin  to  the  fineness  of  the  soil,  producing 
larger  shrinkage  cracks.  This  is  noticeable  in  many 
black  clay  soils,  which  check  excessively.  The  great 
capacity  of  humus  for  moisture  permits  a  wide  range  in 
moisture  content,  which  produces  corresponding  physical 
alteration.  This  wide  swing  from  one  extreme  to  another 
is  a  potent  factor  in  granulative  influences.  The  color 
of  the  humus  affects  the  color  of  the  soil,  and  thereby 
increases  the  rate  of  change  from  the  wet  to  the  dry  state 
by  increased  evaporation  of  moisture.  The  relative 
effects  of  crude  muck,  and  the  ammonia  extract  from  the 
same  muck,  on  the  cohesion  of  a  puddled  clay,  as  indi- 
cated by  the  force  required  for  a  uniform  penetration  of 
a  knife-edge,  is  shown  in  the  following  table ;  the  samples 
were  dried  and  rewetted  twenty  times :  — 

Puddled  Clay  plus  Muck  * 

Percentage 
of  penetration 

100 

82 
73 
58 
50 


1.  Clay 

2.  Clay  plus    5  per  cent  of  muck 

3.  Clay  plus  15  per  cent  of  muck 

4.  Clay  plus  25  per  cent  of  muck 

5.  Clay  plus  50  per  cent  of  muck 


1  Fippin,  E.  O.     Some  Causes  of  Soil  Granulation.      Trans. 
Amer.  Soc.  Agron.,  Vol.  2,  pp.  106-121.     1910. 


192       SOILS:    PROPEUTIES  AND  MANAGEMENT 

Puddled  Clay  plus  Muck  Extract  i 

Percentage 
of  pene- 
tration 

1.  Clay 100 

2.  Clay  plus  1  per  cent  of  extract 85 

3.  Clay  plus  2  per  cent  of  extract 76 

4.  Clay  plus  4  per  cent  of  extract 69 

126.  Action  of  plant  roots  and  animals.  —  The  exten- 
sion of  plant  roots  changes  the  soil  structure  by  forcing 
the  particles  apart  at  each  growing  root  point,  and  possibly 
also  by  some  action  yet  to  be  explained.  Crops  differ 
greatly  in  their  effect  on  soil  structure.  Grass,  millet, 
wheat,  and  other  plants  with  fine  roots  are  more  beneficial 
to  tilth  than  coarse  or  tap-rooted  plants  such  as  corn, 
oats,  and  beets.  Grass  affects  structure  also  by  protecting 
the  surface  of  the  ground.  It  is  advisable  to  establish 
a  rotation  on  clay  soil,  that  plowing  may  be  done  at  fre- 
quent intervals  and  that  plants  with  different  root  devel- 
opments may  be  given  an  opportunity  to  exert  their 
influences.  The  organic  matter  left  in  the  soil  by 
decaying  roots  is  always  in  very  intimate  contact 
with  the  soil  grains  and  has  much  to  do  with  accelerat- 
ing granulation. 

Animals  also  affect  soil  structure.  Earthworms,  by 
carrying  materials  to  the  surface,  exert  a  mixing  effect, 
while  the  lines  of  seepage  and  zones  of  weakness  developed 
through  their  burrowing  proclivities  are  of  no  mean  im- 
portance. Insects,  especially  ants  and  other  burrowing 
creatures,  aid  in  this  and  in  other  ways. 

1  Fippin,  E.  O.  Some  Causes  of  Soil  Granulation.  Trans. 
Amer.  Soc.  Agron.,  Vol.  2,  pp.  106-121.     1910. 


/> 


SOIL   STRUCTURE  193 


127.  Addition  of  lime.  —  One  of  the  important  effects 
of  lime  is  in  its  flocculating  action.  This  agglomeration, 
as  already  explained  under  colloids,  is  the  drawing  to- 
gether of  the  finer  particles  of  a  soil  mass  into  granules. 
When  caustic  lime  is  mixed  with  water  containing  fine 
particles  in  suspension,  there  is  almost  immediately  a 
change  in  the  arrangement  of  the  particles.  They  appear 
first  to  draw  together  in  light,  fluffy  groups,  or  floc- 
cules,  which  then  rapidly  settle  to  the  bottom  so  that 
the  supernatant  liquid  is  left  clear  or  nearly  so.  This 
phenomenon  is  termed  flocculation,  because  of  the 
groups  of  particles.  It  is  not  an  action  limited  to 
caustic  lime  alone,  however,  but  because  of  the  useful- 
ness of  this  compound  in  other  ways,  and  because  of 
its  very  strong  action,  it  is  ordinarily  used  on  soils. 
This  flocculating  tendency  when  caustic  lime  is  added 
goes  on  in  the  soil  as  well  as  with  suspensions,  although 
more  slowly.  In  general  the  lime  serves  to  satisfy  the 
adsorptive  capacity  of  the  colloidal  material,  and  by 
throwing  down  these  colloids  develops  lines  of  weakness. 
The  cohesive  power  of  the  soil  is  thus  localized  and 
granulation  must  necessarily  occur. 

The  various  forms  of  lime  differ  in  their  granulating 
capacities,  calcium  oxide  and  calcium  hydrate  being  very 
active  while  calcium  carbonate  is  relatively  inactive  in 
this  regard.  For  this  reason,  if  flocculation  effects  are 
desired,  the  oxide  or  hydrate  combinations  are  added. 
The  relative  influences  of  lime  on  puddled  clay  as  measured 
by  penetration  is  shown  in  the  following  table ; *  the  soil 
was  dried  once  and  the  untreated  soil  was  used  as  100 
per  cent :  — 

•  »  Fippin,  E.  O.     Some  Causes  of  Soil  Granulation.     Trans, 
Amer.  Soc.  Agron.,  Vol.  2,  pp.  106-121.     1910. 
o 


194       SOILS:    PROPERTIES  AND  MANAGEMENT 

Percentage 
of  pene- 
tration 

1.  Puddled  clay ;  100 

2.  Clay  plus    2  per  cent  CaO 56 

3.  Clay  plus    4  per  cent  CaO 43 

4.  Clay  plus    6  per  cent  CaO 33 

5.  Clay  plus    5  per  cent  CaC03 98 

6.  Clay  plus  10  per  cent  CaC03 Ill 

7.  Clay  plus  25  per  cent  CaC03 95 

In  the  soil  the  oxide  and  hydrate  revert  to  the  carbonate, 
but  before  this  change  occurs  the  flocculating  effects  have 
been  exerted  and  the  lines  of  weakness  so  essential  to 
granulative  processes  have  been  developed.  Lime  really 
does  not  produce  granulation  in  a  normal  soil  through 
its  own  action  alone,  but  is  aided  by  the  other  influences 
already  discussed. 

Warington  !  reports  a  statement  of  an  English  farmer 
to  the  effect  that  by  the  use  of  large  quantities  of  lime  on 
heavy  clay  soil  he  was  enabled  to  plow  with  two  horses 
instead  of  three.  It  is  generally  true  that  soils  rich  in 
lime  are  well  granulated,  and  maintain  a  much  better 
physical  condition  than  soils  of  the  same  texture  that 
are  poor  in  lime. 

128.  Tillage.  —  The  effect  of  tillage  on  soil  structure 
is  to  produce  lines  of  cleavage,  and  these,  when  produced 
by  plowing,  are  multitudinous  and  fairly  uniformly  dis- 
tributed. Plowing,  when  the  moisture  content  is  suit- 
able, tends  to  break  the  soil  into  thin  layers,  which  move 
one  over  the  other  like  the  leaves  of  a  book  when  the  pages 

1  Warington,  R.  Physical  Properties  of  Soil,  p.  33. 
Oxford.     1900. 


SOIL   STRUCTURE  195 

are  bent.  This  disturbance  of  the  existing  arrangement 
of  particles  puts  in  motion  the  two  forces  that  have  al- 
ready been  discussed,  the  pull  of  the  water  film  and  the 
binding  power  of  the  colloidal  matter.  The  strength 
of  cohesion  between  small  particles,  such  as  clay,  can  be 
realized  when  one  considers  the  tenacity  with  which  these 
particles  are  held  together  in  dried  puddled  soil.  This 
cohesive  attraction  is  inversely  proportional  to  the  square 
of  the  distance  between  the  centers  of  the  attracting  bodies. 
Particles  that  can  be  brought  as  closely  together  as  can 
clay  particles  may  be  thus  held  with  great  firmness. 
The  effect  of  tillage  when  an  excess  of  water  is  present 
is  to  force  the  particles  into  large  masses  and  bring  about 
/  a  generalized  exertion  of  the  forces  of  plasticity.  The 
soil  then  becomes  puddled.  Tillage  when  the  soil  is 
too  dry  results  either  in  clodding  or  in  the  soil's  becom- 
ing so  pulverized  that  it  becomes  puddled  on  wetting. 
As  already  emphasized,  proper  pulverization  by  tillage, 
especially  by  plowing,  may  occur  only  when  the  soil  is 
in  optimum  moisture  condition. 

129.  The  action  of  the  plow.  —  The  plow  brings  about  its 
effects  because  of  the  differential  stresses  set  up  in  the  fur- 
row slice  as  it  passes  over  the  share  and  the  moldboard. 
The  soil  in  immediate  contact  with  the  plow  surface  is  re- 
tarded by  friction,  and  the  layers  above  tend  to  slide  over 
one  another  much  as  the  leaves  of  a  book  when  they  are 
bent.  If  the  soil  is  in  just  the  right  condition,  maximum 
granulation  results ;  but  if  the  moisture  is  too  high  or  too 
low,  puddling  or  clodding  may  follow,  especially  on  a 
heavy  soil.  Not  only  does  a  shearing  occur,  but  this 
shearing  is  differential,  due  to  the  slope  of  the  share  and 
especially  to  the  curve  of  the  moldboard.  Where  the 
soil  is  to  be  turned  over  with  the  least  expenditure  of 


196       SOILS:    PROPERTIES   A.Xh    MANAGEMENT 

force,  the  share  is  sloping  and  is  set  to  deliver  a  slanting 
cut,  and  the  mold  board  is  long  and  gently  inclined.  This 
allows  the  furrow  slice  to  be  turned  with  little  granulation 
and  a  minimum  expenditure  of  energy.  When  maximum 
granulation  and  pulverization  are  desired,  the  moldboard 
is  short  and  sharply  turned,  and  the  share  is  less  sloping 
and  the  cutting  edge  is  less  slanting.  Such  conditions 
make  for  the  development  of  more  friction  and  the  genera- 
tion of  those  internal  twisting  and  shearing  stresses  neces- 
sary for  good  granulation.  The  sharper  the  bending  of 
the  furrow  slice,  the  greater  are  the  internal  stresses  set 
up.  While  the  plow  is  the  very  best  pulverizing  agent 
when  optimum  soil  moisture  conditions  prevail,  it  is  also 
a  most  effective  puddling  agent  when  the  soil  is  wet. 
Therefore  care  in  the  judging  of  optimum  conditions  for 
plowing  is  a  most  important  feature  in  the  maintenance 
and  encouragement  of  soil  granulation  and  tilth. 
/  130.  Resume.  —  The  factors  controlling  the  struc- 
tural onnrlitinn  of  any  soil  are  found  to  be  plasticity  and 
.cohesion.  As  these  increase,  the  tendencies  of  a  soil  to 
puddle  when  wet  and  to  clod  when  dry  are  augmented. 
Therefore,  in  heavy  soils  a  decrease  in  these  factors  is 
advisable,  through  a  careful  control  of  moisture  and  a 
bettering  of  the  granular  structure  of  the  soil.  Granu- 
lation, while  due  to  some  extent  to  the  localized  influence 
of  the  water  film,  is  traceable  largely  to  the  colloidal 
matter  which  acts  as  a  binding  agent.  It  is  really  a 
concentration  of  the  forces  of  cohesion  and  plasticity 
around  numberless  localized  foci.  Granulation  takes 
place  under  the  influence  of  wetting  and  drying,  freezing, 
plants  and  animals,  addition  of  humus  and  lime,  and 
tillage  operations,  especially  plowing.  Due  to  the  high 
cohesion  and  plasticity  of  heavy  soils,  the  moisture  zone 


SOIL    STRUCTURE  197 

for  successful  plowing  is  relatively  narrow.  The  ability 
to  detect  when  this  zone  has  been  reached  in  a  clay  soil 
is  one  of  the  essentials  of  successful  soil  management. 
Another  essential  is  the  effective  widening  of  such  a 
zone  by  granulation  operations.  The  optimum  mois- 
ture condition  for  tillage  is  also  the  optimum  condition 
for  plant  growth  —  a  happy  coincident,  since  by  regu- 
lating the  moisture  content  for  plant  development  condi- 
tions are  rendered  most  favorable  for  all  soil  activities. 
It  is  thus  possible  to  realize  that  criterion  in  all  soil  physi- 
cal operations,  a  maximum  tilth. . 


CHAPTER  XI 

THE   FORMS    OF   SOIL    WATER    AND     THEIR 
MOVEMENT 

Under  all  normal  conditions  the  soil  bears  a  certain 
amount  of  moisture,  which  must  be  reckoned  with  in 
any  study  whether  of  a  practical  or  of  a  theoretical  na- 
ture. Moreover,  the  amount  of  water  varies  in  its  char- 
acteristics according  to  its  position.  It  also  has  move- 
ment, which  goes  far  in  determining  its  usefulness  to  plants. 
Before  a  discussion  of  the  different  forms  of  water,  their 
movement,  and  their  availability  to  plants,  may  be  entered 
into,  however,  some  way  of  quantitatively  stating  the 
amounts  present  must  be  determined  upon. 

131.  Methods  of  expressing  soil  moisture.  —  During 
the  many  years  of  soil  investigation,  especially  where  the 
problems  had  to  deal  either  directly  or  indirectly  with 
moisture,  five  methods  of  water  expression  have  been 
evolved,  their  use  depending  on  the  nature  of  the  work 
and  on  the  points  to  be  expressed.  These  may  be  listed 
under  two  general  heads  :  — 

A.  Percentage  expression 

1.  Percentage  on  a  wet  basis 

2.  Percentage  on  a  dry  basis 

B.  Volume  expression 

1 .  Cubic  inches  to  the  cubic  foot  of  soil 

2.  Percentage  by  volume 

3.  Surface  inches 

198 


THE  FORMS   OF  SOIL    WATER  199 

The  simplest  way  of  explaining  the  application  of  these 
methods  for  the  expression  of  the  amount  of  water  in  a 
soil  is  by  a  specific  case.  Suppose  a  certain  soil  in  field 
condition  weighs  100  pounds  to  a  cubic  foot  and  carries 
10  pounds  of  water.  Obviously  it  would  contain  10 
per  cent  of  water  by  the  wet  method  of  calculation,  or 
11.1  per  cent  of  water,  using  the  absolutely  dry  soil  as 
a  basis.  A  pound  of  water  contains  27.6  cubic  inches; 
therefore  the  amount  of  water  carried  by  this  soil  expressed 
by  volume  would  be  276  cubic  inches  for  every  cubic 
foot  of  soil.  The  percentage  by  volume  would  equal 
(276  -f-  172S)  X  100,  or  about  16  per  cent.  An  inch  of  water 
covering  the  top  of  a  cubic  foot  weighs  5.2  pounds.  Ob- 
viously the  number  of  surface  inches  which  this  10  pounds 
of  water  would  occupy  if  placed  on  the  top  of  the  cubic 
foot  of  soil  would  be  10  •*■  5.2  or  1.92  surface  inches. 

The  first  method  of  moisture  expression,  as  percentage 
on  a  wet  basis,  is  open  to  two  serious  objections.  In  the 
first  place,  two  different  percentages  of  water  in  different 
samples  of  the  same  soil  do  not  represent  the  same  degrees 
of  wetness  as  are  expressed  by  the  percentages.  For 
example,  100  grams  of  wet  soil  containing  5  per  cent  of 
water  would  consist  of  5  grams  of  water  and  95  grams  of 
soil,  a  ratio  of  1  to  19.  If  the  soil  contained  instead  25 
per  cent  of  water,  the  ratio  would  be  1—3  instead  of  1—3.8, 
as  the  ratio  of  the  percentages  would  naturally  lead  one 
to  expect.  The  second  objection  is  just  as  serious  and 
arises  from  the  fact  that  soils  have  different  apparent 
weights.  For  example,  5  per  cent  of  water  on  the  wet 
basis  for  a  clay  weighing  when  dry  70  pounds  to  the  cubic 
foot  would  equal  3.68  pounds,  while  5  per  cent  on  a  sand 
weighing  100  pounds  would  give  5.26  pounds  of  the  same 
volume.     The  error  of  such  a  method  of  expression  is 


200       SOILS:    PROPEliTIES  AND  MANAGEMENT 

obvious,  not  only  in  comparing  the  water  content  of  the 
same  soil,  but  in  comparing  different  soils  as  well. 

In  using  a  percentage  of  moisture  based  on  the  dry 
soil  instead  of  on  the  wet,  the  first  of  the  above  objections 
is  eliminated.  Consequently  this  method  of  expression 
is  perfectly  legitimate  as  long  as  soils  having  about  the 
same  apparent  specific  gravity  are  compared.  As  soon 
as  soils  of  different  weights  are  considered,  however,  a 
more  nearly  accurate  method  must  be  employed.  Ob- 
viously, then,  the  only  really  rational  mode  of  moisture 
Statement  is  by  the  volume  method.  In  ordinary  calcu- 
lations of  water,  however,  the  percentage  by  dry  weight 
is  generally  used  beeanse  of  its  simplicity  and  the  facility 
of  expression  that  it  affords.  It  is  also  much  easier  to 
establish  than  a  percentage  based  on  volume. 

The  first  and  second  methods  of  volume  expression  are 
of  about  equal  value  as  far  as  direct  comparison  goes.  For 
the  actual  water  present  the  number  of  cubic  inches  to  a 
cubic  foot  of  soil  is  perhaps  preferable,  as  it  shows  the  exact 
amount  of  water  contained  and  may  easily  be  converted 
to  pounds  to  a  cubic  foot  or  tons  to  an  acre  as  the  case 
may  be.  The  third  volume  statement  is  generally  used  in 
field  practice,  especially  in  irrigated  regions,  where  water 
is  measured  in  inches  in  depth  to  an  acre  of  area.  Such 
a  statement  of  the  available  water  in  a  soil  not  only  is 
convenient,  but  also  gives  a  direct  comparison  with  the 
probable  rainfall  of  the  growing  season. 

132.  Kinds  of  water  in  the  soil.  —  As  has  already  been 
demonstrated,  a  soil  of  a  definite  volume  weight  has  a 
definite  pore  space  which  may  be  occupied  by  air  or  by 
water,  or  shared  by  both,  as  the  case  may  be.  Of  course, 
an  ideal  soil  for  plant  growth  is  one  in  which  there  is 
both  air  and  water,  the  proportions  depending  on  the 


THE  FORMS   OF  SOIL    WATER  201 

texture  and  the  structure  of  the  soil  and  the  character 
of  the  crop.  Assuming  for  the  time  being,  however,  that 
the  pore  space  is  entirely  filled  with  water,  or,  in  other 
words,  that  the  soil  is  saturated,  three  forms  of  water 
are  found  to  be  present  —  hygroscopic,  capillary,  and 
free,  or  gravitational.  These  forms  differ,  not  in  their 
composition,  but  in  the  position  that  they  occupy  in  rela- 
tion to  the  soil  particles. 

The  hygroscopic  and  capillary  water  are  both  film 
forms ;  that  is,  they  surround  the  soil  particle,  being  held 
partly  by  the  attraction  of  the  particle  and  partly  by  the 
molecular  attraction  of  the  liquid  for  itself.  The  hygro- 
scopic film  is  very  thin,  being  water  of  condensation,  or 
adsorption.  When  this  film  is  satisfied  and  moisture  is 
still  present,  the  capillary  water  film  begins  to  form.  The 
line  of  demarcation  between  hygroscopic  and  capillary 
water  is  not  sharp.  '  The  general  difference  between  the 
two  forms  may  be  considered  as  being  not  only  one  of 
position,  but  also  one  of  movement,  this  power  being  pos- 
sessed only  by  the  capillary  film.  With  a  change  in  any 
controlling  condition,  such  as  temperature,  hygroscopic 
water  may  change  to  capillary,  or  capillary  water  to 
hygroscopic,  as  the  case  may  be.  As  the  capillary  water 
continues  to  increase  and  the  film  becomes  thicker  and 
thicker,  a  point  is  at  last  reached  at  which  gravity  over- 
comes the  surface  tension  of  the  liquid  and  drops  of  water 
form  which  tend  to  move  downward  through  the  air 
spaces,  being  now  subject  to  movement  by  the  attrac- 
tion of  gravity.  Free,  or  gravitational,  water  then  also 
becomes  present  in  the  soil.  If  water  is  still  added,  the 
gravitational  water  continues  to  increase  until  the  air 
is  almost  entirely  displaced  and  a  saturated  condition 
results.     There  may  be  a  change  of  capillary  to  free  water 


*202       SOILS:    PROPERTIES  AND  MANAGEMENT 

or  of  free  water  to  capillary  with  a  change  of  structure, 
temperature,  or  pressure,  as  was  seen  to  be  the  case  be- 
tween the  hygroscopic  and  capillary  moisture.  The  forms 
of  water  present  in  a  saturated  soil  may  be  conveniently 
represented  by  the  following  diagram :  — 

HYGROSCOPIC!     CAPILLARY     |      FREE 

Fig.  30.  —  Diagram    representing    the  three  forms  of   water  that  may 
be  present  in  a  soil. 

£1.33.  Hygroscopic  water.  —  The  hygroscopic  water 
a  soil  has  been  spoken  of  as  the  water  of  condensation, 
adsorption.  It  is,  however,  quite  distinct  from  water 
condensed  on  a  surface  colder  than  the  atmosphere  in 
which  it  is  placed.  All  bodies  possess  the  power,  to 
a  greater  or  less  degree,  of  adsorbing  water  even  when 
at  the  same  temperature  as  the  air  with  which  they  are 
in  contact,  provided,  of  course,  that  the  air  contains  water 
vapor.  The  hygroscopic  film  may  be  continuous  or  only 
partly  continuous,  depending  on  the  condition  of  the 
surface.  In  fact,  the  movement  of  water  over  surfaces 
is  often  greatly  facilitated  by  an  already  existing  hygro- 
scopic film.  External  conditions  being  constant,  the 
amount  of  hygroscopic  water  of  various  materials  is 
determined  by  two  factors:  (1)  the  characteristics  of  the 
material  itself,  and  (2)  the  amount  of  surface  it  exposes. 

It  is  a  well-known  fact  that  various  materials  differ 
in  the  amount  of  hygroscopic  water  they  will  hold,  due 
to  the  attraction  of  the  substances  themselves  for  water. 
The  differences  in  the  thickness  of  the  film  is  so  slightly 
altered,  however,  by  differences  in  materials,  that,  other 
factors  being  constant,  the  hygroscopic  water  becomes 
a   function   almost   entirely   of   surface.     Glass   becomes 


THE  FORMS   OF  SOIL    WATER 


203 


far  more  hygroscopic  when  pulverized.  Porous  bodies 
are  especially  high  in  hygroscopic  water,  sometimes 
holding  as  much  as  20  to  30  per  cent  of  moisture.  The 
following  data,  drawn  from  Ammon  x  and  von  Dobeneck,2 
although  no  doubt  faulty,  illustrate  the  differences  in 
hygroscopicity  of  materials  commonly  found  in  soils 
and  make  plain  the  complexity  of  the  question  when 
applied  to  soil  phases  :  — 

Percentage  of  Hygroscopicity  of  Different  Substances 
at  20°  C.  when  Exposed  for  One  Day  to  Saturated 
Air 


Ammon 

Von  Dobeneck 

Humus 

15.96 

18.04 

Ferric  oxide 

19.76 

20.41 

Kaolin 

.47 

3.55 

Limestone 

.29 

.32 

Quartz       

.07 

.17 

One  of  the  characteristics  peculiar  to  colloids  in  partic- 
ular is  a  high  adsorptive  power  for  moisture,  this  giving 
them  properties  not  usually  possessed  by  crystalloids. 
Gelatinous  precipitates  of  silica,  ferric  oxide,  and  alumin- 
ium oxide  are  good  examples.  Colloidal  humus,  gela- 
tin, and  agar  are  noted  for  their  adsorptive  powers.  The 
water  in  such  cases  is  not  simply  adsorbed  oh  the  external 


1  Ammon,  Georg.  Untersuchungen  iiber  das  Condensa- 
tionsvermogen  der  Bodenconstituenten  fur  Gase.  Forsch. 
a.  d.  Gebiete  d.  Agri.-Physik,  Band  II,  Seite  1-46.     1879. 

2  Dobeneck,  A.  F.  von.  Untersuchungen  iiber  das  Absorp- 
tionsvermogen  und  die  Hygroskopizitat  der  Bodenkonstitu- 
enten.  Forsch.  a.  d.  Gebiete  d.  Agri.-Physik,  Band  XV, 
Seite  163-228.     1892. 


204       SOILS:    PROPERTIES  AND  MANAGEMENT 

expanses,  but  is  distributed  over  the  great  internal  sur- 
face exposm*.  Such  water  cannot  be  expelled  by  ordi- 
nary drying,  but  the  material  must  be  subjected  to  a  high 
heat  in  order  to  drive  oft'  even  a  part  of  the  water  so  held. 
The  qUMtiotl  is  greatly  complicated  also  by  the  fact  that 
some  bodies  have  a  chemical  affinity  for  water.  This 
results  in  the  formation  of  hydrates  and  other  salts.  Such 
water  cannot  be  expelled  without  the  breaking-up  of  the 
compounds. 

Ordinary  soil  possesses  to  an  extraordinary  degree 
the  three  characteristics  already  cited  :  that  is,  it  exposes 
a  very  large  amount  of  free  surface ;  it  tends  to  generate 
continuously  large  amounts*  of  colloidal  material  such  as 
ferric  hydrate,  aluminium  hydrate,  silicic  acid,  and  espe- 
cially hninic  materials  in  a  colloidal  state;  and  it  always 
has  present  compounds  having  an  affinity  for  water. 
However,  since  these  compounds  are  easily  satisfied,  and 
also  since  the  adsorptive  power  of  colloids  is  due  to  the 
surface  exposed,  it  may  be  considered  that,  other  condi- 
tions being  equal,  the  hygroscopicity  of  the  soil  is  essen- 
tially a  surface  phenomenon.  Although  for  all  practical 
purposes  hygroscopicity  may  be  considered  as  having 
special  relation  to  surface,  exact  correlation  is  not  easy 
partly  because  of  the  difficulty  of  accurately  determining 
the  surface  exposed  by  a  normal  soil. 

134.  Effect  of  texture  and  humus  on  hygroscopicity.  — 
The  question  being  thus  reduced  to  a  surface  consideration, 
it  is  evident  that  the  texture  of  the  soil,  external  factors 
being  under  control,  is  the  determining  factor.  The  fol- 
lowing figures  from  Loughridge,1  by  whom  the  hygroscopic 

1  Loughridge,  R.  H.  Investigations  in  Soil  Physics. 
California  Agri.  Exp.  Sta.,  Rept.  of  Work  of  the  Agri.  Exp. 
Stations  of  California  for  1892-3-4,  pp.  76-77. 


THE  FORMS   OF  SOIL    WATER 


205 


moisture  was  determined  by  exposing  the  air-dry  soil  at 
15°  C.  to  a  saturated  atmosphere  and  then  drying  at  200° 
C,  illustrate  this  point :  — 

Hygroscopic  Capacity  of  Various  Soils 


Soils 


Per  cent  Clay 
Material  Remain- 
ing in  Suspension 

after  Standing 
for  24  Hours 


Hygroscopic 

Water  Expressed 

in  Percentage 


15  clays      .     . 
7  clay  loams 
9  loams    .     . 
4  sandy  loams « 
4  sands     .     . 


31.97 

17.15 

12.06 

7.39 

2.93 


10.45 
6.06 
5.18 
2.50 
2.21 


Apparently,  the  finer  the  soil,  the  greater  is  the  hygro- 
scopicity.  The  finer  the  soil,  the  higher  also  is  the  per- 
centage of  clay,  and  consequently  the  greater  is  the  amount 
of  material  likely  to  be  present  in  a  colloidal  state.  As  a 
matter  of  fact,  the  hygroscopic  moisture  as  shown  above 
is  roughly  proportional  to  the  clay;  and  as  clay,  espe- 
cially the  finer  forms,  is  largely  colloidal  in  nature,  the 
colloidal  content  of  a  soil  practically  determines  the  hygro- 
scopic content.  This  fact  is  the  basis  for  Mitscherlich's  * 
method  of  colloid  estimation,  in  which  hygroscopic  mois- 
ture determined  under  certain  controlled  conditions  is 
used  as  a  relative  measure  of  colloidal  content.  The  vari- 
ous grades  of  particles  constituting  the  textural  make-up 
of  a  soil,  then,  do  not  possess  the  same  weight  in  the  deter- 
mination of  hygroscopicity,  the  dominant  grade  being 
clay,  especially  that  part  which  has,  by  either  physical 


1  Mitscherlich,  E.  A.     This  text,  paragraph  111. 


206       SOILS:    PROPERTIES  AND  MANAGEMENT 

or  chemical  means  or  both,  been  thrown  into  a  colloidal 
condition.  Especially  do  the  humous  colloids,  as  has 
already  been  shown,  function  in  this  regard,  so  that  the 
organic  matter  must  be  of  very  great  importance  in  deter- 
mining the  hygroscopic  capacity  of  any  soil.  The  finer 
the  soil,  the  greater  is  the  amount  of  hygroscopic  water 
merely  because  of  the  large  area  of  surface  exposed. 
Also,  any  practice  that  will  increase  the  colloidal  material 
—  the  humous  colloids  being  very  susceptible  to  increase 
by  proper  soil  management  —  the  higher  will  be  the  per- 
centage of  this  hygroscopic  moisture.  Texture  and  humus, 
them^goyern  the  hvgroscopicity  of  most  soflsT"^ 

135.  Nature  oTthe^Im.^^Ke"  nature  of  this  thin  film 
which  is  designated  as  hygroscopic  water  has  not  as  yet 
been  determined.  Held  so  strongly  by  a  molecular  force 
averaging  probably  10,000  atmospheres,  generated  by 
adhesion  and  cohesion,  it  is  not  definitely  known  whether 
the  film  exists  as  a  liquid  or  a  vapor.  Consequently  it 
cannot  be  expected  to  conform  to  the  laws  that  are  gen- 
erally found  to  apply  to  capillary  films.  In  many  cases 
the  film  may  not  be  continuous,  and  being  so  very,  very 
thin,  it  may  even  possess  a  negative  surface  tension.  The 
radius  of  influence  of  a  particle  in  water  has  been  shown 
by  Chamberlain1  to  be  about  1.5  X  10-7  centimeters. 
Within  this  zone  the  molecules  of  water  are  much  restricted 
in  their  motions.  The  thickness  of  the  hygroscopic  film  on 
quartz  particles  as  calculated  by  Briggs  2  is  2.66  X  10-6 
centimeters,  showing  that  the  outer  edge  of  the  hygroscopic 


1  Chamberlain,  C.  W.  The  Radius  of  Molecular  Attrac- 
tion.    Physical  Review,  Vol.  31,  pp.  170-182.     1910. 

2  Briggs,  L.  J.  On  the  Adsorption  of  Water  Vapor  and  of 
Certain  Salts  in  Aqueous  Solution  by  Quartz.  Jour.  Phys. 
Chem.,  Vol.  9,  pp.  617-641.     1905. 


THE  FORMS   OF  SOIL    WATER 


207 


film,  where  the  water  to  a  large  extent  loses  its  movement, 
is  considerably  without  this  zone  of  influence.  In  order 
to  give  some  idea  of  the  extreme  minuteness  of  the  hygro- 
scopic film,  it  may  be  said  that  its  thickness  is  less  than 
the  diameter  of  the  smallest  known  soil  bacteria.  In 
moving  from  the  surface  of  a  particle  outward  through 
an  ordinary  water  film,  passage  is  first  made  through  the 
zone  of  influence.  When  the  edge  of  this  is  reached,  an 
area  is  passed  through  which  continues  with  constantly 
increasing  capacity  for  molecular  motion  until  the  outer 
edge  of  the  hygroscopic  film  is  crossed,  where  molecular 
activity  reaches  its  maximum. 

136.  Effect  of  humidity  and  temperature  on  hygro- 
scopic water.  —  Two  external  conditions  seem  to  affect 
the  amounts  of  hygroscopic  water  that  a  soil  may  hold 
under  definite  conditions  —  humidity  and  temperature. 
As  a  general  rule,  the  higher  the  humidity,  the  higher  is 
the  hygroscopic  moisture.  The  experiments  of  von  Dobe- 
neck  1  with  quartz  and  humus  illustrate  this  point :  — 

Percentage  of  Hygroscopic  Water  held  at  Various  Humid- 
ities    AFTER     AN     EXPOSURE      OF     TWENTY-FOUR     HOURS      AT 

20°  C. 


Quartz 
Humus 


30 

Per  cent 

.045 
4.055 


50 
Per  cent 

.053 

7.765 


70 
Per  cent 

.076 
10.589 


90 
Per  cent 

.119 
15.676 


100 
Per  cent 

.175 
18.014 


The  results  as  to  the  effects  of  a  rise  in  temperature 
on  the  hygroscopic  film   are  not  so  definite.     Most  in- 


1  Dobeneck,  A.  F.  von.  Untersuchungen  uber  das  Absorp- 
tionsvermogen  und  die  Hygroskopizitat  der  Bodenkonstitu- 
enten.  Forsch.  a.  d.  Gebiete  d.  Agri.-Physik,  Band  XV,  Seite 
163-228.     1892. 


208       SOILS:    PROPERTIES  AND  MANAGEMENT 

vestigators ]  find  that  as  the  temperature  is  increased 
the  hygroscopicity  becomes  lowered,  thus  following  the 
general  laws  of  adsorption.  Hilgard,  however,  obtained 
opposite1  results  when  the  air  was  saturated,  although  his 
data  agreed  with  previous  results  when  hygroscopicity 
was  studied  in  an  atmosphere  unsatisfied  as  to  its  capac- 
ity for  water  vapor.  King 2  explains  this  discrepancy 
as  being  due  to  the  very  high  vapor  pressure  generated 
by  a  saturated  atmosphere  at  high  temperatures,  causing 
a  more  rapid  taking-up  of  water  by  the  soil  than  was 
lost  from  its  surface.  The  time  necessary  for  a  soil  to 
assume  its  maximum  thickness  of  adsorbed  water  is  un- 
certain. Hilgard3  used  seven  hours  in  his  determina- 
tions, while  Mitselierlich *  exposed  his  soil  for  several 
days.  A  soil  continues  to  increase  in  weight  slowly  as 
its  time  of  exposure  to  moist  air  is  increased,  so  that  a 
sharp  line  of  demarcation  between  capillary  and  hygro- 
scopic water  is  difficult  to  establish.  Capillary  water 
may  even  be  present  in  the  minute  interstices  before  the 
hygroscopic  film  is  elsewhere  satisfied.5 

137.  Determination  of  hygroscopicity.  —  The  method 
of  the  determination  of  the  maximum  hygroscopicity  of  a 
soil,  or,  in  other  words,  the  hygroscopic  coefficient,  is 
simple  in  outline.     The  soil,  in  a  thin  layer,  is  exposed 


1  Patten,  H.  E.,  and  Gallagher,  F.  E.  Adsorption  of  Vapors 
and  Gases  by  Soils.  U.  S.  D.  A.,  Bur.  Soils,  Bui.  51,  p.  33. 
1908. 

2  King,  F.  H.  Physics  of  Agriculture,  pp.  179-180. 
Published  by  the  author,  Madison,  Wisconsin,  1910. 

3  Hilgard,  E.  W.     Soils,  pp.  196-201.     New  York.     1911. 

4  Mitscherlich,  E.  A.  Bodenkunde,  pp.  56-58.  Paul 
Parey,  Berlin.     1905. 

5  Briggs,  L.  J.  The  Mechanics  of  Soil  Moisture.  U.  S. 
D.  A.,  Bur.  Soils,  Bui.  10,  p.  12.     1897. 


THE  FORMS   OF  SOIL    WATER  209 

to  an  atmosphere  of  definite  humidity  under  conditions 
of  constant  temperature  and  pressure.  Complications 
arise  from  the  necessity  of  using  a  very  thin  layer  of  soil, 
from  the  difficulty  of  controlling  humidity,  and  from  the 
tendency  of  capillary  water  to  form  in  the  soil  interstices 
before  the  hygroscopic  film  is  satisfied.  The  question  of 
how  long  the  exposure  should  take  place  is  a  very  serious 
factor,  as  has  already  been  pointed  out.  In  the  drying 
of  the  soil  after  exposure  a  vexnjg^condition  also  is  en- 
countered, in  that  as  the  temperature  is  raised,  the  giving- 
off  of  water  vapor  continues.  It  is  evident,  therefore, 
that  not  only  must  any  method  be  more  or  less  arbitrary, 
but  that  its  value  can  be  only  comparative.  The  method 
of  Mitscherlich,  as  already  described,1  is  probably  the 
most  nearly  accurate.  He  exposes  the  dry  soil  under 
partial  vacuum  over  10  per  cent  sulfuric  acid  and  water. 
The  partial  vacuum  is  to  hasten  adsorption,  and  the  acid 
to  prevent  a  fully  saturated  air,  thereby  cutting  down 
chances  of  dew  deposition. 

138.  Heat  of  condensation.  —  The  amount  of  energy 
necessary  to  expel  the  hygroscopic  film  from  around  a 
soil  particle  is  very  great,  since  its  only  movement  is 
thermal.  As  a  matter  of  fact,  it  is  really  impossible  to 
divest  the  soil  grain  entirely  without  causing  the  loss  of 
moisture  other  than  that  simply  adsorbed.  As  so  much 
energy  is  expended  in  removing  this  film,  it  is  reasonable 
to  expect  that  a  certain  amount  of  heat  of  condensation 
when  the  film  is  resumed  would  become  apparent.  Pat- 
ten2 offers  the  following  quantitative  data  concerning 
this  point :  — 

' 1  Mitscherlich,  A.  E.     This  text,  paragraph  111. 
2  Patten,  H.  E.     Heat  Transference  in  Soils.     U.  S.  D.  A., 
Bur.  Soils,  Bui.  59,  p.  34.     1909. 


210      SOILS:    PROPERTIES  AND  MANAGEMENT 
Heat  Evolved  by  Wetting  Soils  Dried  at   110°  C. 


Soil 

Calories  per 
Kilo  of  Dry  Soil 

Coarse  quartz        

150 

Podunk  fine  sandy  loam 

Norfolk  sand 

200 
347 

Hagerstown  loam 

1108 

Galveston  clay 

3785 

Muck  soil  (25  per  cent  organic  matter)     .     .     . 

6413 

139.  Capillary  water.  —  It  has  been  shown  in  the  pre- 
vious discussion  that  a  large  proportion  of  the  hygroscopic 
film  is  beyond  the  radius  of  influence  of  the  particle  and 
is  not  held  so  rigidly  as  is  the  inner  portion.  In  other 
words,  in  this  film  a  certain  amount  of  molecular  move- 
ment is  possible,  this  movement  depending  on  the  dis- 
tance from  the  particle.  As  soon,  how- 
ever, as  the  boundary  of  the  hygroscopic 
film  is  crossed,  a  comparatively  thick 
film  of  moisture  is  reached  in  which 
molecular  movement,  except  for  the 
influence  of  viscosity,  is  perfectly  free 
and  unimpeded.  These  two  zones  (see 
Fig.  31) — one  in  which  capillary  move- 
ment is  more  or  less  free,  and  a  com- 
paratively thin  film  in  which  molecular 
movement  becomes  increasingly  slug- 
gish as  the  radius  of  influence  of  the 
soil  grain  is  approached  —  are  there- 
differentiated.     The   capillary   water  differs 


Fig.  31.  —  Diagram 
showing  the  rela- 
tionship of  the 
hygroscopic  and 
capillary  water 
films  surrounding 
a  soil  particle, 
(s) ,  particle; 
(0,  zone  of  influ- 
ence of  particle  ; 
(w),  outer  edge  of 
hygroscopic  zone ; 
(c),  capillary  film. 


fore  clearly 

from  the  hygroscopic  moisture  (l)  in  that  it  is  largely  in 
a  liquid  state  and  consequently  is  governed  by  the  ordi- 
nary laws  of  liquids ;  (2)  in  that  it  evaporates  at  ordinary 


THE  FORMS   OF  SOIL    WATER  211 

temperatures,  being  held  with  less  tenacity ;  and  (3)  in 
that  it  has  the  power  of  movement  from  place  to  place 
within  the  film,  hence  the  name  capillary  water. 

140.  Surface  tension  and  the  force  developed  thereby. 
—  The  power  that  tends  to  hold  this  capillary  water  in 
place  against  the  force  of  gravity,  a  constant,  depends 
on  the  surface  tension  of  the  liquid.  This  phenomenon 
of  surface  tension  is  due  to  the  existence  of  certain  molec- 
ular forces  acting  from  within.  In  a  drop  of  water,  for 
example,  the  particles  are  attracted  equally  in  all  direc- 
tions and  consequently  are  able  to  move  with  perfect 
freedom.  The  molecules  on  the  surface  of  the  drop, 
however,  are  not  in  such  an  equilibrium  of  attraction, 
since  the  pull  of  the  water  particles  within  is  greater  than 
that  of  the  air  particles  without.  The  resultant  attrac- 
tion is  therefore  inward,  and  is  directed  along  a  line  per- 
pendicular to  the  surface  at  that  point.  The  result  is 
the  development  of  a  more  or  less  ideal  membrane,  the 
effective  force  of  which  is  not  affected  by  the  amount  of 
the  surface,  but  by  the  curvature.  In  a  sphere  the  force 
or  pressure  developed  by  surface  tension  is  equal  to  twice 
the  surface  tension  divided  by  the  radius.  This  increase 
of  the  effective  force  by  curvature  of  film  is  very  impor- 
tant as  regards  soil  water,  since,  as  will  be  shown  later,  it 
governs  the  movement  of  capillary  water  from  one  particle 
to  another,  the  direction  of  the  movement  being  deter- 
mined by  a  difference  in  pressure  as  developed  by  un- 
equal curvatures  of  film   surfaces. 

As  a  result  of-  this  force  developed  by  surface  tension, 
the  water  film  around  a  soil  particle  tends  to  equalize 
itself  until  this  pressure  is  everywhere  the  same.  On 
this  force  depends  also  the  thickness  of  the  capillary  film. 
Under  any  given  condition  this  capillary  film  will  con- 


212       SOILS:    PROPERTIES  AND  MANAGEMENT 


tinue  to  thicken  until  the  mass  of  the  water  is  so  great 
as  to  allow  gravity  to  come  into  play  and  pull  enough 
water  away  to  again  restore  the  equilibrium.  The  soil 
particle  would  at  this  point  he  maintaining  its  maximum 
thickness  of  capillary  film.  It  is  also  quite  evident  that 
as  the  capillary  Him  is  thinned  —  as,  for  example,  by 
evaporation  —  the  force  developed  by  surface  tension 
would  be  increased,  due  to  increased  curvature  of  the 
film,  and  the  difficulty  of  removing  the  external  layers 
of  the  film  would  naturally  become  greater. 

141.  The  form  of  water  surfaces  between  soil  particles. 
—  In  the  case  of  a  soil,  however,  the  question  of  the 
capillary  film  becomes  more  complex,  since  a  great  num- 
ber of  different-sized  particles  are  present  in  more  or  less 
close  contact  with  one  another.  This  means  that  under 
normal  soil  conditions  the  capillary  film  is  continuous 
from  one  particle  to  another  —  a  very  different  question 
to  consider  from  that  of  a  film  about  a  single  isolated  soil 

grain  more  or  less 
spherical  in  shape. 
Suppose,  for  example, 
that  two  particles, 
each  earning  a  capil- 
lary water  film,  be 
brought  into  such 
contact  that  the  films 
coalesce.  There  are 
now  two  distinct  sur- 
faces—  that  at  A,  A'  (see  Fig.  32),  with  the  curvature 
of  the  original  film,  and  that  at  B,  which  is  very  acute 
and  which  naturally  must  exert  a  very  great  outw7ard 
pull.  Under  the  stress  of  this  pull  developed  by  the 
surface  tension  acting  in  this  film  of  very  great  curvature, 


Fit;.  32.  Diagram  showing  the  coalescence 
aw)  readjustment  to  the  capillary  film  of 
two  soil  particles  when  brought  in  con- 
tact. At  left  is  shown  the  condition  be- 
fore adjustment  with  a  sharp  angle  at  B ; 
at  right  the  films  are  shown  in  equi- 
librium with  a  great  thickening  at  B. 


THE  FORMS   OF  SOIL    WATER  213 

the  water  is  drawn  into  the  space  between  the  particles, 
where  it  becomes  thicker  than  the  capillary  film  about 
the  particles.  This  readjustment  continues  until  the 
forces  developed  by  the  two  films  become  equal.  An 
equilibrium  is  now  established.  It  is  evident,  then,  that 
as  the  capillary  water  becomes  less  in  a  soil  from  any 
cause,  the  moisture  collected  in  the  spaces  between  the 
particles  becomes  less  and  less,  but  still  remains  thicker 
than  the  films  about  the  particles  themselves.  What 
percentage  of  the  capillary  water  is  held  in  the  thickened 
waists  of  the  soil  grains  cannot  be  calculated,  but  it  is 
probable  that  this  moisture  makes  up  the  major  part 
of  the  capillary  water  of  any  soil.  One  of  the  errors 
in  the  determination  of  the  hygroscopic  coefficient  of  a 
soil,  as  already  pointed  out,  arises  from  the  tendency 
toward  the  formation  of  capillary  water  in  these  angles 
between  the  soil  particles  before  the  hygroscopic  film  on 
the  grains  themselves  becomes  satisfied. 

142.  Factors  affecting  amount  of  capillary  water.  — 
As  might  naturally  be  expected,  the  factors  that  tend  to 
vary  the  amount  of  capillary  water  in  a  soil  are  several, 
and  their  study  is  more  or  less  complex,  due  to  the  second- 
ary influences  that  they  may  generate.  These  factors 
may  be  discussed  under  four  heads :  (1)  surface  tension, 
(2)  texture,  (3)  structure,  and  (4)  organic  matter. 

143.  Surface  tension  and  the  amount  of  capillary 
water.  —  Any  condition  that  will  influence  surface  ten- 
sion will  obviously  influence  the  thickness  of  the  capillary 
film,  because  of  a  variation  in  the  forces  thereby  de- 
veloped. A  rise  in  temperature,  by  lowering  the  surface 
tension,  would  consequently  lower  the  capillary  capacity 
of  the  soil,  and  if  the  soil  were  capillarily  saturated  would 
allow  some  of  the  water  to  become  gravitational  in  its 


214     soils:  properties  and  management 

nature.  A  lowering  of  the  temperature  would  eause  a 
change  in  the  opposite  direction.  This  theory  lias  been 
verified  by  certain  experiments  by  King,1  in  which  he 
found,  other  conditions  being  constant,  a  very  decided 
influence  on  capillary  water  through  change  of  tempera- 
ture. Wollny  ■  has  shown  that  a  depression  of  from  .65 
per  cent  in  sand  to  as  high  as  3.7  per  cent  in  kaolin  may 
occur  from  a  rise  in  temperature  of  twenty  degrees.  The 
surface  tension  of  a  liquid  may  also  he  greatly  changed 
by  the  addition  of  salts,  and,  since  the  soil  always  carries 
some  material  in  solution,  the  surface  tension,  and  conse- 
quently the  capillary  capacity,  might  be  expected  to 
increase.  As  a  matter  of  fact,  the  soil  solution  is  very 
dilute,  and  even  if  large  amounts  of  fertilizer  salts  were 
added  the  adsorptive  power  of  the  soil  would  tend  to 
maintain  a  very  dilute  soil  water  at  the  surface  of  the 
films.  Again,  as  humus  decay  is  continuously  going  on, 
oily  materials  are  probably  produced  which  would  tend 
to  spread  over  the  capillary  films  and  greatly  reduce  their 
surface  tension.  Therefore,  as  far  as  is  now  known  of  the 
two  varying  influences,  temperature  change  is  by  far  the 
most  potent  in  its  influence  on  capillary  capacity. 

144.  Texture  and  the  amount  of  capillary  water.  — 
The  finer  the  texture  of  a  soil,  the  greater  is  the  number 
of  angles  between  the  particles  in  which  a  film  of  capillary 
water  may  be  held ;  also,  the  actual  amount  of  surface 
exposed  by  the  particles  is  immensely  larger  than  in  a 


1  King,  F.  H.  Fluctuations  in  the  Level  and  Rate  of  Move- 
ment of  Ground  Water.  U.  S.  D.  A.,  Weather  Bur.,  Bui.  5, 
pp.  59-61.     1892. 

2  Wollny,  E.  Untersuchungen  iiber  die  Wasserkopacitat  der 
Bodenarten.  Forsch.  a.  d.  Gebiete  der  Agri.-Physik,  Band  9, 
Seite  361-378.     1886. 


THE  FORMS   OF  SOIL    WATER 


215 


coarse  soil.  Due  to  these  two  conditions,  a  soil  of  fine 
texture  will  contain  considerably  more  capillary  water 
than  one  of  which  the  texture  is  coarse.  The  maximum 
capillary  capacity  of  a  soil  is  not  directly  proportional  to 
the  surface,  as  was  roughly  proved  to  be  the  case  with 
the  hygroscopic  coefficient.  This  is  probably  because 
the  angle  exposures  between  the  grains  increase  in  number 
as  the  texture  becomes  finer  much  faster  than  the  actual 
surfaces  developed  by  the  particles  are  generated.  The 
capillary  water  in  any  soil  varies  with  the  height  of  the 
column.  This  comes  about  from  the  gravity  effects 
on  the  liquid  surrounding  the  particle.  If  the  liquid  had 
no  weight,  gravity  would  not  be  a  factor  and  the  same 
thickness  of  film  would  be  found  at 
any  point  in  a  soil  column.  Such  a 
condition  would  greatly  simplify  the 
study  of  soil  moisture.  If  a  number  of 
particles  (see  Fig.  33)  carrying  maxi- 
mum capillary  films  are  brought  together 
vertically,  the  weight  of  the  whole  con- 
ducting film  is  thrown  momentarily  on 
the  capillary  surfaces  at  the  top.  The 
capillary  spaces  at  this  point  immediately 
lose  water  downward,  so  that  they  may 
assume  a  greater  curvature  and  thus 
support  this  extra  weight  thrown  on 
them.  This  curvature  must  be  sufficient 
to  balance  the  curvature  pressure  of  the 
particles  below  plus  the  weight  of  the 
water  in  the  connecting  films.  The  par- 
ticles beneath  are  at  the  same  time  un- 
dergoing a  similar  adjustment  with  a  set  of  particles  still 
farther  below,  losing  water  in  order  to  allow  a  change  of 


Fig.  33.  —  Diagram 
showing  the  ad- 
justment of  the 
capillary  film  in  a 
long  column  and 
the  appearance  of 
free  or  gravita- 
tional water  if  the 
weight  is  too 
great  for  the  sup- 
porting films. 


216       SOILS:    PROPERTIES  AND  MANAGEMENT 


curvature.  A  thinning  of  these  films  results,  but  not  to 
such  an  extent  as  in  the  particles  above.  The  action 
continues  in  this  manner  through  each  capillary  surface 
until  equilibrium  is  established,  the  change  in  thickness 
of  film  being  less  and  less  in  each  case  due  to  the  cumu- 
lative support  of  the  films  above.  If  the  amount  of 
capillary  water  present  is  too  great  to  be  supported  by 
the  films,  enough  is  lost  by  gravity  at  the  bottom  to 
bring  about  an  equilibrium.  The  film  is  at  its  maximum 
at  the  bottom  of  the  column,  but  decreases  in  thickness 
as  the  column  is  ascended,  not  only  on  the  particles 
themselves,  but  in  the  angle  interstices  as  well.  This  is 
necessary,  as  each  successive  film  must  support  an  in- 
creased weight  of  water.  It  is,  therefore,  evident  that 
it  is  impossible  to  assign  any  definite  figure  as  to  the 
capillary  water  capacity  of  a  soil.  Only  relative  or 
comparative  data  may  be  quoted.     The  following  diagram 


>u 

44      \ 

''"•'.. 

CLAY 

3 

8S 

1 

\ 

\ 

\ 

1 
30                  \ 

SANDY      '•- 

1 

>• 

\ 

20 

^LOAM 

Hi 

SA/VO 

_____ 

\- 

- 

P 

0                            A 

r              * 

z 

S                        J 

OC/otYATCR. 

Fig.  34.  —  Diagram  showing  the  distribution  of  moisture  in  capillary 
columns  of  soil  of  different  textures.  The  end  of  each  soil  column 
rests  in  free  water. 


THE   FORMS   OF  SOIL    WATER  217 

(see  Fig.  34)  from  Buckingham  l  makes  clear  not  only 
the  influence  of  texture  on  capillary  water,  but  also  the 
distribution  of  water  in  a  capillary  column. 

The  final  mean  water  content  of  these  soils  was  10, 
15,  and  20  per  cent,  respectively,  for  the  fine  sand,  the 
sandy  loam,  and  the  clay ;  showing  that  as  the  texture 
becomes  finer,  the  greater  is  the  average  capillary  content 
even  after  allowing  for  the  differences  in  hygroscopic 
moisture. 

145.  Effect  of  structure  on  the  amount  of  capillary 
moisture.  —  The  structure  of  the  soil,  or,  in  other  words, 
the  arrangement  of  the  particles,  will  become  a  factor  in 
capillary  capacity  in  so  far  as  it  affects  the  amount  of 
effective  capillary  surface.  Any  arrangement  of  parti- 
cles that  will  increase  the  number  of  angles  of  contact  will 
evidently  increase  the  amount  of  capillary  water.  The 
compacting  of  a  loose  soil  will  increase  the  possible  capil- 
lary moisture  until  all  the  interstitial  space  becomes 
capillary  in  its  nature;  further  compacting  will  then 
cause  a  marked  decrease.  The  granulation  of  a  clay  soil, 
by  producing  a  crumb  structure  and  by  actually  increas- 
ing the  effective  surface  exposure,  tends  to  increase  its 
water-holding  capacity.  At  the  same  time  the  compacting 
of  a  sand,  by  increasing  not  only  the  actual  effective  sur- 
face, but  also  the  number  of  angles  possible  for  capillary 
concentration,  will  cause  a  rise  in  the  capillary  capacity 
of  that  soil. 

In  a  study  of  this  kind  it  is  very  evident  that  the  aver- 
age data  of  a  long  column  should  be  considered,  since 
the  percentage  of  moisture  at  any  one  point  is  not  in- 
dicative of  the  true  capillary  capacity  of  a  soil.      Such 

1  Buckingham,  E.  Studies  on  the  Movement  of  Soil 
Moisture.     U.  S.  D.  A.,  Bur.  Soils,  Bui.  38,  p.  32.     1907. 


218       SOILS:    PROPERTIES  AND  MANAGEMENT 

figures  have  been  obtained  by  Buckingham  l  in  his  study  of 
loose  and  compact  soils.  The  following  curves  repre- 
sent the  general  trend  of  his  results  :  — 


30            \, 

V^ 

1 
I 
| 

pa 

\ 

10 

SS<:^a^ 

\ 

V 

0 

JO  °7bWAr£G 


Fig.  35.  —  Diagram  showing  the  effect  of  a  compaction  upon  the  distri- 
bution of  moisture  in  capillary  columns.  (L),  loose  sandy  loam; 
(Z/),  compact  sandy  loam;  (C),  compact  clay;  (C')(  loose  clay. 


While  it  is  evident  that  the  mean  water  content  of  the 
compact  sandy  loam  is  greater  than  that  of  the  less  com- 
pact, the  latter  showed  a  higher  percentage  of  moisture 
up  to  about  the  tenth  inch.  The  clay  shows  a  more 
marked  effect  from  compacting,  dropping  in  the  compact 
sample  almost  as  low  as  the  sand,  on  the  average,  and 
showing  at  about  ten  inches  from  the  end  of  the  column 
a  percentage  of  moisture  considerably  below  that  of  either 
the  loose  or  the  compact  sand.  It  is  obvious  that  the 
farmer  may  do  much  in  the  control  of  capillary  water  by 
promoting  a  proper  physical  condition  of  his  soil. 

146.  Organic  matter  and  the  amount  of  capillary  mois- 
ture. —  Organic  matter,  especially  when  it  has  been 
reduced  to  the  form  of  humus,  has  great  capillary  capac- 
ity, far  excelling  in  this  regard  the  mineral  constituents  of 
the  soil.     Its  porosity  affords  an  enormous  internal  sur- 

1  Buckingham,  E.  Studies  on  the  Movement  of  Soil  Mois- 
ture.    U.  S.  D.  A.,  Bur.  Soils,  Bui.  38,  pp.  34-35.     1907. 


THE  FORMS   OF  SOIL    WATER  219 

face,  while  its  colloids  exert  an  affinity  for  moisture  which 
raises  its  water  capacity  to  a  very  high  degree.  Its  ten- 
dency to  swelf  on  wetting  is  but  a  change  in  condition 
incident  to  an  approach  to  its  maximum  moisture  con- 
tent. The  following  data,  taken  from  a  compilation  by 
Storer,1  give  an  idea  of  the  capillary  capacity  of  the  soil 
organic  matter :  — 

Percentage 
of  water 

1.  Humous  extract  from  peat 1200 

2.  Non-acid  extract  from  peat        .     .     .  .  .     .  645 

3.  Vegetable  mold        309 

4.  Peat       190 

5.  Garden  loam,  7  per  cent  humus     ....  96 

6.  Illinois  prairie  soil 57 

7.  Field  loam,  3.4  per  cent  humus      ....  52 

8.  Mountain  valley  loam,  1.2  per  cent  humus  .  47 

Even  after  allowance  has  been  made  for  the  increased 
hygroscopic  coefficient  incident  to  an  increase  in  organic 
matter,  the  effect  of  the  latter  is  very  strongly  evident 
on  the  capillary  capacity  of  a  soil.  Besides  this  direct 
effect,  organic  matter  exerts  a  stimulus  toward  better 
granulation,  a  condition  in  itself  favorable  to  increased 
water-holding  power. 

147.  Determination  of  capillary  water.  —  The  capillary 
water  in  a  sample  of  field  soil  may  be  determined  by  mak- 
ing a  moisture  test  in  the  ordinary  way  for  the  total  water 
contained.  This  represents  the  hygroscopic  plus  the 
capillary  water.  A  determination  of  the  hygroscopic 
coefficient  on  another  sample  yields  a  figure  which  when 

storer,  F.  H.  Agriculture,  Vol.  I,  p.  106.  New  York. 
1910. 


220      SOILS:    PROPERTIES  AND  MANAGEMENT 

subtracted  from  the  total  water  will  give  the  capillary 
water  present  in  the  sample.  The  capillary  water  at 
various  points  in  a  soil  column  may  be  obtained  by  sub- 
tracting the  hygroscopic  coefficient  from  the  various 
percentages  of  moisture  present,  since  the  thin  hygroscopic 
film  is  not  influenced  by  height  of  column  or  ordinary 
structural  conditions.  In  ordinary  soils,  however,  the 
differences  in  hygroscopicity  are  not  so  great  but  that 
the  total  water  retained  in  a  soil  column  against  gravity 
serves  as  a  very  good  measure  of  relative  capillary 
capacity. 

148.  The  moisture  equivalent  of  soils.  —  Briggs  and 
McLane l  have  perfected  a  method  of  comparing  soils 
on  the  basis  of  their  capacity  to  hold  water  against  a 
definite  and  constant  centrifugal  force  of  one  to  three 
thousand  times  the  force  of  gravity.  The  soils,  in  thin 
layer,  are  placed  in  perforated  brass  cups  which  fit  into 
a  centrifugal  machine  capable  of  developing  the  above 
force,  and  are  whirled  until  equilibrium  is  reached.  The 
resultant  moisture  percentage  is  designated  as  the  mois- 
ture equivalent.  It  really  represents  the  capillary 
capacity  of  a  soil  of  minimum  column  length  when 
subject  to  a  constant  and  known  force  or  pull.  The 
finer  the  soil,  the  greater  of  course  is  the  moisture 
equivalent.  The  authors  also  found  that  1  per  cent  of 
clay  or  organic  matter  represented  a  retentive  power  of 
about  .62  per  cent,  while  1  per  cent  of  silt  corresponded 
to  a  retention  of  .13  per  cent.  Representative  data 
which  show  the  correlation  of  the  moisture  equivalent 
to  the  textural  properties  of  the  various  types  are  given 
in  the  table  on  the  following  page. 

1  Briggs,  L.  J.,  and  McLane,  J.  W.  The  Moisture  Equiv- 
alents of  Soils.     U.  S.  D.  A.,  Bur.  Soils,  Bui.  45.     1907. 


THE  FORMS   OF  SOIL    WATER 


221 


Soil 

Per- 
cent- 
age OF 

Or- 
ganic 

MATT3R 

Per- 
cent- 
age 
of 
Sands 

Per- 
centage 
of  Silt 

Per- 
centage 

OF 

Clay 

Mois- 
ture 
Equiva- 
lent 

1.  Norfolk  coarse  sand     . 

2.  Norfolk  fine  sandy  loam 

3.  Yazoo  loam   .... 

4.  Waverly  silt  laom  .     . 

5.  Houston  clay  loam     . 

6.  Houston  clay      .     .     . 

.9 
1.3 
1.3 
2.0 
3.7 
1.4 

87.9 
73.4 
25.8 
14.9 
30.9 
10.0 

7.3 
18.1 
64.1 
62.9 
42.5 
56.6 

4.8 
8.5 
10.1 
22.2 
26.6 
33.4 

4.6 
6.8 
18.9 
24.4 
32.4 
38.2 

149.  The  maximum  retentive  power  of  a  soil.  —  An- 
other determination  has  been  devised  by  Hilgard  1  and 
used  to  considerable  extent  by  other  investigators.2  It 
is  designated  as  the  maximum  retentive  power  of  a  soil. 
A  small  perforated  brass  cup  is  used,  having  a  diameter 
of  about  5  centimeters  and  capable  of  containing  a  soil 
column  1  centimeter  in  height.  A  short  column  is  used, 
since  it  is  only  under  such  conditions  that  a  soil  may  re- 
tain against  gravity  the  greatest  amount  of  water.  Also, 
the  soil  is  able  to  expand  or  contract,  as  the  case  may  be, 
on  the  assumption  of  water  until  an  equilibrium  is  reached. 
A  filter-paper  disk  is  placed  in  the  metal  cup,  and  the  soil 
is  poured  in,  gently  jarred  down,  and  stroked  off  level 
with  the  top  of  the  cup.  The  cup  is  then  set  in  water 
and  the  soil  is  allowed  to  take  up  its  maximum  moisture. 
After  draining,  the  weight  of  the  wet  soil  plus  the  cup, 
together  with  the  weights  previously  obtained,  will  allow 
the  calculation  of  the  total  water  contained  by  the  soil. 

150.  Capillary  movement.  —  It  has  already  been  shown 
how  different  thicknesses  of  films  on  two  particles  tend 


1  Hilgard,  E.  H.     Soils,  p.  209. 

2  This  text,  paragraph  181. 


New  York.     1911, 


222       SOILS:    PROPERTIES  AND   MANAGEMENT 


Fig.  36.  —  Diagram  show- 
ing the  mechanics  of  the 
capillary  movement  of 
water  in  soil.  The  read- 
justment takes  place  in 
the  direction  of  (.4)  due 
to  the  high  tension  devel- 
oped by  the  sharp  film 
curvature  at  this  point. 


to  become  equal,  due  to  the  pulling  force  developed  by 
♦he  angle  of  curvature  between  the  particles.  It  is  evi- 
dent that  differences  in  curvature  must  be  the  motive 
force  in  the  capillary  movement 
of  soil  water.  Let  it  be  supposed, 
for  convenience,  that  three  equal 
spheres  when  brought  in  contact 
contain  unequal  amounts  of  water 
in  the  angles  of  curvature  (see 
Fig.  36).  In  this  case  the  greater 
pull  would  exist  at  A,  since  the 
angle  here  is  more  acute.  Conse- 
quently water  must  move  through 
the  connecting  film  until  the  pull 
at  A  and  that  at  B  become  the  same.  Such  an  adjust- 
ment might  go  on  over  a  large  number  of  films,  and  if 
one  end  of  the  column  was  exposed  to  an  evaporation 
of  just  the  right  rate  and  the  other  end  was  in  contact 
with  plenty  of  moisture,  large  quantities  of  water  would 
be  pumped  by  capillarity. 

This  capillary  movement  may  go  on  in  any  direction  in 
the  soil,  since  it  is  largely  independent  of  gravity;  yet 
under  natural  field  conditions  the  adjustment  tends  to 
take  place  very  largely  in  a  vertical  direction.  When 
a  soil  is  exposed  to  evaporation  the  surface  films  are 
thinned  and  water  moves  upward  to  adjust  the  ten- 
sion. This  explains  why  such  large  quantities  of  soil 
water  may  be  lost  so  rapidly  from  an  exposed  soil. 
Capillary  adjustment  may  go  on  downward,  also,  as  is 
the  case  after  a  shower.  Here  the  rapidity  of  the  ad- 
justment is  aided  by  the  weight  and  movement  of  the 
water  of  percolation. 
The  capillary  adjustment  in  a  soil  may  go  on  under 


THE  FORMS   OF  SOIL    WATER  223 

two  conditions :  (1)  if  the  soil  column  is  in  contact  with 
free  water;  and  (2)  if  no  gravity  water  is  present,  the 
movement  being  merely  from  a  moist  soil  to  a  drier  one, 
an  inexhaustible  supply  of  water  not  being  present.  In 
the  first  case  the  lower  portion  of  the  soil  is  entirely 
saturated  for  a  short  distance  above  the  free  water  sur- 
face, due  to  the  functioning  of  the  pore  spaces  as  true 
capillary  tubes;  above  this  the  film  movement  becomes 
dominant.  The  second  condition  of  capillary  adjustment 
is  the  one  most  commonly  found  in  a  normal  soil,  since  a 
water  table  a  short  distance  below  the  surface  is  not 
usually  conducive  to  the  best  crop  growth.  In  studying 
the  rate  and  height  of  capillary  rise  in  any  soil,  however, 
the  maintenance  of  a  supply  of  free  water  at  the  lower  end 
of  the  column  is  usually  provided  for,  since  this  allows  a 
near  approach  to  the  maximum  capillary  capacity  for  any 
point  in  the  column. 

151.  Factors  affecting  rate  and  height  of  capillary 
movement.  - —  To  persons  familiar  with  the  habits  of  grow- 
ing plants  it  is  evident  that  capillary  movement  must 
play  an  important  part  in  their  nutrition,  since  the  root- 
lets are  unable  to  bring  their  absorptive  surfaces  in  con- 
tact with  all  the  interstitial  spaces  where  the  bulk  of  the 
available  water  is  held.  Consequently  a  consideration 
of  the  movement  of  capillary  moisture  is  necessary,  not 
only  as  to  its  mechanics,  but  also  as  to  the  factors  influ- 
encing its  rate  and  height  of  movement.  These  factors 
are  four  in  number  :  (1)  thickness  of  water  film ;  (2)  sur- 
face tension;    (3)  texture;    and   (4)  structure. 

152.  Effect  of  thickness  of  water  film  on  capillary 
movement.  —  It  has  been  repeatedly  noticed,  in  the 
study  of  the  capillary  adjustment  between  two  soils,  that 
the  lower  the  percentage  of  water,  the  slower  is  the  rate 


224       SOILS:    PROPERTIES  AND  MANAGEMENT 


of  movement.  This  indicates  that  the  thickness  of  the 
film  covering  the  particles  and  connecting  the  interstices 
containing  the  bulk  of  the  capillary  water  is,  within 
certain  limits,  a  dominant  factor  in  rate  of  movement  at 
least.  Let  it  be  supposed  that  a  withdrawal  of  water 
occurs  at  A  (see  Fig.  37),  the  interstitial  space  between 
two  particles,  the  water  surface  being  represented  by  the 
dotted  line  aa  .     There  is  an  immediate  increase  in  the 

curvature  of  this  surface,  and 
water  tends  to  flow  through 
the  capillary  film  spaces  at  c 
and  c',  toward  this  area  of 
greater  tension.  If  water  con- 
tinues to  be  withdrawn  at  A, 
this  adjustment  continues 
with  considerable  ease  until 
the  film  channel  at  c  and  c 
becomes  so  thin  as  to  cause 
its  surface  (bb ')  to  approach  the  edge  of  the  hygroscopic 
film  surrounding  the  particle.  The  viscosity  of  the 
water  gradually  becomes  a  factor  at  this  point,  imped- 
ing the  capillary  adjustment  toward  A.  This  point  of 
sluggish  capillary  movement  has  been  designated  by 
Widtsoe  l  as  the  point  of  lento-capillarity. 

The  amount  of  capillary  water  delivered  at  any  one 
point,  therefore,  will  obviously  be  influenced  by  the 
thickness  of  the  film  and  may  consequently  be  taken 
as  a  measure  of  rate  of  rise.  A  short  soil  column 
should  deliver  more  water  from  a  constant  source 
than  a  longer  one,  due  to  the  thicker  films  at  the  sur- 


Fig.  37.  —  Diagram  for  the  ex- 
planation of  the  effect  of 
thickness  of  water  fiJm  about 
soil  particles  upon  ease  of 
capillary  movement. 


1  Widtsoe,  J.  A.,  and  McLaughlin,  W.  W.  The  Movement 
of  Water  in  Irrigated  Soils.  Utah  Agr.  Exp.  Sta.,  Bui.  115, 
pp.  223-231.     1912. 


THE  FORMS   OF  SOIL    WATER  225 

face  of  the  former  column.     King 1  shows  this  by  the 
following  data :  — 

Evaporation  from  the  Surface  of  Sand  Columns  of  Dif- 
ferent Lengths,  their  Base  being  in  Contact  with 
Free  Water 


Length  op  Column  in  Inches 


Evaporation  at  Surface 
in  Inches  a  Day 


6 
12 
18 
24 
30 


.114 
.111 
.080 
.034 
.019 


Briggs  and  Lapham  2  found,  in  comparing  the  evapo- 
ration from  tubes  of  different  lengths  (85  and  165  centi- 
meters, respectively)  of  Sea  Island  soil,  that  the  shorter 
column  showed  over  five  times  as  much  evaporation  in  a 
period  of  forty-two  days.  This  diminished  flow  with  the 
thinner  films  is  a  vital  point  in  plant  production,  since 
wilting  must  occur  as  soon  as  capillary  movement  becomes 
too  sluggish  to  supply  moisture  fast  enough  for  normal 
development. 

The  thickness  of  film  is  important  also  in  a  considera- 
tion of  the  height  of  rise  in  dry  and  moist  soil  respectively. 
It  is  evident  that  the  rate  would  be  much  more  rapid  in 
the  latter,  but  what  as  to  total  rise  ?     Stewart,3  in  study- 

1  King,  F.  H.  Principles  and  Conditions  of  tips  Movements 
of  Ground  Water.  U.  S.  Geol.  Sur.,  19th  Ar^Rept.,  Part  II, 
p.  92.     1897-1898. 

2  Briggs,  L.  J.,  and  Lapham,  M.  H.  Capillary  Studies. 
U.  S.  D.  A.,  Bur.  Soils,  Bui.  19,  pp.  24-25.     1902. 

3  Reported  by  Briggs,  L.  J.,  and  Lapham,  M.  H.  Capillary 
Studies.     U.  S.  D.  A.,  Bur.  Soils,  Bui.  19,  p.  26.     1902. 

Q 


226       SOILS:    PROPERTIES  AND  MANAGEMENT 

ing  the  capillary  limits  as  to  the  height  of  rise  in  dry  and 
moist  Michigan  soils,  found  this  limit  much  greater  where 
the  soil  was  damp.  This  vertical  rise  from  a  water  table 
was  almost  three  times  greater,  on  the  average,  in  the  soil 
in  which  the  films  were  originally  thicker.  Briggs  and 
Lapham  l  found  this  ratio  in  Sea  Island  soil  to  be  as  high 
as  four  and  one-half ;  while  Wollny  2  has  shown  sand  with 
9.5  per  cent  of  moisture  to  raise  moisture  from  a  water 
table  one-half  higher  in  six  days  than  did  the  same  sand 
dry.  It  is  evident,  therefore,  that  a  soil  with  a  thick 
capillary  film  will  carry  moisture  faster  than  one  with  a 
thinner  film,  and  also  will  raise  the  moisture  higher  when 
the  final  film  adjustment  has  taken  place. 

In  an  air-dry  soil  it  is  obvious  that  before  capillarity 
may  take  place  a  thicker  film  than  has  already  existed 
must  be  established.  This  is  often  difficult  because  of  the 
presence  of  oily  materials  deposited  on  the  surface  of  the 
particles  during  the  process  of  drying  out.  Such  a  condi- 
tion probably  accounts,  at  least  partially,  for  the  differ- 
ence in  total  rise  of  capillary  water  in  a  dry  and  in  a  moist 
soil,  since,  theoretically,  if  time  enough  were  given  for 
adjustment,  the  total  height  should  be  the  same  in  both 
columns.  This  resistance  of  dry  soil  to  the  resumption 
of  a  capillary  film  is  made  use  of  in  soil  mulches,  where  a 
dry  surface  layer  of  the  soil  checks  evaporation  by  imped- 
ing capillary  rise.  It  is  also  obvious  that  in  a  study  of  the 
rate  and  height  of  capillary  movement  and  the  factors 
affecting  it,  moist  columns  should  be  used,  as  this  is  a 

1  Briggs,  L.  J.,  and  Lapham,  M.  H.  Capillary  Studies. 
U.  S.  D.  A.,  Bur.  Soils,  Bui.  19,  p.  26.     1902. 

2  Wollny,  E.  Untersuchungen  iiber  die  Kapillare  Leitung 
des  Wassers  im  Boden.  Forsch.  a.  d.  Gebiete  d.  Agri.-Physik, 
Band  7,  Seite  269-308.     1884. 


THE  FORMS   OF  SOIL    WATER  227 

near  approach  to  the  conditions  of  a  field  soil.  Since  this 
is  rather  a  difficult  study  to  carry  out,  most  of  the  rate 
and  height  data  on  capillary  movement  have  been  largely 
obtained  with  dry  columns  in  contact  with  free  water  at 
the  bottom.  Such  data  are  comparative,  but  are  far 
from  quantitative  as  regards  the  performance  of  any  soil 
under  normal  conditions. 

153.  Surface  tension  and  capillary  movement.  —  As 
has  already  been  shown,  the  thickness  of  a  maximum 
capillary  film  is  largely  determined  by  surface  tension; 
and  as  surface  tension  with  any  given  curvature  exerts  a 
definite  pressure,  it  is  evident  that  this  pressure  may  be- 
come greater  or  smaller  with  variations  in  the  surface 
tension.  One  of  the  most  potent  factors  having  to  do 
with  this  variation  is  temperature.  If  the  temperature 
of  a  soil. column  in  capillary  equilibrium  and  containing 
its  maximum  capillary  moisture  should  be  raised,  some 
of  the  water  would  be  lost  as  free  water,  since  the  pulling 
power  of  the  films  would  be  decreased.  In  the  same 
way,  the  capillary  capacity  would  be  increased  by  a  lower- 
ing of  the  temperature,  which  of  course  would  mean  a 
higher  capillary  rise  in  either  a  dry  or  a  wet  soil.  The 
rate  of  movement,1  however,  would  be  facilitated  in  the 
first  case,  since  the  viscosity  of  the  water  would  be  much 
reduced,  allowing  the  movement  in  the  film  channels  to 
take  place  with  less  friction. 

King  2  has  verified  these  conclusions  in  his  experiments 


1  Wollny,  E.  Untersuchungen  iiber  die  Kapillare  Leitung 
des  Wassers  im  Boden.  Forsch.  a.  d.  Gebiete  d.  Agri.-Physik, 
Band  8,  Seite  206-220.     1885. 

2  King,  F.  H.  Fluctuations  of  the  Level  and  Rate  of  Flow 
of  Ground  Water.  U.  S.  D.  A.,  Weather  Bur.,  Bui.  5,  pp.  59- 
61.     1892. 


228       SOILS:    PROPERTIES  AND  MANAGEMENT 

with  the  fluctuations  of  the  ground  water  of  a  soil  held 
in  a  large  cylindrical  tank.  He  found  that  with  a  lower- 
ing of  temperature  the  ground  water  was  lowered,  due 
to  the  increased  capillary  capacity  of  the  soil  generated 
by  a  higher  surface  tension.  A  consequent  upward  move- 
ment of  water  took  place.  When  the  temperature  was 
raised,  however,  there  was  a  reverse  movement,  due  to  a 
change  of  capillary  water  to  free  water  brought  about  by 
a  lowered  surface  tension. 

The  surface  tension  may  also  be  varied  by  materials  in 
solutions,  most  salts  tending  to  cause  increased  tension. 
The  addition  of  soluble  fertilizer  salts  to  a  soil  would 
therefore  be  expected  to  exert  some  influence.  It  must 
be  remembered  in  this  connection  that  all  soils  contain  a 
certain  amount  of  oily  substances,  produced  during  the 
processes  of  organic  decay.  It  is  probable  that  the  lower- 
ing effect  of  such  material  would  largely  overbalance 
any  marked  influence  from  fertilizer  salts.  Moreover, 
as  such  salts  are  strongly  adsorbed  by  the  soil  particles, 
their  effect  on  the  concentration  of  the  surface  film  would 
probably  be  light  even  if  undisturbed  by  the  soil  resins. 
Wollny x  has  shown  that  adsorbed  salts  produce  little 
effect  on  capillarity,  while  non-adsorbed  salts  cause  a 
depression  increasing  with  concentration. 

Briggs  and  Lapham  2  found  that  with  Sea  Island  soil 
dissolved  salts  in  dilute  solution  had  no  appreciable  effect 
except  in  the  case  of  sodium  carbonate.  The  increased 
rise  in  this  case  they  ascribe  to  the  saponification  of  the 

1  Wollny,  E.  Untersuchungen  liber  die  Kapillare  Leitung 
des  Wassers.  Forsch.  a.  d.  Gebiete  d.  Agri.-Phvsik,  Band  7, 
Seite  269-308.     1884. 

2  Briggs,  J.  B.,  and  Lapham,  M.  H.  Capillary  Studies. 
U.  S.  D.  A.,  Bur.  Soils,  Bui.  19,  pp.  5-18.     1902. 


THE  FORMS   OF  SOIL    WATER  229 

oils  on  the  particles,  and  a  consequent  exposure  of  clean 
surfaces  for  capillary  movement.  These  authors  found 
also  that  concentrated  solutions  reduced  the  rate  of 
capillary  movement.  Davis,1  in  working  with  a  silt  loam, 
obtained  variable  results,  some  salts  depressing  and  some 
accelerating  capillary  rise.  Potassium  acid  phosphate 
caused  the  maximum  retardation,  while  ammonium  ni- 
trate most  markedly  increased  the  rate.  Since  only  one 
soil  was  used  and  the  greatest  observed  capillary  rise  was 
less  than  twelve  inches,  additional  data  must  be  presented 
before  it  is  clear  that  the  concentration  of  salts  may  be- 
come a  very  important  factor  in  humid  soils.  In  alkali 
soils,  in  which  the  concentration  of  the  salts  is  very  great, 
there  is  no  doubt  that  considerable  retardation  may  occur. 

154.  Effect  of  texture  on  capillary  movement.  —  In 
soils  of  fine  texture,  not  only  is  the  amount  of  film  surface 
exposed  greater  than  in  coarse  soils,  but  the  curvature  of 
the  films  is  also  greater,  due  to  the  shorter  radii.  The. 
effective  pressure  exerted  by  the  films  is  consequently 
much  higher  in  fine-grained  soil.  The  greater  exposure 
of  surface  and  the  increased  pressure  both  serve  to  raise 
the  friction  coefficient  and  retard  the  rate  of  flow.  The 
finer  the  texture  of  the  soil,  other  factors  being  equal,  the 
slower  is  the  movement  of  capillary  water.  Water  should 
therefore  rise  less  rapidly  from  a  water  table  through  a 
column  of  clay  than  through  a  sand  or  a  sandy  loam. 

The  height  to  which  water  may  be  drawn  by  the  effec- 
tive capillary  power  of  a  soil,  equilibrium  being  estab- 
lished, depends  on  the  number  of  interstitial  angles.  The 
greater  the  number  of  angles,  the  greater  is  the  total 

1  Davis,  R.  O.  E.  The  Effect  of  Soluble  Salts  on  the  Physi- 
cal Properties  of  Soils.  U.  S.  D.  A.,  Bur.  Soils,  Bui.  82,  pp. 
23-31.     1911. 


230       SOILS:    PROPERTIES  AND  MANAGEMENT 

supporting  power  of  the  films.  As  a  silt  soil  contains  a 
larger  number  of  such  angles,  its  capillary  pull  is  greater 
than  that  of  a  sand,  and  consequently  the  ultimate  move- 
ment would  be  of  greater  scope.  The  finer  the  texture, 
then,  the  slower  is  the  rate  of  capillary  movement  but  the 
greater  is  the  distance.1 

The  relation  of  texture  to  rate  and  height  of  capillary 
movement  in  dry  soil  is  shown  by  the  following  un- 
published data,  obtained  in  the  laboratory  of  the  Depart- 
ment of  Soil  Technology,  Cornell  University :  — 

Effect   of   Texture   on   Rate   and   Height   of   Capillary 
Rise  from  a  Water  Table  through  Dry  Soil 


Soil 

1  Hour 

1  Day 

2  Days 

3  Days 

4  Days 

5  Days 

Sand      .     .     . 
Clay       .     .     . 
Silt    .... 

Inches 

3.5 

.5 

2.5 

Inches 
5.0 
5.7 

14.5 

Inches 

5.9 

8.9 

20.6 

Inches 

6.8 
10.9 
24.2 

Inches 

6.8 
12.2 
26.2 

Inches 

6.9 
13.3 
27.4 

It  is  seen  that  the  movement  in  sand  is  rapid,  one-half 
of  the  total  rise  being  attained  in  one  hour.  The  maxi- 
mum height  is  reached  in  about  three  days.  The  silt  in 
this  case  seems  to  be  of  just  about  the  right  textural  con- 
dition for  a  fairly  rapid  rise,  yet  it  exerts  enough  capil- 
lary pull  to  attain  a  good  distance  above  the  water  table. 
The  friction  in  the  clay  is  greater,  however,  and  this 
results  in  a  slower  rate.  Whether  the  clay  would  ever  be 
able  to  exhibit  a  rise  comparable  with  its  tremendous  pull- 


1  Wollny,  E.  Untersuchungen  iiber  die  Kapillare  Leitung 
des  Wassers  im  Boden.  Forsch.  a.  d.  Gebiete  d.  Agri.-Physik, 
Band  7,  Seite  269-308.  1884.  Also,  Forsch.  a.  d.  Gebiete  d. 
Agri.-Physik,  Band  8,  Seite  206-220.     1885. 


THE  FORMS   OF  SOIL    WATER      .  231 

ing  capacity  is  doubtful,  because  of  the  resistance  offered 
by  the  dry  soil. 

155.  Texture  and  the  capillary  pull  of  soils.  —  An  ingen- 
ious method  for  measuring  quantitatively  the  capillary 
pull  exerted  by  a  moist  soil  has  been  devised  by  Lynde 
and  Dupre.1  The  apparatus  consists  of  a  glass  funnel 
joined  to  a  thick-walled  capillary  tube  by  means  of  a  piece 
of  rubber  tubing,  a  water  seal  being  used  at  this  point. 
The  lower  end  dips  into  mercury.  The  soil  to  be  studied 
is  placed  in  the  funnel,  and  after  being  saturated  is  con- 
nected by  means  of  a  wick  of  cheesecloth  or  filter  paper 
to  the  water  column  previously  established  in  the  capil- 
lary tube.  If  no  break  occurs  between  the  soil  and  the 
capillary  water  column,  the  apparatus  is  ready  for  use. 

The  excess  water  having  drained  away,  there  is  a 
thinning  of  the  films  on  the  soil  surface  due  to  evapora- 
tion. Equilibrium  adjustments  now  take  place,  which 
result  in  the  drawing  upward  of  the  water  column.  The 
mercury  follows,  and  the  strength  of  the  pull  may  be  meas- 
ured by  the  height  of  the  mercury  column.  The  old 
method  of  measuring  capillary  power  by  the  water  move- 
ment through  a  dry  soil  is  vitiated  by  two  conditions 
—  the  length  of  time  necessary,  and  the  fact  that  the 
maximum  lift  cannot  be  obtained  due  to  excessive  fric- 
tion. This  new  method  uses  a  wet  soil,  requires  only 
a  short  time,  and  gives  a  more  nearly  accurate  idea  of  the 
power  of  the  capillary  pull.  It  does  not,  however,  yield 
data  regarding  rate  of  movement,  —  a  factor  of  vital 
importance  to  plant  growth,  as  will  be  shown  later. 

Lynde  and  Dupre,   in  their  results,  confirm  the  state- 

1  Lynde,  C.  J.,  and  Dupre,  H.  A.  On  a  New  Method  of 
Measuring  the  Capillary  Lift  in  Soils.  Jour.  Amer.  Soc.  Agron., 
Vol.  5,  No.  2,  pp.  107-116.     1913. 


232       SOILS:    PROPERTIES  AND  MANAGEMENT 

merits  already  made  regarding  the  relation  of  texture 
to  capillary  power :  — 

The  Capillary  Lift  of  Soil  Separates  as  Determined  by 
Lynde  and  Dupre 


Soil 


Diameter  of  Grain, 
in  Millimeters 


Lift  of  Water 
Column,  in  Feet 


Medium  sand 
Fine  sand 
Very  fine  sand 
Silt        .     .     . 
Clay      .     .     . 


.5     -.25 
.25  -.10 
.10  -.05 
.05  -.005 
.005- 


.98 

1.78 

4.05 

9.99 

26.80 


The  capillary  pull  may  also  be  established,  at  least 
comparatively,  by  the  height  of  the  wetted  soil  and  the 
amounts  of  water  at  various  points  in  a  soil  column  that 
has  reached  a  capillary  equilibrium  when  its  base  is  in 
contact  with  a  constant  supply  of  water.  The  curves 
from  Buckingham  1  (Fig.  34,  p.  216)  determined  after  the 
soil  had  stood  for  sixty-eight  days,  illustrate  this. 

156.  Effect  of  structure  on  capillary  movement.  — 
Structure  has  already  been  shown  to  affect  capillary 
capacity  by  its  influence  on  the  angle  interstices.  Evi- 
dently, therefore,  it  may  alter  both  the  rate  and  the  height 
of  capillary  rise.  The  loosening  of  a  clay  soil  or  the 
compacting  of  a  sandy  soil  will  lessen  the  effective  film 
friction,  while  at  the  same  time  it  will  strengthen  the 
capillary  pull  resulting  in  a  faster  and  a  higher  capillary 
flow  of  water.  The  exact  structural  condition  of  any  soil 
in  which  this  result  is  realized  to  its  highest  efficiency  it 
is  impossible  to  judge  exactly.     In  general,  however,  this 

1  Buckingham,  E.  Studies  on  the  Movement  of  Soil  Mois- 
ture.    U.  S.  D.  A.,  Bur.  Soils,  Bui.  38,  p.  32.     1907. 


THE  FORMS   OF  SOIL    WATER  233 

point  is  approached  when  the  soil  is  in  the  best  physical 
condition  for  crop  growth.  Tillage  operations  in  general, 
tile  drainage,  and  the  addition  of  lime  and  organic  matter, 
operate  toward  this  result  by  their  granulating  tend- 
encies; while  rolling,  by  compacting  a  too  loose  surface, 
may  accomplish  the  same  effect  but  by  an  opposite  process. 
At  certain  seasons  of  the  year  capillarity  must  be 
impeded  near  the  surface,  as  it  continually  pumps  val- 
uable water  upward  to  be  lost  by  evaporation.  This 
movement  may  be  checked  by  producing  on  the  soil  sur- 
face, by  appropriate  tillage,  a  layer  of  dry,  loose  soil. 
This  layer,  called  a  soil  mulch,  affords  much  resistance 
to  wetting  because  of  its  dryness,  while  at  the  same  time 
it  affords  but  little  surface  and  few  angle  interstices  for 
effective  capillary  pull.  Thus  it  is  that  a  farmer,  in  order 
to  meet  his  immediate  or  future  needs,  may  alter  and 
control  capillary  movement  by  careful  attention  to  phys- 
ical conditions,  especially  those  at  the  surface  where 
evaporation  is  always  active. 

157.  Gravitational  water.  —  As  soon  as  the  capillary 
capacity  of  a  soil  column  is  satisfied,  further  addition  of 
moisture  will  cause  the  appearance  of  free  water  in  the  air 
spaces.  By  the  attraction  of  gravity,  this  water  moves 
downward  through  the  earth  at  a  rate  varying  with  soil 
and  climatic  conditions.  In  general  the  flow  is  governed  by 
four  factors — pressure,  temperature,  texture,  and  structure. 
An  understanding  of  the  operation  of  these  forces  is  im- 
portant, since  the  rapid  elimination  of  free  water  from  the 
soil  is  necessary  for  optimum  plant  growth.  The  actual 
procedure,  however,  is  considered  under  the  head  of  "  Land 
Drainage,"  a  distinct  phase  of  soil  management  in  itself. 

158.  Pressure  and  the  movement  of  gravity  water.  — 
It  is  very  evident  that  any  pressure  exerted  on  a  water 


234       SOILS:    PROPERTIES  AND  MANAGEMENT 

column  will  accelerate  the  rate  of  flow.  Under  normal 
conditions  pressure  may  arise  from  two  sources,  baro- 
metric pressure  and  the  weight  of  the  water  column. 
Changes  in  barometric  pressure  are  communicated  to 
gravitational  water  through  a  movement  of  the  soil  air. 
As  the  mercury  column  rises,  more  air  is  forced  into  the 
soil  and  the  pressure  on  the  soil  water  increases.  Such 
a  change  has  been  observed  by  King1  to  produce  as  high  as 
a  15  per  cent  decrease  in  the  flow  of  drains.  King  observed 
also  that  the  level  of  wells  fluctuated  from  time  to  time  for 
the  same  cause.  The  expansion  of  the  air  of  the  soil  due  to 
daily  heatings  was  also  observed  to  produce  diurnal  oscil- 
lations in  the  level  and  the  rate  of  flow  of  ground  water. 
Perhaps  of  greater  import  in  the  rate  of  percolation  of 
water  is  the  pressure  produced  by  the  weight  of  the  free- 
water  column.  Working  along  this  line,  Welitschkowsky  2 
has  shown  rather  conclusively  that  with  an  ideal  length  of 
column  the  flow  varies  directly  with  the  pressure.  His 
ideal  column  with  the  sand  with  which  he  experimented 
was  75  centimeters  in  length.  With  a  longer  column  the 
flow  did  not  increase  as  fast  as  the  pressure ;  while  with 
a  shorter  column,  doubling  the  pressure  more  than  doubled 
the  flow.  These  results  have  been,  verified  by  Wollny  3 
and  ably  reviewed  by  King.4 


1  King,  F.  H.     The  Soil,  p.  180.     New  York.     1906. 

2  Welitschkowsky,  D.  von.  Experimentelle  Untersuchun- 
gen  iiber  die  Permeabilitat  des  Bodens  fur  Wasser.  Archiv  f. 
Hygiene,  Band  II,  Seite  499-512.     1884. 

3  Wollny,  E.  Untersuehungen  iiber  die  Permeabilitat  des 
Bodens  fur  Wasser.  Forsch.  a.  d.  Gebiete  d.  Agri.-Physik, 
Band  14,  Seite  1-28.     1891. 

4  King,  F.  H.  Principles  and  Conditions  of  the  Movements 
of  Ground  Water.  U.  S.  Geol.  Survey,  19th  Ann.  Rept.,  Part 
II,  pp.  67-206.     1897-98. 


THE  FORMS   OF  SOIL    WATER  235 

159.  Effect  of  temperature  on  the  flow  of  gravity  water. 
—  A  rise  in  temperature  of  the  soil  not  only  varies  the 
amount  of  capillary  water  and  thus  increases  the  possible 
free  water  present,  but  at  the  same  time  it  increases  the 
fluidity  and  thus  facilitates  percolation.  The  expansion 
of  the  soil  air  also  tends  to  increase  such  movement.  This 
can  be  noticed  in  the  operation  of  a  tile  drain  in  early 
spring  as  compared  with  summer  conditions.  Calculated 
effects  of  temperature  change  have  been  verified  by  con- 
trolled experimental  results. 

160.  Effect  of  texture  and  structure  on  the  flow  of 
gravity  water.  —  Of  much  more  practical  importance 
than  temperature,  in  the  flow  of  gravitational  water, 
are  the  size  and  the  arrangement  of  the  soil  particles. 
In  working  with  sands  of  varying  grades,  Welitschkowsky,1 
Wollny,2  and  others  have  shown  that  the  flow  of  water 
varies  with  the  size  of  particle,  or  texture.  King  3  has 
demonstrated  that  in  general  with  such  materials  the 
rate  of  flow  is  directly  proportional  to  the  square  of  the 
diameter  of  the  particle.  By  the  use  of  the  effective 
mean  diameter  4  of  a  sand  sample,  he  was  able  to  calculate 
a  theoretical  flow  which  compared  very  closely  to  observed 
percolations.  In  sandy  soils  this  law  holds  in  a  very 
general  way,  but  in  clays  it  fails  entirely.     For  instance, 

1  Welitschkowsky,  D.  von.  Experimentelle  Untersuchungen 
iiber  die  Permeabilitat  des  Bodens  fur  Wasser.  Archiv  f. 
Hygiene,  Band  II,  Seite  499-512.     1884. 

2  Wollny,  E.  Untersuchungen  liber  den  Einfluss  der 
Struktur  des  Bodens  auf  dessen  Feuchtigkeits-  und  Tempera- 
turverhaltnisse.  Forsch.  a.  d.  Gebiete  d.  Agri.-Physik,  Band 
5,  Seite  167.     1882. 

3  King,  F.  H.  Principles  and  Conditions  of  the  Movements 
of  Ground  Water.  U.  S.  Geol.  Survey,  19th  Ann.  Rept.,  Part 
II,    pp.    222-224.     1897-98. 

4  This  text,  paragraph  87. 


236     soils:  properties  and  management 

if  such  a  law  was  in  force  a  sand  having  a  diameter  of 
.5  millimeter  would  exhibit  a  flow  10,000  times  greater 
than  that  through  a  clay  loam  with  a  diameter,  say,  of 
.005  millimeter;  whereas  the  actual  ratio,  as  observed 
experimentally  by  King,  was  less  than  200. 

Evidently,  therefore,  while  it  can  be  stated  as  a  general 
thesis  that  the  flow  varies  with  the  texture,  no  governing 
law  can  be  deduced  for  soils  since  structure  exerts  such  a 
modifying  influence.  The  percolation  in  a  heavy  soil 
takes  place  largely  through  lines  of  seepage,  which  are 
really  large  channels  developed  by  various  agencies. 
If  in  the  drainage  of  average  soil,  the  farmer  depended 
on  the  movement  of  water  through  the  individual  pore 
spaces,  the  soil  would  never  be  in  a  condition  for  crop 
growth.  These  lines  of  seepage  are  developed  by  the 
ordinary  forces  that  function  in  the  production  of  soil 
granulation,  as  freezing  and  thawing,  wetting  and  drying, 
lime,  humus,  plant  roots,  and  tillage  operations. 

A  clear  understanding  of  the  factors  governing  the 
flow  of  gravitational  water  is  of  especial  importance  in 
tile  drainage  operations,  particularly  regarding  the  depth 
of  and  interval  between  tile  drains.  Since  percolation 
is  so  slow  in  a  heavy  soil,  it  is  evident  that  the  tile  must 
be  near  the  surface  in  order  to  secure  efficient  drainage. 
In  a  sand  the  depth  may  be  increased,  because  of  the 
slight  resistance  offered  to  water  movement.  The  depths 
for  laying  tile  in  a  heavy  soil  range  from  one  and  a  half 
to  two  and  a  half  feet,  while  in  a  sand  the  tile  may  often 
be  placed  as  deep  as  four  feet  below  the  surface.  It  is 
evident  also  that  the  less  deep  a  tile  drain  is  laid,  the 
less  distance  on  either  side  it  will  be  effective  in  removing 
the  water;  consequently  on  a  clay  soil  the  laterals  must 
be  relatively  close,  as  compared  to  the  interval  generally 


THE  FORMS   OF  SOIL    WATER  237 

recommended  for  a  sandy  soil.  A  rational  understanding 
of  the  movements  of  gravitation  water  is  clearly  necessary 
in  the  installation  of  tile  drains,  not  only  that  the  system 
may  be  fully  effective,  but  also  that  a  minimum  effective 
cost  may  be  realized. 

161.  Determination  of  the  quantity  of  free  water  that 
a  soil  will  hold.  —  While  there  is  no  particular  advantage 
in  finding  the  quantity  of  gravitational  water  that  a  soil 
will  hold,  since  a  normal  soil  should  never  remain  saturated 
for  any  length  of  time,  it  is  nevertheless  of  interest  to 
know  by  what  methods  such  data  may  be  obtained.  One 
method  is  to  saturate  a  column  of  known  weight,  and 
then,  by  exposing  it  to  percolation,  measure  the  amount 
of  water  that  is  lost.  The  gravitational  water  can  then 
be  expressed  in  terms  of  dry  soil.  The  disadvantage  in 
this  method  lies  in  the  fact  that  it  is  extremely  difficult 
to  free  a  soil  entirely  of  air,  so  that  a  determination  made 
in  this  way  would  yield  low  results.  Again,  a  very  long 
time  must  elapse  before  a  soil  will  give  up  all  its  gravita- 
tional water.  King  *  found  that  with  even  a  sand  the 
draining-away  of  the  free  water  continued  over  a  space 
of  two  and  one  half  years.  It  must  also  be  noted  here 
that  because  of  the  lessening  of  the  capillary  water  as  a 
column  of  soil  is  ascended,  the  space  for  possible  free 
water  increases,  thus  accounting  for  the  ready  entrance 
of  rain  into  a  soil  which  on  the  average  may  contain  a 
relatively  high  water  content. 

162.  The  calculation  of  the  free  water  of  a  soil.  —  A 
more  nearly  accurate  idea  of  the  possible  free-water 
capacity  of  soil  may  be  obtained  by  calculation.  If  the 
absolute  and  the  apparent  specific  gravity  of  a  soil,  and 

1  King,  F.  H.  Physics  of  Agriculture,  pp.  134-135.  Pub- 
lished by  the  author,  Madison,  Wisconsin.     1910, 


238       SOILS:    PROPERTIES  AND  MANAGEMENT 

its  percentage  of  moisture  when  capillarily  satisfied,  are 
known,  the  following  formulas  may  be  used :  — 

«  „    .  ,1       [percentage  of  pore  space 

Percentage  of  air  space  when  M      _  (percentage  of  h20 

capillarily  saturated  w  \ 

:  J       I      X  ap.  sp.  gr.) 

Percentage  of  free  water  pos-  1  _  [percentage   of  air  space 
sible  ap.  sp.  gr. 


163.  Value  of  studying  flow  and  composition  of  gravita- 
tional water.  —  While  the  determination  of  the  possible 
free  water  that  a  soil  will  hold  is  of  little  real  value,  a 
knowledge  of  its  movement  and  its  composition  is  of 
vital  importance.  It  has  already  been  shown  how  the 
rate  of  movement  of  such  water  is  a  factor  in  efficient 
drainage.  The  amount  likely  to  be  thus  lost  is  of  interest 
in  plant  production  from  two  standpoints:  first,  the 
role  that  water  plays  as  a  food  and  a  regulator;  and 
secondly,  the  losses  of  nutritive  elements  that  always 
occur  with  drainage.  It  is  quite  evident  that  these 
questions  should  be  studied  only  on  soil  in  a  normal  field 
position.  Consequently  two  methods  of  procedure  are 
open  —  the  use  of  an  efficient  system  of  tile  drains,  and 
the  construction  of  lysimeters. 

164.  The  study  of  gravity  water  by  means  of  tile  drains. 
—  In  the  first  method  an  area  should  be  chosen  where 
the  tile  drain  receives  only  the  water  from  the  area  in 
question  and  where  the  drainage  is  efficient.  A  study 
of  the  amounts  of  flow  throughout  a  term  of  years  will 
yield  much  valuable  data  concerning  the  factors  already 
discussed.  An  analysis  of  the  drainage  water  will  throw 
light  on  the  ordinary  losses  of  plant-food  from  a  normal 
soil  under  a  known  cropping  system.     The  advantage 


THE  FORMS   OF  SOIL    WATER 


239 


of  such  a  method  of  attack  lies  not  only  in  the  fact  that 
a  large  area  of  undisturbed  soil  is  considered,  but  also 
in  the  opportunity  to  study  practical  field  treatments 
in  relation  to  the  movement  and  composition  of  drainage 
water. 

165.  The  lysimeter  method  of  studying  gravitational 
water.  —  The  lysimeter  method,  however,  has  been  the 
usual  mode  of  attacking  these  problems.  In  this  method 
a  small  block  of  soil  is  used,  being  entirely  isolated  by 
appropriate  means  from  the  soil  surrounding  it.  Effective 
and  thorough  drainage  is  provided.  The  advantages  of 
this  method  are  that  the  variations  found  in  a  large  field 
are  avoided,  the  work  of  carrying  on  the  study  is  not  so 
great  as  in  a  large  field,  and  the  experiment  is  more  easily 
controlled.  One  of  the  best-known  sets  of  lysimeters 
was   that   established    at   the   Rothamsted    Experiment 


G>rac/e 


Fig.  38.  —  Cross  section  of  a  lysimeter  at  the  Rothamsted  Experiment 
Station,  England,  in'),  soil  column  under  study;  (p),  outlet  for 
collected  drainage  water. 


240       SOILS:    PROPERTIES  AND  MANAGEMENT 

Station1  in  England.  (See  Fig.  38.)  Here  blocks  of 
soil  one  one-thousandth  of  an  acre  in  surface  area  were 
isolated  by  means  of  trenches  and  tunnels,  and,  supported 
in  the  meantime  by  perforated  iron  plates,  were  separated 
from  the  surrounding  soil  by  masonry.  The  blocks  of  soil 
were  twenty,  forty,  and  sixty  inches  in  depth,  respectively. 
Facilities  for  catching  the  drainage  were  provided  under 
each  lysimeter.  The  advantage  of  such  a  method  of 
construction  lies  in  the  fact  that  the  structural  condition 
of  the  soil  is  undisturbed  and  consequently  the  data  are 
immediatelv  trustworthv. 


O  6~Sewer7?/e  '  '  \J 


Fig.  39.  —  Cross   section   of  a  soil   tank    at  Cornell   University,   New 
York,     (a),  soil  under  investigation  ;  (p),  outlet  of  drainage  pipe. 


1  Lawes,  J.  B.,  Gilbert,  J.  H.,  and  Warington,  R.  On  the 
Amount  and  Composition  of  the  Rain  and  Drainage  Waters 
Collected  at  Rothamsted.  Jour.  Roy.  Agr.  Soc.,  Ser.  II,  Vol. 
17,  pp.  269-271.     1881. 


THE  FORMS   OF  SOIL    WATER  241 

At  Cornell  University  1  a  system  of  cement  tanks  sunk 
in  the  ground  has  been  constructed.  Each  tank  is  about 
four  and  a  half  feet  square  and  four  feet  deep.  A  sloping 
bottom  is  provided,  with  a  drainage  channel  opening  into 
a  tunnel  beneath  and  at  one  side.  As  the  tanks  are  ar- 
ranged in  two  parallel  rows,  one  tunnel  suffices  for  both. 
(See  Fig.  39.)  The  sides  of  the  tanks  are  treated  with 
asphaltum  in  order  to  prevent  solution.  The  soil  must 
of  course  be  placed  in  the  tanks,  this  causing  a  disturb- 
ance of  its  structural  condition.  As  a  consequence  data 
as  to  rate  of  flow  and  composition  of  the  drainage  water 
are  rather  unreliable  for  the  first  few  years.  Such  an 
experiment  must  necessarily  be  one  of  long  duration. 

166.  Thermal  movement  of  water.  —  Little  has  been 
said  as  yet  regarding  this  third  mode  of  water  movement, 
the  vapor  flow,  which  is  not  peculiar  to  one  form  of  soil 
water  but  affects  all  alike.  It  is  at  once  apparent  that 
the  movement  of  water  vapor  can  be  of  little  importance 
within  the  soil  itself,  since  it  depends  so  largely  on  the 
diffusion  and  convection  of  the  soil  air.  While  the  soil 
air  is  no  doubt  practically  always  saturated  with  water 
vapor,  the  loss  of  moisture  by  this  means  is  slight.  Buck- 
ingham 2  has  shown  that,  while  sand  allows  such  a  move- 
ment to  the  greatest  degree,  the  loss  occurring  in  a  soil 
with  any  appreciable  depth  of  layer  is  almost  negligible. 

The  question  of  the  thermal  movement  of  water  at  the 
soil  surface,  however,  is  vital- in  farming  operations.  At 
this  point  the  water  films  are  exposed  to  sun  and  wind, 
and  drying  goes  on  rapidly,  the  free,  capillary,  and  a 

1  Lyon,  T.  L.  Tanks  for  Soil  Investigation  at  Cornell  Uni- 
versity.    Science,  N.  Ser.,  Vol.  29,  No.  746,  pp.  621-623.     1909. 

2  Buckingham,  E.  Studies  on  the  Movement  of  Soil 
Moisture.     U.  S.  D.  A.,  Bur.  Soils.,  Bui.  38,  pp.  9-18.     1907. 

R 


242       SOILS:    PROPERTIES  AND   MANAGEMENT 

part  of  the  hygroscopic  water  vaporizing  in  the  order 
named.  If  the  loss  of  the  surface  moisture  were  the 
only  consideration,  the  problem  would  not  be  serious; 
but  the  capillarity  of  the  soil  must  be  considered  also. 
As  the  films  at  the  surface  become  thin  a  capillary  move- 
ment begins,  and  if  the  evaporation  is  not  too  rapid  a 
very  great  loss  of  water  may  occur  in  a  short  time. 

The  evaporation  from  a  bare  soil  in  the  Rothamsted 
lysimeters l  averaged  about  seventeen  inches  a  year, 
with  a  rainfall  ranging  from  twenty-two  to  forty-two 
inches.  This  means  that  from  one-third  to  one-half  of 
the  effective  rainfall  was  entirely  lost  as  thermal  water. 
The  necessity  of  checking  such  a  loss  becomes  apparent, 
especially  in  regions  where  rainfall  is  slight  or  drought 
periods  are  likely  to  occur.  As  no  country  is  free  from 
one  or  the  other  of  such  contingencies,  the  great  promi- 
nence that  methods  of  moisture  conservation  hold  in 
systems  of  soil  management  is  understandable.  While 
means  of  checking  losses  by  leaching  and  run-off  are 
advocated,  effective  retardation  of  surface  evaporation 
is  always  particularly  emphasized. 

1Warington,  R.  Physical  Properties  of  the  Soil,  p.  109. 
Clarendon  Press,  Oxford.     1900. 


CHAPTER  XII 

THE    WATER    OF    THE    SOIL   IN   ITS    RELA- 
TION  TO  PLANTS 

Water,  as  has  already  been  shown,  is  one  of  the  external 
factors  in  plant  growth  in  that  it  is  necessary  in  the 
processes  of  weathering,  which  results  in  the  simplifica- 
tion of  compounds  for  plant  utilization.  It  also  func- 
tions as  an  internal  factor  in  plant  development,  inasmuch 
as  it  maintains  the  turgidity  of  the  plant  cells,  acts  as  a 
carrier  of  food  materials,  functions  as  a  regulator,  and 
can  actually  be  utilized  as  a  source  of  hydrogen  and 
oxygen.  These  direct  or  indirect  relations  of  water 
to  plant  growth  may  be  considered  under  three  heads, 
as  follows :  — 

167.    Relations  of  water  to  the  plant.  — 

1.  Water  acts  as  a  solvent  and  a  carrier  of  plant-food 

materials.  It  is  therefore  a  medium  of  transfer 
for  the  mineral  and  gaseous  elements  from  the 
soil  to  their  proper  places  within  the  plant. 

2.  As  a  food  water  either  becomes  a  part  of  the  cell 

without  change,  or  is  broken  down  and  its  ele- 
ments are  utilized  in  new  compounds. 

3.  Water  in  maintaining   turgidity,   in   equalizing  the 

temperature  by  evaporation  from  the  leaves,  and 
in  facilitating  quick  shifts  of  food  from  one  part 
of  the,  plant  to  another,  acts  as  a  regulator  during 
assimilation  and  while  synthetic  and  metabolic 
processes  are  going  on. 
243 


244       SOILS:    PROPERTIES  AND  MANAGEMENT 

Soil  moisture,  therefore,  in  proper  amounts,  becomes 
one  of  the  controlling  factors  in  crop  growth  and  must 
be  looked  to  before  the  maximum  utilization  of  the 
primary  elements  can  be  expected.  The  amount  of 
water  held  within  the  plant  is  not  large,  however,  in 
comparison  with  the  amount  lost  by  transpiration,  al- 
though green  plants  contain  from  60  to  90  per  cent  of 
moisture.  Although  the  main  cause  of  the  high  trans- 
piration of  most  crops  is  not  traceable  to  the  dilute  con- 
dition of  the  soil  solution,  certain  regulatory  functions 
may,  however,  also  come  into  play. 

Because  of  the  readiness  with  which  moisture  passes 
from  plants  into  the  atmosphere,  large  quantities  of 
water  must  be  taken  from  the  soil  in  order  that 
the  plant  may  maintain  its  proper  turgidity.  This 
excess  water  is  largely  lost  or  disposed  of  by  trans- 
piration, at  the  same  time  performing  its  regulatory 
functions. 

168.  The  water  requirement  of  plants.  —  As  might  be 
expected,  the  pounds  of  water  transpired  for  every  pound 
of  dry  matter  produced  in  the  crop  is  very  large.  This 
figure,  called  the  transpiration  ratio,  or  water  require- 
ment, ranges  from  200  to  500  for  crops  in  humid  regions, 
and  almost  twice  as  much  for  crops  in  arid  climates. 
An  accurate  determination  of  the  transpiration  ratio  of 
a  crop  is  somewhat  difficult,  due  to  the  methods  of  pro- 
cedure necessary  and  also  to  the  difficulty  of  controlling 
the  numerous  factors  that  may  vary  the  transpiration. 
For  really  reliable  figures  the  plants  must  be  grown  in 
cans  or  pots,  in  order  that  the  water  lost  may  be  deter- 
mined accurately  by  weighing.  If  there  is  no  percolation, 
the  water  ordinarily  lost  from  a  cropped  soil  includes 
both  that   evaporated   from  the   soil   surface   and   that 


WATER   OF  SOIL  IN  ITS  RELATION  TO  PLANTS      245 

transpired  from  the  leaves.  The  former  loss  may  be 
eliminated  from  calculations  in  two  ways  :  (1)  by  covering 
the  soil  in  some  way  so  that  evaporation  is  absolutely 
checked  and  the  only  loss  is  by  transpiration ;  or  (2)  by 
determining  the  evaporation  from  a  bare  pot  and,  by 
subtracting  this  from  the  total  water  loss  from  a  cropped 
soil,  rinding  the  loss  due  to  transpiration  alone. 

An  objection  to  the  former  method  is  that  any  covering 
which  interferes  with  evaporation  interferes  with  proper 
soil  aeration  also  and  may  render  soil  conditions  abnormal. 
In  the  second  method,  however,  an  even  more  serious 
error  enters,  since  the  evaporation  from  a  bare  soil  is 
not  the  same  as  that  from  a  soil  covered  by  vegetation 
because  of  the  shading  effects.  Also,  due  to  the  action 
of  the  roots,  less  water  is  likely  to  be  allowed  to  move 
to  the  surface  by  capillary  attraction  in  the  cropped  soil. 
Therefore,  any  data  that  may  be  quoted  can  be  only 
general  in  its  application,  not  only  because  of  the  errors 
of  determination  but  also  because  of  the  great  number  of 
factors  that  under  normal  conditions  may  vary  the 
transpiration  ratio.  The  data  on  the  following  page, 
drawn  from  various  investigators  working  by  the  gen- 
eral methods *  already  outlined,  give  some  idea  of  the 
water  transpired  by  different  crops,  due  allowance  being 
made  for  various  disturbing  factors.  Below  the  data 
regarding  transpiration  will  be  found  the  citations  to 
the  work  of  the  various  authors  as  well  as  a  few 
notes  regarding  their  experimental  procedure. 

1  A  brief  discussion  of  the  various  methods  is  found  as  follows  : 
Montgomery,  E.  G.  Methods  of  Determining  the  Water 
Requirements  of  Crops.  Proc.  Amer.  Soe.  Agron.,  Vol.  3, 
pp.  261-283.  1911.  Also  Briggs,  L.  J.,  and  Shantz,  H.  L., 
The  Water  Requirement  of  Plants.  U.  S.  D.  A.,  Bur.  Plant 
Ind.,  Bui.  285.     1913. 


246       SOILS:    PROPERTIES  AND  MANAGEMENT 


Water  Requirements  of  Plants  by  Different 
Investigators 


Crop 


-  '  - 

te  w  z 

5  o  •< 

1 «  J 


Barley  . 
Beans 
Buckwheat 
Clover  . 
Maize 
MiUet  . 
Oats 
Peas 
Potatoes 
Rape 
Rye  .  . 
Wheat  . 


H 


258 
209 

269 


259 


247 


[m  B'-l 


774 

646 

233 
447 
665 
416 

912 


w  O 


310 
282 
363 
310 


376 
273 


353 
338 


464 


576 
271 

503 

477 
385 


M 

3 


468 


337 

469 
563 


544 


<  SO05 


534 
736 
578 
797 
368 
310 
597 
788 
636 
441 
685 
513 


1  Lawes,  J.  B.  Experimental  Investigation  into  the  Amount 
of  Water  Given  off  by  Plants  during  their  Growth.  Jour. 
Hort.  Soc.  London,  Vol.  5,  pp.  38-63.     1850. 

Pots  holding  42  pounds  of  field  soil  were  used.  Evaporation 
from  soil  was  reduced  to  a  very  low  degree  by  perforated  glass 
covers  cemented  on  the  pots.  The  figures  quoted  are  from 
unfertilized  soil. 

2  Wollny,  E.  Der  Einfluss  der  Pflanzendecke  und  Beschat- 
lung  auf  die  Physikalischen  Eigenschaften  und  die  Frucht- 
barkeit  des  Bodens,  Seite  125.     Berlin,  1877. 

Wollny  grew  plants  in  humous  sand  in  amounts  ranging  from 
5  to  12  kilograms.  Evaporation  was  reduced  to  a  very  low 
degree  by  perforated  covers.  Actual  evaporation  from  un- 
cropped  cans  was  observed,  however. 

3  Hellriegel,  H.  Beitrage  zur  den  Naturwissenschaftlichen 
Grundlagen  des  Ackerbaus,  Seite  663.     Braunschweig,  1883. 

Hellriegel  grew  plants  in  4  kilograms  of  clean  quartz  sand 
and  supplied  them  with  nutrient  solutions.  The  loss  by  evap- 
oration from  uncropped   pots  was  used  in  determining  losses 


WATER   OF  SOIL  IN  ITS  RELATION   TO  PLANTS     247 

169.  Factors  affecting  transpiration.  —  These  figures 
serve  to  indicate  not  only  the  variation  between  crops, 
but  also  the  great  effect  of  climate  and  soil  on  transpira- 
tion.1 The  factors  may  be  listed  under  three  heads,  as 
follows :  — 


1  A  complete  review  of  the  literature  concerning  the  climatic 
and  soil  factors  in  their  effect  on  transpiration  may  be  found 
as  follows :  Briggs,  L.  J.,  and  Shantz,  H.  L.  The  Water 
Requirement  of  Plants.  U.  S.  D.  A.,  Bur.  Plant  Ind.,  Bui. 
285.     1913. 


by   transpiration.     In  later   experiments   covers   were   used  in 
order  to  cut  down  evaporation. 

4  King,  F.  H.  Physics  of  Agriculture,  p.  139.  Published 
by  author,  Madison,  Wisconsin,  1910.  Also,  The  Number  of 
Inches  of  Water  Required  for  a  Ton  of  Dry  Matter  in  Wis- 
consin. Wisconsin  Agr.  Exp.  Sta.,  11th  Ann.  Rept.,  pp.  240- 
248.  1894 ;  and  The  Importance  of  the  Right  Amount  and 
Right  Distribution  of  Water  in  Crop  Production.  Wisconsin 
Agr.  Exp.  Sta.,  14th  Ann.  Rept.,  pp.  217-231.     1897. 

King  used  cans  holding  about  400  pounds  of  soil.  Some  were 
set  down  into  the  earth  while  others  were  not.  Part  of  the 
work  was  carried  on  in  the  field;  the  remainder  was  run  in 
vegetative  houses.  Normal  soils  were  used.  Evaporation 
from  soil  was  very  low,  water  being  added  from  beneath.  The 
data  quoted  are  the  average  of  a  large  number  of  tests. 

5  Leather,  J.  W.  Water  Requirements  of  Crops  in  India. 
Memoirs,  Dept.  Agr.,  India,  Chem.  Series,  Vol.  I,  No.  8,  pp. 
133-184,  1910,  and  No.  10,  pp.  205-281.     1911. 

Jars  containing  from  12  to  48  kilograms  of  soil  were  used. 
Loss  by  evaporation  was  determined  on  bare  pots.  The  plants 
were  grown  in  culture  houses  or  in  screened  inclosures. 

6  Briggs,  L.  J.,  and  Shantz,  H.  L.  Relative  Water  Require- 
ment of  Plants.  U.  S.  D.  A.,  Jour.  Agr.  Research,  Vol.  Ill, 
No.  1,  pp.  1-63.  1914.  Also,  The  Water  Requirements  of 
Plants.     U.  S.  D.  A.,  Bur.  Plant  Ind.,  Bui.  284.     1913. 

Plants  were  grown  in  cans  holding  250  pounds  of  soil.  Evap- 
oration from  soil  was  prevented  by  means  of  a  paraffin  covering. 
Work  was  conducted  in  screened  inclosures.  The  data  are  the 
average  of  several  years'  work. 


248       SOILS:    PROPERTIES  AND  MANAGEMENT 

1.  Crop.  —  Differences  due  to  different  crops  and  to 

variations  of  the  same  crop. 

2.  Climate.  —  Rain,   humidity,   sunshine,   temperature, 

and  wind. 

3.  Soil.  —  Moisture  and  general  fertility. 

170.  Effect  of  crop  and  climate  on  transpiration.  — 
Not  only  do  different  crops  show  a  variation  of  tran- 
spiration in  the  same  season,  but  the  same  crop  may  give 
a  totally  different  transpiration  in  different  years.  This 
is  due  in  part  to  inherent  differences  in  the  crop  itself. 
For  example,  leaf  surface  or  root  zone  would  totally 
alter  the  transpiration  relationship  under  any  given 
condition.  However,  a  great  deal  of  the  variation  ob- 
served in  the  ratios  already  quoted  arises  from  differences 
in  climatic  conditions.  As  a  general  thing,  the  greater 
the  rainfall,  the  higher  is  the  humidity  and  the  lower  is 
the  relative  transpiration.  This  accounts  for  the  high 
figures  obtained  by  Widtsoe l  in  arid  Utah.  Mont- 
gomery 2  found,  in  studying  the  water  requirements  of 
corn  under  greenhouse  conditions,  that  an  increase  in 
the  percentage  humidity  from  42  to  65  lowered  the  tran- 
spiration ratio  from  340  to  191.  In  general,  temperature, 
sunshine,  and  wind  vary  together  in  their  effect  on  tran- 
spiration. That  is,  the  more  the  sunshine,  the  higher  is 
the  temperature,   the  lower   is   the  humidity,    and   the 

1  Widtsoe,  J.  A.  Production  of  Dry  Matter  with  Differ- 
ent Quantities  of  Irrigation  Water.  Utah  Agr.  Exp.  Sta., 
Bui.  116.  1912.  Also,  Irrigation  Investigations.  Factors  In- 
fluencing Evaporation  and  Transpiration.  Utah  Agr.  Exp. 
Sta.,  Bui.  105.     1909. 

2  Montgomery,  E.  G.,  and  Kiesselbach,  T.  A.  Studies  in 
Water  Requirements  of  Corn.  Nebraska  Agr.  Exp.  Sta., 
Bui.   128,  p.  4.     1912. 


WATER   OF  SOIL   IN  ITS  RELATION  TO  PLANTS      249 


greater  is  likely  to  be  the  wind  velocity.     All  this  would 
tend  to  raise  the  transpiration  ratio. 

171.  Effect  of  soil  moisture  on  transpiration.  —  From 
the  soil  standpoint,  however,  the  factors  inherent  in  the 
soil  itself  are  of  more  vital  importance  as  regards  tran- 
spiration, since  they  can  be  controlled  to  a  certain  extent 
under  field  conditions.  An  increase  in  the  moisture  con- 
tent of  a  soil  usually  results  in  an  increased  transpiration 
ratio.  The  work  of  Hellriegel l  with  barley  grown  in 
quartz  sand  containing  a  nutrient  solution  may  be  cited 
in  this  regard,  together  with  the  data  obtained  by  Mont- 


gomery2  at 
loam  soil :  — 


Lincoln,   Nebraska,   with  corn  grown  in  a 


Effect  of  Soil  Moisture  on  Transpiration 


Barley  —  Hellriegel 

Corn  —  Montgomery 

Soil  Moisture  Per- 
centage of  Total 
Capacity 

Transpiration 
Ratio 

Soil  Moisture  Per- 
centage of  Total 
Capacity 

Transpiration 
Ratio 

80 
60 
40 
30 
20 
10 

277 
240 
216 
223 
168 
180 

100 

80 

60 

45 

35 

290 
262 
239 
229 
252 

These  data  show  clearly  that  an  excessive  amount  of 
water  in  the  soil  is  not  a  favorable  condition  for  the 


1  Hellriegel,  H.  Beitrage  zu  den  Naturwissenschaftlichen 
Grundlagen  des  Ackerbaus,  Seite  639.     Braunschweig.     1883. 

2  Montgomery,  E.  G.  Methods  of  Determining  the  Water 
Requirements  of  Crops.  Proc.  Amer.  Soc.  Agron.,  Vol.  3, 
p.    276.     1911. 


250       SOILS:    PROPERTIES  AND  MANAGEMENT 

economic  use  of  water,  as  the  plant,  in  order  to  supply 
itself  properly  with  food,  must  transpire  excessive  amounts 
of  water.  As  soil  moisture  may  be  controlled,  this  waste 
may  to  a  certain  extent  be  eliminated. 

172.  The  influence  of  fertility  on  transpiration.  —  The 
amount  of  available  plant-food  is  also  concerned  in  the. 
economic  utilization  of  water.  In  general  the  data  along 
these  lines  show  that  the  richer  the  soil,  the  lower  is  the 
transpiration  ratio.  Therefore  a  farmer,  in  raising  the 
general  fertility  of  his  soil  by  drainage,  lime,  good  tillage, 
green  manures,  barnyard  manures,  and  fertilizers,  provides 
at  the  same  time  for  a  greater  amount  of  plant  production 
for  every  unit  of  water  utilized.  Again,  quoting  from 
Hellriegel  and  Montgomery,  the  following  figures  are 
available :  — 


Effect  of  the  Supply  of  Plant-food  Materials  on  the 
Transpiration  Ratio  of  Barley  grown  in  Quartz  Sand 
with  a  Nutrient  Solution;  Calcium  Nitrate  being  in 
the  Minimum.     Hellriegel  x 


Units2  of  Ca(N0s>2 
Applied 

Dry  Matter  Produced 
per  Pot  (Grams) 

Transpiration  Ratio    . 

0 

4 

8 

12 

16 

20 

1,111 

8,479 
13,936 
18,288 
23,026 
25,504 

724 
399 
347 
345 
302 
292 

1  Hellriegel,    H.      Beitrage   zu   den   Naturwissenschaftlichen 
Grundlage  des  Ackerbaus,  p.  629.     Braunschweig.     1883. 

2  A   unit    of    Ca(N03)2    equals    1    mg. -equivalent.      A    mg.- 
equivalent  of  Ca(N03)2  equals  82.1  mg. 


WATER   OF  SOIL  IN  ITS  RELATION   TO  PLANTS      251 


Relative    Water    Requirements    of    Corn    on    Different 
Types  of  Nebraska  Soils,  1911.     Montgomery1 


Soil 

Dry  Weight  of  Plants 
in  Grams  per  Pot 

Transpiration  Ratio 

Manured 

Unmanured 

Manured 

Unmanured 

Poor  (15  bushels)      .     . 
Medium  (30  bushels)     . 
Fertile  (50  bushels)  .     . 

376 
413 
472 

113 
184 
270 

350 
341 
346 

549 
479 
392 

173.  Effect  of  texture  on  transpiration.  —  The  effects 
of  texture  have  been  investigated  by  a  number  of  men, 
the  work  of  Von  Seelhorst2  and  of  Widtsoe3  being  per- 
haps the  most  reliable.  While  these  investigators  found 
in  general  that  crops  on  heavy  soils  exhibited  a  lower 
transpiration  ratio,  hasty  conclusions  must  not  be  drawn. 
Since  the  fine-textured  soils  contain  more  plant-food 
materials,  it  is  probable  that  this  is  the  balancing  factor 
rather  than  texture  alone. 

174,  Actual  amounts  of  water  necessary  to  mature  a 
crop.  —  Although  it  can  be  seen  from  the  transpiration 
ratio  that  the  amount  of  water  necessary  to  bring  an 
average  crop  to  maturity  is  very  large,  a  concrete  example 
may  be  cited  to  advantage.  A  fair  estimate  of  the  dry 
matter  produced  in  raising  a  forty-bushel  crop  of  wheat 
would  be  about  two  tons.     Assuming  the  transpiration 


1  Montgomery,  E.  G.  Water  Requirements  of  Corn. 
Nebraska  Agr.  Exp.  Sta.,  25th  Ann.  Rept.,  p.  xi.     1912. 

2  Seelhorst,  C.  von.  Uber  den  Wasserverbrauch  von 
Roggen,  Gerste,  Weizen,  und  Kartoffeln.  Jour.  f.  Land- 
wirtschaft,  Band  54,  Heft  4,  Seite  316-342.     1906. 

3  Widtsoe,  J.  A.  Irrigation  Investigations.  Factors  Influ- 
encing Evaporation  and  Transpiration.  Utah  Agr.  Exp.  Sta., 
Bui.  105.     1909. 


252       SOILS:    PROPERTIES  AND  MANAGEMENT 

ratio  to  be  300,  the  amount  of  water  actually  used  by 
the  plant  would  amount  to  600  tons  to  the  acre,  or  about 
5.2  inches  of  rainfall.  This  does  not  include  the  evapora- 
tion that  is  continually  going  on  from  the  soil  surface, 
which  might  very  easily  amount  to  as  much  more.  More- 
over, this  draft  on  the  soil  water  is  not  a  uniform  one,  but 
increases  gradually  as  the  crop  develops,  until  at  heading 
time  great  quantities  must  be  supplied  in  a  short  period. 
The  necessity  of  moisture  conservation  in  order  to  meet 
the  plant  requirements  and  preserve  its  normal  develop- 
ment, even  in  humid  regions,  becomes  obvious. 

175.  Role  of  capillarity  in  the  supplying  of  the  plant 
with  water.  —  A  query  arises  at  this  point  regarding  the 
mode  by  which  this  immense  quantity  of  water  is  supplied 
to  the  plant.  The  plant  rootlets,  especially  their  absorb- 
ing surfaces,  are  few  in  number  as  compared  with  the 
interstitial  angles  that  contain  most  of  the  water  retained 
in  the  soil.  How,  then,  does  the  plant  avail  itself  of 
water  not  in  immediate  contact  with  its  rootlets?  This 
question  has  been  anticipated  in  the  discussion  concern- 
ing the  capillary  equilibrium  which  tends  to  occur  in  all 
soils.  As  soon  as  the  rootlet  begins  to  absorb  at  one 
point,  the  film  in  that  interstitial  angle  (see  Fig.  36)  is 
thinned.  A  considerable  convexity  of  the  water  surface 
occurs  at  that  point,  resulting  in  a  great  outward  pull 
which  causes  the  water  to  move  in  all  directions  toward 
that  point.  Thus,  a  feeding  rootlet,  by  absorbing  some 
of  the  soil  solution  with  which  it  is  in  contact,  creates  a 
condition  of  instability  which  results  in  considerable 
film  movement.  It  can  therefore  be  said  that  capillarity 
is  the  important  factor  in  any  soil  in  supplying  the  plant 
with  proper  quantities  of  moisture. 

Many  of  our  early  investigators  have  overestimated 


WATER   OF  SOIL  IN  ITS   RELATION  TO  PLANTS      253 

the  distances  through  which  this  movement  may  be 
effective  in  properly  supplying  the  plant.1  It  must  be 
understood,  however,  that  the  rate  of  water  supply  is 
the  controlling  factor  in  plant  nutrition.  It  has  been 
shown  also  that  the  longer  the  capillary  column,  the  less 
is  the  amount  of  water  delivered  from  a  water  table  to 
any  given  point.  Therefore  capillarity,  although  it 
may  act  through  a  distance  of  ten  feet,  may  be  important 
for  only  three  feet  as  far  as  plant  nutrition  is  concerned, 
since  water  beyond  that  point  is  moved  too  slowly  to  be 
of  any  great  value  in  time  of  need.  No  reliable  data  are 
available  as  to  this  particular  phase,  but  the  knowledge 
of  the  factors  governing  capillary  movement  clearly 
indicates  that  capillarity  of  the  soil  is  of  greatest  im- 
portance in  a  restricted  zone  immediately  around  each 
absorbing  root  surface. 

176.  Influence  of  water  on  the  plant.2  —  In  general, 
as  the  amount  of  water  available  to  a  crop  is  increased, 
the  vegetative  growth  also  is  increased,  the  plant  be- 
coming more  succulent.  The  percentage  of  moisture  in 
the  crop,  even  at  harvest  time,  is  usually  high.  Quality 
practically  always  suffers  with  such  a  stimulation  of 
vegetative  activity.  This  is  especially  noticeable  with 
such  crops  as  barley  and  peaches.  Shipping  qualities 
also  are  depressed  with  increased  water,  especially  if 
the  water  available  is  excessive.  With  an  enlargement 
of  the  plant  cell  a  change  probably  occurs  in  the  cell 
contents,  tending  toward  a  greater  susceptibility  to 
disease.     Ripening    is    delayed,    tillering    is    diminished, 

1  Warington,  R.  Physical  Properties  of  Soil,  p.  105.  Ox- 
ford.    1900. 

2  Mitscherlich,  E.  A.  Das  Wasser  als  Vegetationsfaktor. 
Landw.  Jahr.,  Band  42,  Seite  701-717.     1912. 


254       SOILS:    PROPERTIES  AND  MANAGEMENT 

and  the  percentage  of  protein  content  of  the  crop  is  de- 
creased. It  is  a  curious  fact,  as  will  be  shown  later,  that 
many  of  the  general  and  morphological  effects  of  large 
quantities  of  available  water  on  plant  growth  are  the 
same  as  those  from  the  presence  of  too  much  soluble 
nitrogen.  In  cereals  the  stimulation  of  increased  water 
is  shown  especially  in  the  ratio  of  grain  to  straw.  Widt- 
soe's1  results  in  this  regard  are  representative  of  the 
data 2  available  on  this  point :  — 

Distribution  of  Dry  Matter  between  Grain  and  Straw 
with  Varying  Amounts  of  Water 


Inches  op  Water 

Grain  in  Percentage  op 
Total  Dry  Matter 

5 

44.4 

7| 

43.2 

10 

42.8 

15 

40.8 

25 

38.6 

35 

37.5 

50 

32.9 

As  a  general  rule  this  depression  of  the  ratio  of  grain 
to  straw  is  not  due  to  an  actual  decrease  in  the  grain,  but 
to  a  correspondingly  greater  production  of  dry  matter  in 


1  Widtsoe,  J.  A.  The  Production  of  Dry  Matter  with 
Different  Quantities  of  Irrigation  Water.  Utah  Agr.  Exp. 
Sta.,  Bui.  116,  p.  49.     1912. 

2  Bunger,  H.  Uber  den  Einfluss  Verschieden  Hohen  Was- 
sergehalts  des  Bodens  in  den  Einzelhen  Vegetationsstadien 
bei  Verschiedenem  Nahrstoffreichtum  auf  die  Entwicklung 
des  Haferpflanzen.  Landw.  Jahrb.,  Band  35,  Seite  941-1051. 
1906.  Also,  Seelhorst,  C.  von,  und  Freekmann,  W.  Der 
Einfluss  des  Wassergehaltes  des  Bodens  auf  die  Ernten  und 
die  Ausbilding  Verschiedener  Getriedevarietaten.  Jour.  f. 
Landw.,  Band  51,  Seite  253-269.     1903. 


WATER   OF  SOIL  IN  ITS  RELATION    TO  PLANTS      255 

the  vegetative  parts.  As  available  water  increases  this 
dry  matter  ascends  until  a  maximum  is  reached.  The 
general  relationships  are  well  exemplified  by  data  from 


/6 

L 
1 

I* 

r 

■ 


"ca/2/v 

~~W/i£AT 

/ 

rs 

so  eo  30  4o  so  6om.   fiaO 

Fig.  40.  —  The  effect  of  increased  water  supply  on  the  production  of 
dry  matter  by  various  crops. 


Widtsoe,1  tabulated  on  the  following  page,  although 
other  equally  valuable  figures  may  be  obtained  from  Von 
Seelhorst 2  and  Atterberg.2  The  curves  above  (Fig.  40) 
illustrate  Widtsoe's  data  and  the  general  trend  in  the 
dry  matter  produced  as  the  moisture  is  increased. 


1  Widtsoe,  J.  A.  The  Production  of  Dry  Matter  with  Dif- 
ferent Quantities  of  Irrigation  Water.  Utah  Agr.  Exp.  Sta., 
Bui.   116,  pp.   19-25.     1912. 

2  Seelhorst,  C.  von,  und  Krzymowski,  Dr.  Versuch  iiber 
den  Einfluss,  welchen  das  WTasser  in  dem  Versehiedenem  Vegeta- 
tionsstadien  des  Hafers  auf  sein  Wachstum  ausiibt.  Jour.  f. 
Landw.,  Band  53,  Seite  357-370.  1905.  Also,  Atterberg,  A. 
Die  Variationen  der  Nahrstoffgehalte  bei  dem  Hafer.  Jour.  f. 
Landw.,  Band  49,  Seite  97-113.     1901. 


256       SOILS:    PROPERTIES  AND  MANAGEMENT 


Crop   Yield   in   Pounds   to   the   Acre    as    Influenced    by 
Different  Amounts  of  Water.     Widtsoe 


Dry 

Inches 

Dry 

Inches 

Dry 

Inches 

Matter 

OF 

Matter 

op 

Matter 

op  Water 

Wheat 

Water 

Corn 

Water 

Potato 

18.74 

4,969 

13.04 

10,757 

11.17 

2310 

21.24 

5,545 

15.54 

12,762 

13.67 

2730 

23.74 

5,684 

20.54 

13,092 

16.17 

2925 

28.74 

6,279 

25.54 

13,856 

21.17 

3405 

38.74 

6,672 

30.54 

14,606 

26.17 

4005 

48.74 

7,229 

35.54 

15,294 

36.17 

3660 

63.74 

7,999 

60.54 

12,637 

51.17 

3797 

177.  Availability  of  the  water  in  the  soil.  —  From  the 
discussion  already  presented  regarding  the  forms  of  water 
in  the  soil,  the  ways  in  which  they  are  held,  and  their 
movements,  it  is  evident  that  all  the  moisture  present 
in  a  soil  is  not  available  for  plant  growth.  Three  divisions 
of  the  soil  water  may  be  made  on  this  basis :  unavailable, 
available,  and  super-available. 

178.  Unavailable  soil  water.  —  As  has  been  shown  in 
a  previous  paragraph,  free  or  capillary  water  may  become 
of  little  use  to  a  plant  through  distance,  since  capillarity 
is  unable  to  pump  the  water  fast  enough  to  supply  ordinary 
crop  needs.  Water  near  at  hand  or  in  the  immediate 
zone  of  the  rootlet  may  also  become  unavailable  through 
the  obstruction  of  capillarity,  friction  instead  of  distance 
being  the  cause  in  this  case.  As  the  rootlet  thins  the 
interstitial  film  at  any  point,  capillarity  occurs  and 
water  moves  toward  the  absorbing  surface.  This  move- 
ment is  rapid  enough  for  plant  needs  until  the  film  chan- 
nels on  the  particles  become  thin.  (See  Fig.  37.)  As  the 
zone  of  hygroscopic  influence  of  the  particle  is  approached 


WATER   OF  SOIL  IN  ITS  RELATION  TO  PLANTS      257 

the  viscosity  increases  very  rapidly  and  cuts  down  the 
capillarity  to  such  a  point  that  the  needs  of  the  plant 
are  unsatisfied.  Wilting  therefore  occurs  simply  be- 
cause the  soil  is  unable  to  move  the  water  rapidly 
enough  for  crop  needs.  As  the  viscosity  of  the  water 
increases  very  rapidly  after  the  point  of  lento-capillarity 
is  reached,  the  wilting  coefficient  is  a  figure  somewhat 
less  than  the  percentage  representing  the  lento-capillarity ; 
also,  it  is  greater  than  the  hygroscopic  coefficient,  since 
wilting  due  to  viscosity  occurs  before  it  is  possible 
for  the  film  to  become  thinned  to  the  zone  of  hygro- 
scopicity.  Not  only  all  the  hygroscopic  water  is  unavail- 
able, then,  but  also  a  certain  small  quantity  of  the 
capillary  water  lying  between  the  point  of  wilting  and 
the  hygroscopic  film.  This  relationship  is  shown  by 
data  from  the  work  of  Heinrich  and  of  Briggs  and  Shantz ; 
men  working  at  widely  different  times  and  under  entirely 
different  conditions. 


Relation  of  the  Wilting  Point  to  the  Hygroscopic  Co- 
efficient.    Heinrich  x 


Soil 

Wilting  Point 

Percentage  op 
Hygroscopic  Water 

Coarse  sandy  soil 
Sandy  garden  soil 
Fine  humous  sand 
Sandy  loam    .     . 
Calcareous  soil    . 
Peat      .... 

1.5 
4.6 
6.2 

7.8 

9.8 

49.7 

1.15 
3.00 
3.98 
5.74 
5.20 
42.30 

1  Heinrich,  R.  Ueber  das  Vermogen  der  Pflanzen  den  Boden 
an  Wasser  zu  erschopfen.  Jahresbericht  der  Agri.-chem., 
Band  18,   Seite  368-372.     1875. 


258       SOILS:    PROPERTIES  AND  MANAGEMENT 


Relation  of  the  Wilting  Point  to  the   Hygroscopic  Co- 
efficient.      BltlGGS   AND   SHANTZ  l 


Soil 

Hygroscopic 
Coefficient 

Wilting  Point 

Coarse  sand 

Fine  sand 

Fine  sand 

Sandy  loam 

Sandy  loam         .... 
Fine  sandy  loam     .     .     . 

Loam 

Loam 

Clay  loam 

.5 
1.5 
2.3 
3.5 
4.4 
6.5 
7.8 
9.8 
11.4 

.9 

2.6 

3.3 

4.8 

6.3 

9.7 

10.3 

13.9 

16.3 

179.  The  wilting  coefficient  of  plants.  —  It  has  been 
known  for  many  years  that  the  common  plants  possess 
different  capacities  for  resisting  drought.  This  has 
usually  been  ascribed  to  one  or  more  of  three  causes: 
(1)  difference  in  root  extensions;  (2)  difference  in  ability 
to  become  adjusted  to  a  slow  intake  of  water ;  and  (3)  dif- 
ference in  pulling  power  against  the  viscosity  of  the  water 
film.  The  last  two  capabilities  would  argue  for  different 
wilting  coefficients  for  different  crops  on  the  same  soil. 
The  most  extended  work  on  this  subject  has  been  by 
Briggs  and  Shantz,2  who  found  that  the  permanent  wilting 
point  in  a  saturated  atmosphere  is  practically  the  same 
for  all  plants.     Later  Caldwell 3  demonstrated  that  this 

1  Briggs,  L.  J.,  and  Shantz,  H.  L.  The  Wilting  Coefficient 
for  Different  Plants  and  its  Indirect  Determination.  U.  S. 
D.  A.,  Bur.  Plant  Indus.,  Bui.  230,  p.  65.     1912. 

2  Briggs,  L.  J.,  and  Shantz,  H.  L.  The  Wilting  Coefficient 
for  Different  Plants  and  its  Indirect  Determination.  U.  S. 
D.  A.,  Bur.  Plant  Indus.,  Bui.  230.     1912. 

3  Caldwell,  J.  S.  The  Relation  of  Environmental  Condi- 
tions to  the  Phenomenon  of  Permanent  Wilting  in  Plants. 
Physiological  Researches,  Station  N,  Baltimore.  U.  S.  D.  A., 
Vol.  I,  No.  1.  .  July,  1913. 


WATER   OF  SOIL  IN  ITS  RELATION   TO  PLANTS      259 

relationship  of  the  physical  constants  of  the  soil  to  the 
wilting  point  depends  on  the  rate  at  which  the  plant 
loses  water,  showing  that  the  soil  factors  are  not  entirely 
dominant  in  this  respect.  This  work  seemed,  neverthe- 
less, to  indicate  that  the  conclusions  of  Briggs  and  Shantz 
were  correct  for  plants  of  humid  regions,  where  the  wilt- 
ing occurred  in  a  saturated  atmosphere.  If  such  is  the 
case,  it  can  be  accounted  for  only  by  the  fact  that  the 
soil  forces  in  their  effect  on  the  wilting  point  are  so  power- 
ful as  to  override  any  distinguishing  characteristics  that 
the  plant  itself  may  possess,  or  at  least  reduce  such  an 
influence  within  the  error  of  actual  experimentation. 

180.  Determination  of  the  wilting  point.  —  Briggs  and 
Shantz,1  in  their  investigations,  devised  a  very  accurate 
method  for  making  determinations  of  the  wilting  point. 
Glass  tumblers  holding  about  250  cubic  centimeters  of 
soil  in  an  optimum  condition  were  used.  The  seeds  were 
placed  in  this  soil,  after  which  soft  paraffin  was  poured 
over  the  surface  in  order  to  stop  evaporation,  thus  re- 
moving this  disturbing  factor  in  the  capillary  equilibrium 
of  the  moisture.  The  seedlings  on  germination  were 
able  to  push  through  this  paraffin.  While  the  plants 
were  developing,  the  tumblers  were  kept  standing  in  a 
constant-temperature  vat  of  water  in  order  to  prevent 
condensation  of  moisture  on  the  inside  of  the  glass.  The 
vegetative  room  was  under  temperature  control.  When 
definite  wilting  occurred,  as  determined  in  a  saturated 
atmosphere,  a  moisture  test  was  made  on  the  soil.  The 
resulting  figure,  within  experimental  error,  represents 
the  wilting  point  for  the  soil  used. 

1  Briggs,  L.  J.,  and  Shantz,  H.  L.  The  Wilting  Coefficient 
for  Different  Plants  and  its  Indirect  Determination.  U.  S. 
D.  A.,  Bur.  Plant  Indus.,  Bui.  230,  pp.  10-14.     1912. 


260       SOILS:    PROPERTIES  AND  MANAGEMENT 

181.  Calculation  of  the  wilting  point.  —  In  studying 
the  correlation  of  this  wilting  coefficient  to  soil  conditions, 
Briggs  and  Shantz  1  advanced  the  following  relationships. 
Expressed  as  formula?  they  represent  methods  of  at 
least  approximating  the  wilting  point  from  other  soil 
factors.  These  formulae  are  arranged  in  the  order  of 
their  reliability,  based  on  the  data  obtained  by  the 
authors :  — 


1.  Wilting  point  =  Moisture  equivalent  (em)r  2  Q  per  cent) 

2.  Wilting  point  =  Hygroscopic  coefficient  (em)r  71  per  oent) 

.68 

3.  Wilting  point 

=  Water-holding  capacity  (Hilgard  method)  ~  21  (orror  8  3  per  cent) 


182.  Relation  of  texture  to  the  wilting  point.  —  From 
the  data  already  quoted  2  from  Heinrich  and  from  Briggs 
and  Shantz  regarding  the  hygroscopic  coefficient  and  the 
wilting  point,  it  is  evident  that  a  very  close  relationship 
exists  between  the  texture  of  the  soil  and  the  percentage 
of  moisture  at  which  plants  wilt.  The  finer  the  soil 
texture,  the  higher  is  the  wilting  point.  The  following 
figures,  from  Briggs  and  Shantz,3  bring  out  the  point  very 
clearly :  — 


1  Briggs,  L.  J.,  and  Shantz,  H.  L.  The  Wilting  Coefficient 
for  Different  Plants  and  its  Indirect  Determination.  U.  S.  D. 
A.,  Bur.  Plant  Indus.,  Bui.  230,  pp.  56-77.     1912. 

2  This  text,  paragraph  178. 

3  Briggs,  L.  J.,  and  Shantz,  H.  L.  The  Wilting  Coefficient 
for  Different  Plants  and  its  Indirect  Determination.  U.  S.  D. 
A.,  Bur.  Plant  Indus.,  Bui.  230,  pp.  26-33.     1912. 


WATER   OF  SOIL  IN  ITS  RELATION  TO  PLANTS      261 


Relation  of  Texture  to  the  Wilting  Point  of  Kubanka 

Wheat 


Soil 

Moisture 
Equivalent 

Wilting  Point  of 
Kubanka  Wheat 

Sand 

1.55 

.86 

Fine  sand 

4.66 

2.60 

Fine  sand 

5.50 

3.33 

Fine  sand 

6.74 

3.70 

Sandy  loam 
Sandy  loam 
Sandy  loam 
Loam    .     . 

9.70 
14.50 
18.60 
23.80 

4.80 

9.60 

8.84 

12.40 

Loam    .     . 

25.00 

13.90 

Clay  loam 
Clay  loam 

27.40 
29.30 

14.50 
17.10 

Briggs  and  Shantz  have  attempted  to  express  this 
correlation  by  a  formula  which,  while  very  inaccurate, 
shows  in  general  the  relationships  already  expressed. 
The  correlation  in  this  case  is  made  between  the  wilting 
point  and  the  mechanical  composition  of  the  soil :  — 

Wilting  point  =  .01  sand  +  .12  silt  +  .57  clay  (error  10 
per  cent) 

183.  Available  and  super-available  water.  —  Advanc- 
ing from  the  wilting,  or  critical,  moisture  content  of  a 
soil,  all  the  remaining  capillary  water  is  found  to  be  avail- 
able for  normal  plant  use.  However,  when  free  water 
begins  to  appear,  a  condition  adverse  to  plant  growth  is 
established,  and  as  the  saturation  point  is  approached 
this  condition  becomes  more  adverse.  This  free  water 
is  designated  as  the  super-available  water,  since  it  is 
beyond  the  available  and  its  presence  generates  condi- 
tions unfavorable  to  plant  growth.     The  upper  limit  of 


262       SOILS:    PROPERTIES  AND  MANAGEMENT 

the  capillary  water  is  called  the  maximum  water  content 
for  plant  growth.  The  bad  effects  of  free  water  on  the 
plant  arise  largely  from  the  poor  aeration  that  results 
from  its  presence.  Not  only  are  the  roots  deprived  of 
their  oxygen,  but  toxic  materials  tend  to  accumulate. 
Favorable  bacterial  activities,  such  as  nitrification  and 
ammonification,  are  much  retarded  also. 

The  various  forms  of  water  in  the  soil  and  their  avail- 
ability to  the  plant  are  illustrated  diagrammatically  in 
the  following  figure. 

HYGROSCOPIC    WP0INT       LENTOC/iPILLARtTY  MAX/MUM  WATER 

COEEE/C/ENTk      I  POINT  CONTENT 

mW05C0P/(>[\    ^OPTIMUM    WATER   CONTENT        \  \EREE«JUPEfm/VIBLE 


AVAILABLE    MOISTURE 


Fig.  41. — Diagram  showing  the  forms  of  water  in  the  soil  and  their 
relationship  to  the  plant. 

184.  Optimum  moisture  for  plant  growth.—  It  is 
very  evident  that  there  must  be  some  moisture  condition 
of  a  soil  which  is  best  for  plant  development.  This  is 
usually  designated  as  the  optimum  content.  It  is  not 
to  be  assumed,  however,  that  the  total  range  of  the 
available  soil  water  represents  this  condition  for  optimum 
plant  growth.  ■  Nor  is  this  optimum  water  content  in 
any  particular  soil  to  be  designated  by  a  definite  per- 
centage. In  reality  the  moisture  in  a  soil  may  undergo 
considerable  fluctuation  and  yet  allow  the  plant  to  develop 
normally.  This  is  because  the  physical  condition  of  the 
soil  changes  with  varying  water  content  and  the  plant  is 
able  to  accommodate  itself  to  such  a  fluctuation  without 
a  disturbance  in  its  normal  development  occurring. 
Wollny  1  has  shown  that  the  optimum  moisture  for  com- 

1  Wollny,  E.  Untersuchung  iiber  den  Einfluss  der  Wachs- 
thumsfaktoren    auf    des     Produktionsvermogen     der     Kultur- 


WATER    OF  SOIL   IN  ITS   RELATION    TO   PLANTS      263 

mon  field  crops  in  general  covers  a  range  from  60  to  80 
per  cent  of  the  water  capacity  of  the  soil.  Mayer x 
placed  the  optimum  moisture  content  of  wheat  at  80 
per  cent  of  the  water  capacity  of  the  soil,  rye  at  75  per 
cent,  barley  at  75  per  cent,  and  oats  at  from  85  to  90 
per  cent.  Such  estimates  not  only  emphasize  the  range 
of  optimum  moisture  conditions,  but  at  the  same  time 
show  the  relatively  high  percentage  of  moisture  necessary 
for  maximum  crop  growth. 

Granulation  has  considerable  influence  on  the  range  of 
optimum  moisture  conditions,  since  the  better  the  granu- 
lation, the  better  is  the  soil  able  to  accommodate  itself 
to  changes  in  water  content  without  disturbance  of 
plant  growth.  The  poorer  the  tilth  of  any  soil,  the 
narrower  does  this  fluctuating  in  optimum  moisture  be- 
come. In  moisture  conservation  and  control  a  granular 
soil  is  one  of  the  first  improvements  to  be  aimed  at. 
Drainage,  liming,  addition  of  organic  matter,  and  tillage, 
by  leading  up  to  such  a  condition,  increase  the  effective- 
ness and  economy  of  utilization  of  soil  moisture. 


pflanzen.       Forsch.    a.    d.    Gebiete   d.    Agri.-Physik,    Band  20, 
Seite  53-109.     1897. 

1  Mayer,  A.  Uber  den  Einfluss  kleinerer  oder  Grosserer 
Mengen  von  Wasser  auf  die  Entwickelung  einiger  Kultur- 
pflanzen.     Jour.  f.  Landw.,  Band  46,  Seite  167-184.     1898. 


CHAPTER   XIII 


THE  CONTROL  OF  SOIL  MOISTURE 


In  the  discussion  of  the  water  requirements  of  plants, 
it  was  apparent  that  for  a  normal  yield  of  any  crop  the 
amount  used  by  the  plant  alone  was  very  great,  varying 
from  five  to  ten  acre  inches  according  to  conditions.  Were 
this  the  only  loss  of  water,  the  question  of  raising  crops 
with  given  amounts  of  rainfall  would  be  a  simple  one. 
Three  further  sources  of  water  loss,  however,  are  usually 
found  functioning  in  the  soil  and  tending  to  lower  the 
water  that  would  go  toward  transpiration,  a  loss  absolutely 
necessary  for  proper  plant  growth.  The  various  ways  by 
which  water  finds  an  exit  from  a  soil  are  (1)  transpiration, 
(2)  run-off  over  the  surface,  (3)  percolation,  and  (4)  evap- 
oration. The  following  diagram  makes  clear  their  re- 
lationships. 


Transpiration 


.» Transpiration. 
O-    A         )r~/yo/f 
S    Fi/aporation' 

/77m 


Fig.  42. — Diagram  illustrating  the  ways  by  which  water  may  be  lost 

from  a  soil. 

264 


THE  CONTROL   OF  SOIL  MOISTURE  265 

It  is  immediately  obvious  that  as  the  losses  by  run-off, 
leaching,  and  evaporation  increase,  the  amount  of  water 
left  for  crop  utilization  decreases.  In  arid  and  semiarid 
regions  this  is  fatal  to  plant  growth,  while  in  humid  regions 
it  may  be  such  a  factor  in  periods  of  drought  as  to  se- 
riously reduce  the  harvest.  Control  of  moisture  is  there- 
fore necessary  in  all  regions,  and  this  really  consists  in 
so  adjusting  run-off,  leaching,  and  evaporation  as  to 
maintain  optimum  moisture  conditions  in  the  soil  at  all 
times.  This  control  should  result  in  a  proper  and  eco- 
nomic utilization  of  the  soil  water  by  the  plant. 

185.  Run-off  losses.  —  In  regions  of  heavy  rainfall 
or  in  areas  where  the  land  is  sloping  or  rather  impervious 
to  water,  a  considerable  amount  of  the  moisture  received 
as  rain  is  likely  to  be  lost  by  running  away  over  the  sur- 
face. Under  such  conditions  two  considerations  are  im- 
portant :  (1)  by  not  entering  the  soil  the  water  is  lost  for 
plant  use;  and  (2)  washing  of  the  soil  may  occur,  which 
if  allowed  to  proceed  may  entirely  ruin  the  land.  The 
amount  of  run-off  varies  with  the  rainfall,  the  slope,  and 
the  character  of  the  soil.  In  some  regions  it  may  rise 
as  high  as  50  per  cent  of  the  rainfall,  while  in  arid  regions 
it  is  of  course  very  nearly  zero.  As  a  general  thing,  this 
loss  is  estimated  with  the  losses  by  leaching,  the  two  being 
expressed  as  one  figure. 

186.  Percolation  losses.  —  When  at  any  time  the 
amount  of  rainfall  entering  a  soil  becomes  greater  than 
the  water-holding  capacity  of  the  soil,  losses  by  percola- 
tion will  result.  The  losses  will  depend  largely  on  the/ 
amount  and  distribution  of  the  rainfall  and  the  capa- 
bility of  the  soil  to  hold  moisture.  The  bad  effects  of 
excessive  percolation  are  twofold :  (1)  the  actual  loss  of 
water,  and  (2)  the  leaching-out  of  salts  that  may  function 


266       SOILS:    PROPERTIES  AND  MANAGEMENT 

as  plant-food.  The  quantity  of  nutrient  elements  lost 
annually  from  the  average  soil  in  a  humid  region  more 
than  equals  that  withdrawn  by  the  crops.  The  results 
from  the  Rothamsted  drain  gauges  x  from  1871  to  1904 
on  a  clay  loam  soil  of  three  different  depths  are  inter- 
esting as  to  the  light  that  they  afford  regarding  actual 
drainage  losses :  — 


Drainage  through 

Proportion  of 
Rainfall  Drained 

through  Soil 

Rain- 
fall 

Depth  in  Inches 

Per  Cent 

20 

40 

60 

20 

40 

60 

January       .... 

2.32 

1.82 

2.05 

1.96 

78.5 

88.4 

84.5 

February     . 

1.97 

1.42 

1.57 

1.48 

72.2 

80.0 

75.2 

March    .     . 

1.83 

0.87 

1.02 

0.95 

47.6 

55.6 

52.0 

April       .     . 

1.89 

0.50 

0.57 

0.53 

26.5 

30.0 

28.0 

May       .     . 

2.11 

0.49 

0.55 

0.50 

23.2 

26.1 

23.6 

June        .     . 

2.36 

0.63 

0.65 

0.62 

24.0 

27.6 

26.3 

July        .     . 

2.73 

0.69 

0.70 

0.65 

25.3 

25.6 

23.8 

August   .     . 

2.67 

0.62 

0.62 

0.58 

23.2 

23.2  21.7 

September 

2.52 

0.88 

0.83 

0.76 

35.0 

32.8  30.0 

October 

3.20 

1.85 

1.84 

1.68 

57.8 

57.5,  52.5 

November  . 

2.86 

2.11 

2.18 

2.04 

76.7 

76.3 

72.4 

December  . 

ear 

2.52 

2.02 

2.15 

2.04 

80.3 

85.4 

81.0 

Mean  total  per  j 

28.98 

13.90 

14.73 

13.79 

48.2 

51.0 

48.0 

Winter,    October    to 

March     .... 

14.70 

10.09 

10.81 

10.15 

68.6 

72.8 

69.0 

Summer,     April     to 

September   .     .     . 

14.28 

3.81 

3.92 

3.64 

26.6 

27.4  25.4 

The  rainfall  and  relative  loss  through  the  40-inch  depth 
of  soil  is  shown  graphically  in  the  following  diagram :  — 


1Hall,  A.  D. 
pJ  23.     London. 


The  Book  of  the  Rothamsted  Experiments, 
1905. 


THE   CONTROL    OF  SOIL   MOISTURE 


267 


K 

esf/A/F 

4LL^ 

*s 

\ 
\ 

/ 

S 

/ 

\ 
\ 

J7/P4// 

1AG£ 

/ 
/ 

\ 

\ 



^ 

Fig.  43. — Rainfall  and  percolation  losses  through  a  40-inch  soil  column. 
Lysimeter  records,  Rothamsted  Experiment  Station,  England. 


It  appears  from  these  figures  that  about  50  per  cent 
of  the  rainfall  in  such  a  climate  as  that  of  England  is 
lost  by  percolation  alone.  It  appears  also  that  the  loss 
is  much  lower  in  summer  than  in  winter,  the  ratio  being 
about  one  to  three.  Also,  the  longer  the  soil  column, 
the  less  is  the  percolation,  due  to  the  greater  water- 
holding  capacity  possessed  by  the  longer  column. 

187.  Methods  of  checking  loss  by  run-off  and  leaching. 
—  It  must  not  be  inferred  that  the  soil  is  never  in  such 
a  condition  that  percolation,  and  even  run-off,  are  not 
advantageous.  Often  in  winter  the  excess  water  may 
be  drained  over  the  surface  with  no  damage  whatsoever. 
Also,  when  the  soil  becomes  filled  with  free  water,  either 
in  winter  or  in  the  growing  season,  drainage  must 
take  place  in  order  to  establish  optimum  soil  conditions. 
The  control  of  the  free  water  of  the  soil  may  be  brought 


268       SOILS:    PROPERTIES  AND  MANAGEMENT 

about  by  drainage  operations  or  by  methods  that  will 
increase  the  water-holding  capacity  of  the  soil.  The 
former  is  really  a  matter  of  engineering  technique  and 
will  be  treated  in  a  separate  chapter.  The  latter  is  a 
function  of  the  soil  itself  and  must  be  specifically  con- 
sidered at  this  point. 

The  necessity  of  giving  attention  to  losses  due  to 
run-off  and  leaching  varies  with  climatic  conditions. 
In  very  humid  regions  these  losses  are  of  grave  importance, 
while  in  arid  regions  they  are  insignificant  as  compared 
with  losses  by  evaporation.  For  example,  in  England 
the  losses  by  percolation  and  run-off  in  many  cases  are 
as  high  as  60  per  cent  of  the  rainfall.  In  the  Mississippi 
River  basin  the  loss  is  50  per  cent,  in  the  Missouri  it  is 
about  20,  while  in  the  Great  Basin  it  drops  to  zero.  This 
does  not  indicate  that  drainage  is  not  practiced  in  the 
last-named  region,  however,  for,  owing  to  over-irrigation, 
seepage,  and  other  conditions,  drainage  operations  often 
become  as  important  as  in  humid  climates. 

The  quantity  of  water  entering  a  soil  is  determined 
almost  entirely  by  the  physical  condition  of  the  soil. 
If  the  soil  is  loose  and  open,  the  water  enters  readily  and 
little  is  lost  over  the  surface  as  run-off.  If,  on  the  other 
hand,  the  soil  is  compact,  impervious,  and  hard,  most 
of  the  rainfall  runs  away,  and  not  only  is  there  a  serious 
loss  of  water,  but  considerable  erosion  may  also  result. 
The  first  step  in  checking  run-off  losses,  therefore,  is 
strictly  physical  in  nature.  As  the  water  that  has  entered 
the  soil  moves  downward  it  is  continually  being  changed 
to  capillary  water  in  its  passage.  If  the  capillary  capacity 
of  the  soil  is  high,  a  greater  percentage  of  this  rain  water 
becomes  capillary  and  a  less  percentage  is  left  to  be  carried 
away  as  gravitational  water.     The  secret  in  the  control 


THE   CONTROL    OF  SOIL   MOISTURE  269 

of  run-off  and  percolation, rthen,  is  first,  to  have  a  loose, 
open  structure  of  soil  in  order  to  facilitate  ready  entrance 
of  the  water jj  and  secondly,  to  promote  and  encourage 
a  physical  condition  of  soil  which  provides  high  capillary 
capacity^  Drainage,  lime,  humus,  and  good  tillage  en- 
courage granulation,  which  has  so  much  to  do  with  the 
proper  entrance  of  water  into  the  soil  and  its  proper 
handling  and  utilization  therein.  The  benefits  of  drain- 
age are  felt  only  when  free  water,  superavailable  to 
plants,  becomes  present.  Its  quick  removal,  therefore, 
not  only  betters  the  physical  condition  of  the  soil,  but 
also  aids  in  the  maintenance  of  the  optimum  moisture 
conditions  for  the  plants. 

Fall  and  early  spring  plowing  is  always  recommended 
as  a  means  of  increasing  the  moisture  capacity  of  the 
soil,  particularly  where  organic  matter  is  well  supplied. 
It  provides  a  deep  soil,  and  should  establish  the  best 
conditions  for  the  storage  of  moisture,  as  well  as  food, 
for  the  plant.  If  organic  matter  is  not  supplied,  deep 
plowing  is  not  advisable  on  light  sandy  soil ;  but  on 
clay  soil  it  is  beneficial  because  of  the  loosening  and  granu- 
lating effect.  Fall  plowing  in  particular  is  to  be  recom- 
mended for  such  soil,  as  the  loose  condition  produced 
facilitates  the  entrance  of  surface  water  while  the  granu- 
lation that  the  soil  undergoes  during  the  winter  increases 
its  water-holding  power.  A  soil  in  excellent  physical 
condition  may  contain  considerably  more  water  than 
the  soil  of  the  same  texture  but  in  poor  tilth,  and  yet 
present  better  conditions  for  crop  growth.  Where  fall 
plowing  cannot  be  done,  early  spring  plowing  is  the  next 
best  procedure. 

188.  Evaporation  losses.  —  Evaporation  of  soil  water 
takes   place   almost   entirely   at   the   surface,   exceptions 


270       SOILS:    PROPERTIES  AND  MANAGEMENT 

being  where  deep,  large  cracks  occur,  which  allow  thermal 
loss  directly  from  the  subsoil.  This  loss  of  water  by 
direct  evaporation  from  the  soil  may  be  excessive  and 
may  result  in  direct  reduction  of  the  crop  yield  —  a  type 
of  loss  so  familiar  that  examples  hardly  need  be  cited. 
In  the  results  with  the  Rothamsted  rain  gauges  about 
50  per  cent  of  the  annual  rainfall  was  regained  in  the 
drainage  water.  Since  the  gauges  bore  no  crop,  the 
remaining  50  per  cent  must  have  been  lost  by  evapora- 
tion. It  should  be  noted  that  in  the  summer  months 
under  warm  temperature  this  loss  was  greatest,  amount- 
ing to  75  per  cent  of  the  rainfall.  Correspondingly,  in 
the  semiarid  and  arid  sections  of  the  country,  where 
there  is  little  or  no  drainage,  the  rainfall  is  all  lost  by 
evaporation.  Investigations  indicate  that  about  70  per 
cent  of  the  precipitation  on  the  land  surface  is  derived 
from  evaporation  from  land  surface.  Even  in  humid 
regions,  where  the  annual  rainfall  is  ample  for  maximum 
crop  production,  the  crops  are  frequently  reduced  below 
the  profit  point  by  prolonged  periods  of  dry  weather  in 
the  growing  season,  during  which  the  loss  of  water  from 
the  plants,  coupled  with  the  loss  from  the  soil,  exhausts 
the  moisture  supply. 

While  run-off  and  percolation  are  directly  proportional 
to  the  rainfall,  loss  by  evaporation  does  not  vary  to  such 
a  degree.  The  loss  by  percolation  depends  almost 
directly  upon  the  amount  of  rainfall  above  the  retentive 
power  of  the  soil.  In  years  of  heavy  precipitation, 
losses  by  percolation  must  increase.  Evaporation  from 
the  soil  depends  largely  upon  the  time  that  the  soil 
surface  is  moist,  and  this  will  not  vary  markedly  from 
year  to  year.  The  following  figures  from  the  Rothamsted 
drain  gauges  may  be  quoted  in  this  regard :  — 


THE   CONTROL    OF  SOIL   MOISTURE 

Records  from  Rothamsted  (1870-1878)  x 


271 


Rainfall  Inches 

Evaporation  Inches 

Percolation  Inches 

22.9 

17.3 

5.6 

26.3 

18.4 

7.9 

29.3 

18.1 

11.2 

30.8 

18.3 

12.5 

31.6 

16.6 

15.0 

32.6 

18.0 

14.6 

34.2 

18.0 

16.2 

35.8 

18.3 

17.5 

42.7 

17.2 

25.5 

A  rough  calculation  may  be  made  which  will  show  the 
apportionment  of  the  yearly  rainfall  in  a  humid  region  of 
the  temperate  zone  between  the  three  forms  of  losses  — 
run-off  and  percolation,  evaporation,  and  transpiration. 
The  percolation  under  a  rainfall,  say,  of  28  inches,  as 
shown  by  the  Rothamsted  work,  is  roughly  14  inches,  or 
50  per  cent.  The  water  requirement  of  an  ordinary 
crop  is  about  7  inches.  This  leaves  a  loss  of  7  inches 
to  be  credited  to  evaporation.  In  other  words,  one- 
half  the  rainfall  goes  as  run-off  and  percolation,  while 
the  other  half  is  divided  about  equally  between  the  plant 
and  loss  by  evaporation.  While  run-off  and  percolation 
may  be  checked  to  some  extent,  not  enough  conservation 
can  occur  in  this  direction  to  tide  a  crop  over  a  period 
of  drought.  Paramount  attention  should  therefore  be 
directed  toward  the  checking  of  losses  by  evaporation, 
since  moisture  thus  saved  means  just  that  amount  added 
to  the  water  available  for  crop  use.  It  should  be  remem- 
bered that  over  a  large  proportion   of  cultivated  lands 

1  Warington,  R.  Physical  Properties  of  the  Soil,  p.  109. 
1900. 


272       SOILS:    PROPERTIES  AND  MANAGEMENT 

the  crop  yields  are  controlled  more  directly  by  lack  than 
by  excess  of  water.  It  is  a  common  observation  that 
soils  which  ordinarily  give  a  low  yield  in  seasons  of  nor- 
mal or  low  rainfall  give  good  yields  in  a  wet  season, 
indicating  how  dominant  is  this  influence  of  moisture  on 
soil  fertility. 

189.  Methods  of  checking  evaporation  losses.  —  All 
methods  for  the  reduction  or  elimination  of  evaporation 
losses  depend  on  one  or  both  of  two  functions :  (1)  the 
actual  control  of  evaporation  as  it  occurs  at  the  surface ; 
and  (2)  the  prevention  of  the  movement  of  capillary 
water  upward  to  take  the  place  of  the  moisture  already 
lost.  It  has  been  shown  that  as  water  is  lost  at  the  sur- 
face of  a  soil,  movement  is  induced  and  capillarity  is 
set  up.  Such  action,  if  allowed  to  continue,  must  ulti- 
mately bring  about  great  losses.  The  obstruction  of 
capillarity  would  obviously  lower  these  losses  to  a  marked 
degree.  As  it  is  difficult  and  often  impracticable  to  en- 
tirely eliminate  evaporation,  the  most  successful  methods 
of  water  control  usually  include  a  change  in  the  structural 
condition  of  the  soil  which  tends  toward  a  lower  capil- 
larity, especially  at  the  surface.  Of  all  the  methods  of 
moisture  conservation,  the  use  of  a  n^ulch  has  been  found 
most  satisfactory.  The  consideration  of  mulches  is 
therefore  one  of  the  most  important  phases  in  the  study 
of  moisture  control. 

190.  Mulches  for  moisture  control.  —  Any  material 
applied  to  the  surface  of  a  soil  primarily  to  prevent  loss 
by  evaporation  may  be  designated  as  a  mulch.  It  may 
at  the  same  time  fulfill  other  useful  functions,  such  as 
the  keeping  down  of  weeds  and  the  maintaining  of  a 
uniform  soil  temperature.  By  the  conservation  of  the 
moisture,  more  water  remains  in  the  soil  for  the  solution 


THE-   CONTROL   OF  SOIL   MOISTUBE  273 

of  the  essential  elements,  and  bacterial  activity  is  en- 
couraged. As  a  general  rule,  more  soluble  plant-food  is 
likely  to  be  found  under  a  mulched  soil,  other  conditions 
being  equal,  than  under  a  soil  not  so  treated. 

191.  Kinds  of  mulches.  —  Mulches  are  of  two  general 
sorts,  artificial  and  natural.  In  the  former  case,  foreign 
material  is  merely  spread  over  the  soil  surface  and  evapora- 
tion is  obstructed  thereby.  Manure,  straw,  leaves,  and 
the  like,  may  be  used  successfully.  Such  mulches,  while 
very  effective,  are  not  generally  applicable  to  field  crops 
where  intertillage  is  practiced,  since  they  would  make 
cultivation  absolutely  impossible  by  cumbering  the  soil 
surface  with  a  large  amount  of  inert  material.  Their 
use  is  therefore  limited  to  intensive  crops  such  as  are 
found  in  trucking  operations.  Leaves,  including  pine 
needles,  and  sawdust  are  very  effective  as  a  mulch,  but 
some  precautions  should  be  observed  in  their  application. 
For  example,  the  oak  is  rich  in  tannic  acid,  which  may 
be  washed  out  of  the  mulch  into  the  soil  and  by  its  effect 
on  the  growing  plant  may  cause  a  lowering  of  productivity. 
In  some  European  countries,  as  well  as  in  a  few  localities 
in  America,  stones  have  been  drawn  on  the  soil  to  serve 
as  a  mulch,  particularly  in  orchard  and  vineyard  culture, 
with  markedly  beneficial  effects.  Particularly  is  this 
true  on  such  lands  as  are  too  steep  to  permit  cultivation. 
As  further  evidence  of  the  utility  of  this  practice,  it  has 
been  observed  in  the  fruit-growing  section  of  the  Ozark 
Mountains,  and  doubtless  in  other  regions,  that  the 
removal  of  stones  from  the  land  not  only  results  in  the 
soil's  becoming  harder,  but  also  reduces  crop  yield  by 
increasing  loss  of  moisture.  It  is  therefore  necessary 
for  the  farmer  to  decide  whether  the  inconvenience 
to  tillage    or   other   operations   due   to  the   presence   of 


274       SOILS:    PROPERTIES  AND   MANAGEMENT 

stones  may  not  be  more  than  offset  by  their  beneficial 
effects. 

The  materials  for  mulching  mentioned  above  are  all 
strictly  artificial,  and  their  application  is  greatly  limited, 
due  to  the  lack  of  material  and  the  expense  involved. 
They  are  therefore  used  only  under  special  conditions. 
The  second  type  of  mulch  is  almost  universal  in  its  prac- 
tical availability. 

By  proper  cultivation  almost  any  soil  surface  may  be 
brought  into  such  a  condition  that  evaporation  of  mois- 
ture is  more  rapid  than  the  upward  capillary  movement. 
This  is  because  surface  tillage  produces  a  loose,  open 
structure,  which,  while  increasing  the  rate  of  thermal 
movement  of  the  water,  at  the  same  time  obstructs 
capillary  action.  The  surface  layer,  therefore,  quickly 
becomes  air-dry  and  is  in  a  condition  designated  as  a 
soil  mulch.  As  it  differs  from  the  soil  below  only  in 
structure,  it  has  numerous  advantages  over  artificial 
mulches,  at  the  same  time  performing  successfully  all 
the  functions  of  the  latter.  Since  not  only  the  water  in 
the  mulch  is  sacrificed  but  also  a  small  quantity  pumped 
upward  by  capillarity  during  the  operation,  speed  in 
formation  is  of  importance.  The  tillage  implements 
that  give1  the  maximum  looseness  and  granulation  will 
prove  the  most  successful.  A  spike-tooth  harrow  or  a 
weeder  is  the  instrument  ordinarily  employed. 

192.  The  functions  of  a  mulch.  —  A  soil  mulch  depends 
for  its  effectiveness  on  two  functions  —  (1)  the  shutting- 
off  of  evaporation,  and  (2)  the  checking  of  capillary  move- 
ment upward.  It  has  already  been  shown  that  thermal 
movement  of  water  through  dry  soil  layers  is  practically 
nil;1  therefore,  as  long  as  the  soil  is  dry,  evaporation  is 
1  This  text,  paragraph  166. 


THE  CONTROL    OF  SOIL   MOISTURE  275 

very  low.  Moreover,  any  layer  of  air-dry  soil  resists 
wetting,  principally  because  of  the  resins  and  oils  that 
become  deposited  on  the  surface  of  the  soil  particles. 
This  material,  called  "  agricere,"  has  a  low  surface  tension 
and  the  capillary  water  film  is  not  easily  resumed  under 
such  conditions.  Again,  if  the  soil  is  well  granulated 
it  is  able  to  assume  a  looser  and  more  open  structure. 
The  interstitial  angles,  which  afford  spaces  for  capillary 
surfaces,  are  cut  down,  and  the  capillary  pulling  power 
of  the  layer  is  much  reduced  even  if  it  should  assume  a 
film  of  water.  It  is  evident  that  looseness  and  dryness 
are  the  essentials  in  the  efficiency  of  a  soil  mulch.  As 
long  as  a  mulch  is  dry,  texture  is  not  a  very  important 
factor  in  efficiency,  a  dry  sand  being  about  as  effective 
as  a  dry  clay.  Texture  is  important,  however,  in  the 
length  of  time  that  a  mulch  will  remain  effective.  Due 
to  the  fact  that  the  capillary  power  of  a  clay  is  so  great, 
it  will  become  moist  from  below  after  a  few  days ;  while 
a  sand  mulch,  if  there  is  no  rain,  will  remain  dry  for  an 
indefinite  period.  On  a  heavy  clay  soil  in  fine  tilth  a 
mulch  may  be  destroyed  by  moist,  foggy  weather,  or 
by  a  number  of  days  of  very  humid  atmosphere ;  such  a 
condition,  by  causing  condensation  of  moisture  on  the 
clay,  hastens  the  reestablishment  of  capillarity  with  the 
subsoil,  thus  allowing  moisture  to  be  pumped  up  and 
lost. 

193.  The  soil  mulch  versus  the  dust  mulch.  —  A  few 
words  will  not  be  amiss  at  this  point  regarding  the  term 
"  dust  mulch,"  which  is  observed  so  commonly  in  soil 
literature.  This  term  would  indicate  that  the  mulch  is 
in  a  very  fine  condition,  its  granulation  having  been 
broken  down.  Such  a  condition  would  not  be  conducive 
to  efficiency,  as  it  would  encourage  capillarity,  while  at 


276       SOILS:    PROPERTIES  AND  MANAGEMENT 

the  same  time  it  would  become  puddled  on  wetting  — 
certainly  a  very  undesirable  condition.  As  a  matter  of 
fact,  efficient  mulches  are  not  in  a  dust  form,  but  are 
granulated  and  much  looser  than  could  be  obtained  were 
they  finely  divided.  It  is  evident  that  the  term  "  dust 
mulch  "  is  incorrect  and  should  be  superseded  by  "  soil 
mulch,"  a  figure  of  speech  which  more  exactly  expresses 
the  true  field  conditions. 

194.  Formation  of  a  mulch.  —  It  has  already  been 
stated  that  a  mulch  should  be  formed  as  quickly  as 
possible.  This  would  not  be  such  a  factor  were  the 
mulch  adjusted  only  once  in  a  season.  It  is  necessary, 
however,  especially  in  humid  regions,  to  re-form  a  mulch 
every  week  or  ten  days.  The  cutting-down  of  formation 
losses  therefore  becomes  important.  In  general  the 
mulch  should  be  made  just  as  soon  after  a  rain  as  it  is 
possible  to  work  the  land,  since  the  most  rapid  evapora- 
tion occurs  during  the  few  hours  immediately  after  a 
rain,  when  the  soil  is  very  moist.  Even  after  light  showers 
the  soil  should  be  quickly  cultivated,  since  the  rain  may 
have  established  a  capillary  communication  with  the 
surface  and  thus  provided  for  a  rapid  loss  of  the  water 
already  conserved  by  previous  work.  Under  arid  con- 
ditions, where  the  atmosphere  is  dry  and  hot  and  in  free 
circulation,  the  surface  soil  is  quickly  dried  out  after  a 
rain.  This  drying  takes  place  so  rapidly  that  the  capil- 
lary films  quickly  become  so  thin  that  movement  is 
stopped  and  no  more  water  is  brought  to  the  surface. 
The  soil  may  be  ever  so  hard  and  compact,  but  as  long 
as  it  is  kept  dry  it  very  effectively  conserves  the  moisture 
below.  The  more  rapid  the  loss,  the  more  quickly  will 
the  mulch  condition  be  created,,  and  therefore  the  less  the 
total  loss  of  water  is  likely  to  be.     This  has  been  demon- 


THE  CONTROL   OF  SOIL   MOISTURE 


277 


strated  by  Buckingham  1  in  some  experiments  in  which 
arid  climate  conditions  were  created  at  the  surface  of  a 
capillary  column  forty-six  inches  in  height.  The  soil 
was  a  fine  sandy  loam.  At  first  the  loss  of  water  under 
the  arid  conditions  was  very  rapid  and  exceeded  that 
under  the  humid  conditions;  but  the  rate  of  loss  soon 
dropped  considerably  below  that  of  the  humid  column, 
and  continued  to  fall  behind  during  the  twenty  days 
of  the  experiment.  The  differences  in  this  case  were 
due  to  self-mulching,  a  very  common  phenomenon  of 
arid  land  soils,  particularly  those  of  a  loamy  character. 
This  self-mulching  is  often  seen  in  sands  in  humid 
regions.  The  under  layers  of  a  sand  pile  are  always 
moist,  due  to  the  self-mulching  tendencies  of  the  sur- 
face. The  results  of  Buckingham  are  shown  in  the  fol- 
lowing curves : — 


/O  /S  ZO  0*Y<5     £J.4P&£0 


Fig.  44. — Evaporation  curves  on  a  sandy  loam  under  humid  and  arid 
conditions.  Self-mulching  has  occurred  under  the  arid  conditions 
and  a  reduction  in  evaporation  has  resulted. 


1  Buckingham,  E.     Studies  on  the  Movement  of  Soil  Mois- 
ture.    U.  S.  D.  A.,  Bur.  Soils,  Bui.  38,  pp.  18-24.     1907. 


27.8       SOILS:    PROPERTIES  AND  MANAGEMENT 

195.  Depth  of  a  mulch.  —  The  depth  of  a  mulch  is  an 
important  question  in  humid  regions.  Not  only  must 
all  the  water  in  the  layer  be  sacrificed  in  order  to  make 
the  mulch  effective,  but  the  plant-food  of  that  layer 
is  temporarily  withdrawn  from  use.  In  humid  areas, 
where  the  surface  soil  is  usually  not  over  eight  or  ten 
inches  in  depth,  the  latter  consideration  is  vital,  since 
the  fertility  of  the  soil  would  be  greatly  depressed  by  a 
deep  soil  mulch.  Another  factor  to  be  considered  here 
is  the  possible  root  pruning  that  may  occur  while  the 
mulch  is  being  formed.  While  not  of  importance  early 
in  the  season,  it  is  worthy  of  considerable  attention  when 
the  intertilled  crop  attains  some  age.  It  has  been  shown, 
with  such  crops  as  corn,  that  considerable  depression  in 
yield  may  result  from  the  maintenance  of  a  mulch  at  too 
great  a  depth,  some  of  the  feeding  roots  being  cut  off 
thereby.  For  such  reasons  the  average  depth  of  mulch 
for  humid  regions  and  in  dry-farming  operations  has 
become  regulated  to  about  three  inches,  although  in  the 
late  cultivation  of  corn  a  less  depth  than  this  is  advocated. 
In  irrigated  regions  where  little  rainfall  occurs  and  where 
the  soil  is  very  deep  and  uniformly  fertile,  mulches  as 
deep  as  ten  or  twelve  inches  are  sometimes  found,  es- 
pecially in  orchards.  As  rainfall  occurs  but  few  times 
during  the  season,  such  a  mulch  often  needs  no  attention 
except  for  its  original  formation.  With  crops  having 
shallow  roots  a  thinner  mulch  layer  must  of  course  be  used. 

196.  Resume  of  mulch  control.  —  To  summarize  briefly, 
the  cardinal  points  in  mulch  control  are :  (1)  mulches  are 
more  effective  and  more  easily  maintained  in  an  arid  than 
in  a  humid  climate;  (2)  their  efficiency  depends  directly 
on  their  dryness,  looseness,  and  granulation ;  (3)  sandy 
soil  is  more  easilv  maintained  as  a  mulch  than  clav  soil ; 


THE  CONTROL    OF  SOIL   MOISTURE 


279 


(4)  from  two  to  three  inches  is  ordinarily  the  most  effec- 
tive depth;  (5)  after  a  heavy  rain,  the  soil  mulch  must 
be  renewed  by  tillage,  and  this  is  much  more  urgent  on 
clay  than  on  sandy  soil ;  even  without  rain,  a  clay  mulch 
may  become  inefficient;  (6)  tillage  for  mulch  purposes 
must  ordinarily  be  more  frequent  in  spring  or  during 
periods  of  heavy  rain,  than  at  other  times  of  the  year; 
(7)  the  use  of  foreign  materials  as  mulches  may  be  justi- 
fied under  special  circumstances. 

197.  Water  saved  by  a  mulch.  —  It  is  very  difficult 
to  quote  data  regarding  the  capacity  of  a  mulch  to  con- 
serve moisture,  since  conditions  vary  to  such  a  degree 
from  one  region  to  another.  Again,  water  may  not  be  the 
limiting  factor  in  crop  growth,  and  even  if  moisture  were 
saved  there  might  be  but  little  influence  on  crop  yields. 
As  a  general  rule,  mulches  are  most  easily  maintained 
and  most  effective  in  arid  and  semiarid  regions.  Since 
there  is  no  doubt  that  moisture,  under  such  conditions,  is 
the  limiting  factor  in  plant  growth,  data  from  such  regions 
should  be  especially  significant. 

Moisture  Content  of  Mulched  and  Unmulched  Eastern 
Montana  Soils.     Average  of  Three  Years.1     October  6 


Mulched 

Unmulched 

First  foot 

16.8 
16.4 
13.2 
10.1 
9.6 

10.8 

Second  foot 

9.4 

Third  foot 

9.5 

Fourth  foot 

8.9 

Fifth  foot 

8.5 

Average  : 

13.2 

9.4 

1  Buckman,   H.   O.     Moisture    and    Nitrates    in    Dry    Land 
Agriculture.     Proc.  Amer.  Soc.  Agron.,  Vol.  2,  p.  131.     1910. 


280      SOILS:     PROPERTIES  AND  MANAGEMENT 

If  the  wilting  point  of  this  soil  is  6  per  cent,  the  mulched 
area  contains  more  than  twice  as  much  available  moisture. 
This  3.8  per  cent  of  available  moisture  by  which  the 
mulched  soil  excels  the  unmulched  is  equivalent  in  a  five- 
foot  depth  to  about  250  tons  of  water,  enough  to  increase 
the  crop  by  a  ton  of  dry  matter  —  certainly  not  an  insig- 
nificant saving  where  crop  yield  and  moisture  are  so  very 
closely  correlated. 

A  considerable  amount  of  experimentation  *  is  available 
which  seems  to  indicate  that  mulching  a  soil  does  not  in- 
crease its  yield  over  a  soil  not  so  treated.  One  reason  for 
this,  as  already  suggested,  may  be  in  the  fact  that  water 
may  not  have  been  the  limiting  factor,  the  rainfall  having 
been  just  right  in  amount  and  distribution.  Again,  the 
roots  may  have  so  intercepted  the  capillary  water  as  to 
have  allowed  no  more  evaporation  from  the  unmulched 
soil  than  from  the  mulched.  In  some  soils  hard  layers 
often  form  which  act  in  repelling  capillary  movement. 
Such  a  condition  would  function  as  successfully  in  check- 
ing losses  as  if  a  true  mulch  were  present.  In  the  study 
of  mulches  and  their  value  in  increasing  a  crop,  decided 
opinions  should  not  be  advanced  until  every  phase  has 
been  thoroughly  investigated  regarding  the  exact  factors 
dominant  in  the  determination  of  yield.  The  extended 
use  of  soil  mulches  in  the  Corn-Belt  and  in  dry-farming 
operations  argues  for  their  benefits. 

198.  Effect  of  mulches  other  than  on  moisture.  —  That 
mulching  a  soil  has  other  effects  besides  the  conserving  of 
moisture  is  universally  evident.  In  general  the  physical 
condition  of  the  soil  is  always  better  after  a  crop  that  has 

1  Cates,  J.  Si,  and  Cox,  H.  R.  The  Weed  Factor  in  the 
Cultivation  of  Corn.  U.  S.  D.  A.,  Bur.  Plant  Indus.,  Bui. 
257.     1912. 


THE   CONTROL    OF  SOIL   MOISTURE 


281 


been  intertilled.  Not  only  has  the  surface  been  kept 
well  granulated,  but  the  presence  of  optimum  moisture 
below  has  allowed  the  granulating  agents  to  become 
more  active.  The  following  of  potatoes  by  corn  is,  at 
least  partially,  an  attempt  to  take  advantage  of  the 
better  tilth  of  the  soil  with  a  crop  that  is  particularly 
benefited  thereby.  Again,  a  mulch  not  only  tends  to 
allow  a  ready  entrance  of  water  into  the  soil,  but  at  the 
same  time  increases  the  water-holding  capacity  —  factors 
already  emphasized  in  the  discussion  of  control  of  losses 
by  percolation  and  run-off.  By  keeping  down  weeds * 
another  saving  is  effected,  not  only  in  moisture  but  also 
in  plant-food.  Some  results  from  an  experiment 2  con- 
ducted at  Cornell  University  serve  to  illustrate  the  re- 
lation of  mulches  and  weeds  to  soil  moisture  and  crop 
production  in  a  humid  region  in  a  season  of  good  rainfall. 
The  crop  grown  was  maize.  Every  third  plot  was  a 
check  and  was  given  normal  treatment :  — 


Check  plot 

Weeds  removed,  but  not  cultivated    . 

Mulched  with  straw 

Check  plot 

No  cultivation  ;  weeds  allowed  to  grow 
One   cultivation ;     weeds   allowed   to 

grow 

Check  plot 


Yields  Calcu- 
lated to  Basis 
of  100  ON 
Check  Plots 


Soil  Moisture 

during  August 

Per  Cent 


1  Cates,  J.  S.,  and  Cox,  H.  R.  The  Weed  Factor  in  the  Culti- 
vation of  Corn.     U.  S.  D.  A.,  Bur.  Plant  Indus.,  Bui.  257.     1912. 

2  Craig,  C.  E.  The  Cause  of  Injury  to  Maize  by  Weeds. 
Presented  as  a  thesis  for  the  degree  of  M.  S.  A.,  Cornell  Uni- 
versity.    Unpublished.     June,  1908. 


282       SOILS:    PROPERTIES   AND  MANAGEMENT 

The  application  of  a  soil  mulch  is  not  confined  to 
intertilled  crops  such  as  maize,  potatoes,  vineyards, 
fallow,  and  the  like.  Under  some  conditions  it  may  be 
applied  to  grain  fields  with  good  results.  In  those  sections 
of  the  country  where  dry  farming  is  practiced,  it  is  not 
uncommon  to  drag  the  grainfield  with  a  sharp-tooth 
harrow,  the  teeth  pointing  backward.  This  is  begun  when 
the  plants  are  small,  and  may  be  continued  until  they 
attain  a  considerable  size  or  until  they  sufficiently  shade 
the  ground  to  greatly  reduce  surface  evaporation. 

199.  General  usefulness  of  a  mulch.  —  While  a  soil 
mulch  is  used  primarily  in  order  to  conserve  moisture, 
its  relationships  are  different  in  different  regions  accord- 
ing to  climatic  and  cropping  peculiarities.  In  dry- 
farming  regions  a  mulch  is  maintained  as  nearly  as  possible 
the  year  round,  since  moisture  must  be  carried  from  the 
previous  summer  and  winter  to  the  growing  season  in 
order  to  supplement  the  rainfall  occurring  at  that  time. 
In  irrigated  regions  a  mulch  is  useful  in  two  ways  —  by 
conserving  the  rainfall  and  by  checking  the  loss  of  irriga- 
tion water;  after  the  latter  is  once  in  the  soil  less  addi- 
tional water  need  be  applied  and  the  consequent  cost  of 
irrigation  is  much  less.  Again,  in  arid  regions  where 
there  is  an  excess  of  soluble  salts,  rapid  evaporation  is 
detrimental  since  these  salts  tend  to  concentrate  near 
the  surface  and  become  harmful  to  plants.  The  pre- 
vention of  the  rise  of  alkali  is  therefore  a  very  important 
function  of  the  soil  mulch  in  such  regions. 

In  humid  regions  the  utilization  of  a  soil  mulch  is 
much  less  intense,  since  the  conservation  of  moisture 
over  long  periods  is  unnecessary,  due  to  the  rainfall. 
However,  during  the  growing  season  periods  of  drought 
occur,  when  if  available  water  is  lacking  in  the  soil,  the 


THE   CONTROL    OF  SOIL   MOISTURE  283 

crop  suffers.  The  amount  of  moisture  conserved  by  a 
mulch  will  usually  keep  the  plant  growing  normally 
through  such  periods,  while  crops  on  soils  not  so  treated 
may  suffer  greatly.  The  tiding  of  crops  over  short  periods 
of  light  rainfall  is  the  chief  function  of  mulches  in  humid 
climates. 

200.  Other  practices  affecting  evaporation  losses.  — 
Although  the  control  of  water  by  mulches  is  such  an 
important  consideration,  other  means  of  checking  losses 
are  available.  These  may  be  grouped  under  five  heads : 
(1)  fall  and  early  spring  plowing,  (2)  rolling,  (3)  shelters, 
(4)  level  cultivation,  (5)  plants. 

201.  Fall  and  early  spring  plowing.  —  Fall  and  early 
spring  plowing  owe  much  of  their  efficiency  to  the  con- 
servation of  moisture  effected  through  the  creation  of  a 
mulch  over  the  surface.  Fall  plowing  may  be  practiced 
for  a  number  of  reasons,  but  in  regions  of  deficient  rain- 
fall, particularly  in  winter,  the  conservation  of  the  mois- 
ture in  the  soil  at  the  close  of  the  growing  season  is  an 
important  consideration.  This  practice  is  well  adapted 
to  those  soils  in  semiarid  sections  that  do  not  blow  too 
badly  when  fall-plowed,  and  where  the  winter  rain  is 
not  sufficient  to  saturate  the  soil.  If  the  soil  is  left  in 
the  bare,  hard  condition  resulting  from  the  removal  of 
a  crop  of  maize,  wheat,  or  barley,  a  large  quantity  of 
water  may  be  lost  by  evaporation  during  the  fall  months. 

For  the  average  farmer  in  humid,  regions  where  the 
winter  rainfall  is  sufficient  to  saturate  the  soil,  early 
spring  plowing,  coupled  with  tillage,  is  very  important. 
Not  only  may  moisture  be  conserved,  but  the  soil  is 
worked  at  the  stage  when  it  yields  most  readily  to  pul- 
verization. Fallow  land,  and  bare  stubble  land  of  fine- 
textured    soil,    are   most   benefited,    since    they    become 


284       SOILS:    PROPERTIES  AND  MANAGEMENT 

compact  to  the  very  surface  as  a  result  of  the  winter 
rain  and  snow,  and  are  therefore  in  condition  for  the 
most  rapid  loss  of  water.  They  should  be  plowed  as 
early  as  practicable  without  injury  to  their  structure. 
At  the  Wisconsin  Experiment  Station !  two  adjacent 
pieces  of  land  very  uniform  in  character  were  plowed 
seven  days  apart.  At  the  time  when  the  second  plot 
was  plowed,  it  was  found  to  have  lost  1.75  inches  of 
water  from  the  surface  four  feet  in  the  previous  seven 
days ;  while  the  piece  plowed  earlier  had  actually  gained, 
doubtless  by  increased  capillarity,  a  slight  amount  of  water 
over  what  it  had  contained  when  plowed.  There  was  a 
conservation  of  nearly  two  inches  of  water  in  the  root  zone 
as  a  result  of  plowing  one  week  earlier  —  enough  to 
produce  1500  pounds  of  dry  matter  in  maize  to  the  acre, 
if  properly  utilized. 

202.  Rolling.  —  Very  often  in  the  spring,  when  the 
seed  bed  is  very  loose,  rolling  is  resorted  to,  in  order  to 
bring  about  a  compaction  of  the  soil.  At  the  same 
time  capillarity  is  established  with  the  firmer  earth 
beneath,  and  as  the  moisture  moves  upward  a  rapid 
germination  of  the  seed  is  induced.  Care  must  be  taken 
that  this  capillarity  be  checked  once  it  has  performed 
this  office,  as  great  losses  from  evaporation  may  occur 
at  the  surface  and  the  crop  be  robbed  of  much  available 
water.  It  is  an  economic  procedure  in  such  cases  to 
follow  the  roller  after  a  few  days  with  a  harrow,  in  order 
that  a  mulch  may  be  established  and  this  loss  checked. 

203.  Shelters.  —  Shelters  of  any  kind,  whether  natural 
or  artificial,  tend  to  break  the  wind  velocity  and  thereby 
check  losses  by  evaporation.     Strips  of  timber  are  com- 

1  King,  F.  IL,  The  Soil,  p.  189.     New  York.     1906. 


THE  CONTROL   OF  SOIL   MOISTURE  285 

monly  grown  or  retained  for  this  purpose.  Wooden  fences 
and  walls  of  one  sort  or  another  have  a  similar  effect. 
Windbreaks,  composed  of  growing  plants  have  the  dis- 
advantage that  for  a  considerable  distance  beyond  the 
spread  of  their  branches  their  roots  penetrate  the  soil 
and  use  the  moisture,  which  is  one  reason  for  the  smaller 
growth  of  crops  near  trees.  Bearing  on  the  efficiency  of 
windbreaks,  results  by  King  *  show  that  when  the  rate 
of  evaporation  at  twenty,  forty,  and  sixty  feet  to  the 
leeward  of  a  black  oak  grove  fifteen  to  twenty  feet  high 
was  11.5,  11.6,  and  11.9  cubic  centimeters,  respectively, 
from  a  wet  surface  of  twenty-seven  square  inches,  the 
evaporation  was  14.5,  14.2,  and  14.7  cubic  centimeters, 
at  two  hundred  and  eighty,  three  hundred,  and  three 
hundred  and  twenty  feet  distant  —  or  24  per  cent  greater 
at  the  outer  stations  than  at  the  inner  ones.  A  scanty 
hedgerow  reduced  evaporation  30  per  cent  at  twenty 
feet  and  7  per  cent  at  one  hundred  and  fifty  feet,  below 
the  evaporation  at  three  hundred  feet  from  the  hedge. 

Very  often  tent  shelters  are  used  in  the  growing  of 
tobacco.  The  commonest  form  of  the  tent  is  a  frame 
eight  or  nine  feet  high,  over  which  is  spread  a  loosely 
woven  cloth.  Investigations  by  Stewart  2  in  Connecticut 
showed :  (1)  That  the  tent  greatly  reduced  the  velocity 
of  the  wind.  This  reduction  amounted  to  93  per  cent 
when  the  outside  velocity  was  seven  miles  an  hour,  and 
85  per  cent  when  the  outside  velocity  was  twenty  miles 
an  hour,  there  being  a  small  regular  decrease  in  relative 
efficiency  with  increased  velocity  of  the  wind.  (2)  The 
relative  humidity  under  the  tent  was  higher  than  outside, 

1  King,  F.  H.     The  Soil,  p.  205.     New  York.     1906. 

2  Stewart,  J.  B.  Effects  of  Shading  on  Soil  Conditions. 
U.  S.  D.  A.,  Bur.  Soils,  Bui.  39,     1907. 


286       SOILS:    PROPERTIES  AND  MANAGEMENT 

and  during  a  good  part  of  the  time  attained  a  difference 
of  10  per  cent.  The  effect  of  this  was  to  reduce  evapora- 
tion by  from  53  to  63  per  cent  on  different  days  in  July, 
in  spite  of  a  higher  temperature  inside  the  tent.  (3)  The 
direct  effect  of  this  was  to  increase  the  moisture  content 
in  the  soil  in  spite  of  a  larger  crop  growth  under  the  tent. 
These  differences  are  shown  by  the  following  curves  (see 
Fig.  45),  which  represent  the  percentage  of  water  in  the 
soil  to  a  depth  of  nine  inches  from  June  13  to  August  1. 


f 

J 

fO 

0CG/OD     OF   OaS£ZVAT/OM 

IS                  20                2S-                 JO                  3 

■ 

40 

*S  0* 

Vs 

i 

<iO 

^> 

f. 

T-^\ 

-\ 

V 

/'" 

S- 

J 1 

* 

V. 

r\ 

* 

', 

6 

"-■■■ 

\ 

V 

\ 

\ 

} 

\ 

\ 

; 

J 

Fig.  45. — Curves  showing  the  percentage  of  moisture  in  a  sandy  soil  to 
the  depth  of  nine  inches  inside  and  outside  of  a  loosely  woven  tent 
over  a  period  of  about  fifty  days.  Heavy  line,  moisture  inside  of 
tent ;  broken  line,  moisture  curve  of  soil  outside  of  tent. 


Not  only  was  the  tent  effective  in  preventing  evapora- 
tion and  thereby  increasing  the  average  moisture  Content 
of  the  soil,  but  the  soil  was  able  to  maintain  a  more  uni- 
form content,  due  to  the  freer  movement  and  adjustment 
of  the  capillary  water  under  the  tent  —  conditions  more 
conducive  to  rapid  crop  growth. 

204.  Level  cultivation.  —  The  velocity  of  the  wind 
next  to  the  ground  may  be  checked  by  ridging  the  soil. 
It  is  doubtful  whether  this  practice  conserves  moisture, 


THE  CONTROL    OF  SOIL  MOISTURE  287 

because  a  greater  amount  of  surface  is  exposed  over 
which  evaporation  may  take  place.  On  the  other  hand, 
wide  experience,  as  well  as  investigation,  indicates  that, 
for  the  conservation  of  water,  level  culture  is  better  than 
ridged  culture.  This  principle  has  led  to  the  gradual 
abandonment  of  the  practice  of  "  laying  by  "  corn  and 
potatoes  with  a  high  ridge.  In  all  regions  of  deficient 
rainfall,  the  best  practice  prescribes  level  tillage  and  a 
fine,  dry  mulch,  both  of  which  are  attained  by  the  frequent 
use  of  shallow-running  small-tooth  cultivators.  Many 
experiments  have  demonstrated  the  larger  crop  yields 
to  be  obtained,  on  the  average,  from  this  practice. 

205.  Plants.  —  Plants  growing  on  the  soil  tend  to 
check  evaporation  from  two  causes — (1)  their  shading 
effects,  and  (2)  the  tendency  of  the  roots  to  intercept 
capillary  water  as  it  moves  upward  and  to  appropriate 
it  for  plant  growth.  Plants,  however,  tend  to  intercept 
a  certain  amount  of  rain  and  prevent  its  ever  reaching 
the  soil.  The  amount  of  water  wasted  in  this  way  by 
forests  ranges  from  15  to  30  per  cent.  In  general  this 
tendency  just  about  offsets  the  saving  that  occurs  from 
shading. 

206.  Summary  of  moisture  control.  —  It  is  clearly  seen 
from  the  discussion  of  moisture  control  that  the  structural 
condition  of  the  soil  is  the  secret  of  successful  operation. 
Run-off  and  leaching  are  reduced  by  increased  capillary 
capacity,  a  structural  relationship.  Evaporation  is 
checked  by  a  soil  mulch,  which  depends  for  its  effective- 
ness on  its  physical  condition.  Drainage,  lime,  addition 
of  organic  matter,  and  tillage  in  perfecting  granulation 
function  in  increasing  the  ease  and  effectiveness  with 
which  soil  moisture  may  be  controlled.  It  must  be 
clearly  kept  in  mind  that  all   such   control   is  directed 


288       SOILS:    PROPERTIES  AND  MANAGEMENT 

toward  the  regulation  of  the  soil  moisture  in  such  a  way 
that  an  optimum  water  supply  may  be  held  constantly 
in  the  soil  during  the  growing  season.  If  this  can  be 
accomplished,  the  largest  crop  yields  may  be  expected  that 
are  possible  under  the  existing  fertility  conditions  of  any 
soil. 


CHAPTER  XIV 
SOIL   HEAT1 

Normal  plant  growth  is  practically  suspended  below  a 
temperature  of  about  40°  F.,  while  proper  germination  of 
seeds  does  not  proceed  much  below  that  temperature. 
As  a  rule  it  is  not  desirable  to  place  either  seeds  or  plants 
in  a  soil  in  which  active  growth  does  not  take  place  almost 
immediately,  since  certain  molds  and  fungi,  active  at 
low  temperature,  may  sap  their  vitality  and  ultimately 
cause  their  destruction.  The  desirable  chemical  reactions 
in  the  soil  are  checked  to  a  certain  extent  by  lack  of  heat, 
while  the  important  biological  activities  are  greatly  im- 
peded, if  not  brought  entirely  to  a  standstill,  when  the 
soil  temperature  approaches  32°  F.  Such  functions  as 
the  decay  and  putrefaction  of  organic  matter,  the  forma- 
tion of  ammonia  from  simple  humic  bodies,  the  building-up 
of  this  ammonia  into  the  nitrate  form,  and  the  fixation 
of  the  free  nitrogen  either  by  free-fixing  or  symbiotic 
bacteria,  depend  on  an  optimum  soil  temperature. 

A  knowledge  of  the  functions  of  heat,  therefore,  es- 
pecially as  to  its  relationship  to  plant  growth  and  bac- 
terial activities,  becomes  important;  for  the  farmer  can 
to  a  certain  extent  control  soil  temperature.     He  is  able 

1  For  bibliography  of  the  literature  of  soil  heat,  see  Bouyoucos, 
G.  J.     An  Investigation  of  Soil  Temperature  and  Some  of  the 
Most  Important  Factors  Influencing   It.     Michigan  Agr.  Exp. 
Sta.,  Technical  Bui.  17,  pp.  194-196.     1913. 
u  289 


290       SOILS:    PROPERTIES  AND  MANAGEMENT 


also  to  govern  the  time  when  his  sowing  and  planting  are 
performed  in  such  a  way  that  the  soil  will  be  fitted,  at 
least  as  far  as  heat  is  concerned,  for  proper  seed  germina- 
tion and  plant  growth. 

207.  Relation  of  heat  to  germination  and  growth.  — 
In  order  to  show  the  exact  relationship  of  heat  to  ger- 
mination of  seeds  and  to  the  growth  of  plants,  the  follow- 
ing data  from  Haberlandt l  are  given.  While  these 
tables  are  not  exact,  they  show  clearly  the  necessity  of 
careful  control  of  temperature  in  the  propagation  of 
plants :  — 

The    Relation   of   Temperature   to   the    Germination   op 
Certain  Seeds  (in  Degrees  Fahrenheit) 


Minimum 

Optimum 

Maximum 

Corn 

Scarlet  bean 

Pumpkins 

Wheat 

Barley 

49 
49 
52 

41 
41 

93 
93 
93 

84 
84 

115 
115 
115 

108 
99 

The  Relation  of  Temperature  to  the  Growth  of  Certain 
Plants  (in  Degrees  Fahrenheit) 


Wheat  . 
Barley  . 
Corn 
Peas  .  . 
Buckwheat 
Melon  . 
Pumpkin 


Minimum 


32-40 
32-40 
40-51 
32-40 
32-40 
60-65 
51-60 


Optimum 


77-88 
77-88 
88-98 
77-88 
77-88 
88-98 
98-111 


Maximum 


88-98 
88-98 
98-111 
88-98 
98^-111 
111-122 
111-122 


1  Haberlandt,  F.  Die  Oberen  und  Unteren  Temperatur- 
grenze  fur  die  Keimung  der  Wichtigeren  Landwirthschaftlichen 
Samereien.  Landw.  Versuchs.  Stat.,  Band  17,  Seite  104-116. 
1874. 


SOIL  HEAT  291 

It  is  noticeable  that  there  are  here  three  groups  of 
plants  as  far  as  temperature  conditions  for  optimum 
growth  are  concerned.  Wheat  represents  the  crops  that 
germinate  and  grow  at  a  relatively  low  temperature. 
Corn  requires  a  medium  high  temperature  for  proper 
growth,  while  melons  and  pumpkins  represent  crops  the 
temperature  requirements  of  which  are  very  high.  These 
needs  must  be  supplied  for  a  proper  development  of  such 
plants,  and  must  of  course  be  considered  in  crop  adapta- 
tion as  well  as  in  soil  management  in  general. 

208.  Chemical  and  physical  changes  due  to  heat.  — 
In  the  soil  a  certain  amount  of  chemical  action  is  going  on, 
no  matter  what  the  temperature  may  be ;  but  it  is  with- 
out doubt  true  that  this  activity  is  greatly  accelerated  by 
an  increase  in  soil  heat.  This  arises  from  two  causes : 
(1)  because  heat  increases  the  solubility  of  the  soil  con- 
stituents; and  (2)  because  the  activity  of  the  soil  or- 
ganisms is  stimulated  to  such  an  extent  as  to  in 
turn  influence  chemical  reaction.  The  increased  pro- 
duction of  carbon  dioxide  is  a  good  example  of  this  re- 
lationship. The  warming  of  the  soil  in  spring  and 
summer,  therefore,  by  stimulating  the  amount  of  solu- 
tion, increases  to  a  marked  extent  the  constituents  avail- 
able for  plant  growth. 

The  effect  of  temperature  is  less  marked  in  a  direct 
way  on  the  structure  of  the  soil  than  on  its  chemical  or 
biological  nature  unless  the  freezing  point  is  reached. 
At  this  point,  if  moisture  is  present,  the  soil  mass  is  dis- 
rupted and  may  become  rather  granulated  if  the  freezing 
process  is  often  repeated.  The  practice  of  fall-plowing 
in  order  to  better  the  tilth  of  the  soil  is  really  taking  advan- 
tage of  this  natural  phenomenon.  A  change  in  tem- 
perature also  causes  the  expansion  or  contraction  of  the 


292     soils:  properties  and  management 

soil  gases  and  may  greatly  facilitate  their  movement. 
This  is  essentially  a  physical  relationship.  It  must  be 
kept  in  mind,  however,  that  with  heat  as  with  other  soil 
factors,  no  clear-cut  and  distinct  discussion  of  its  effects 
in  one  direction  may  be  made  without  considering  the 
indirect  influences  that  are  continually  opening  up  avenues 
which  lead  to  phases  more  or  less  foreign  to  the  one  under 
discussion.  This  serves  to  emphasize  the  close  correla- 
tion of  the  various  factors  and  conditions  that  must  be 
dealt  with  in  a  study  of  soils. 

209.  Sources  of  soil  heat.  —  The  soil  may  receive 
heat  directly  or  indirectly  from  three  general  sources: 
(1)  from  the  sun,  (2)  from  the  stars,  and  (3)  by 
conduction  from  the  heated  interior  of  the  earth. 
The  tw6  last-named  sources  are  so  unimportant  as 
to  warrant  no  further  discussion,  since  the  amount  of 
heat  received  by  the  soil  therefrom  is  so  small  as  to  be 
negligible. 

The  sun,  then,  either  directly  or  indirectly  supplies 
all  the  heat  and  energy  that  make  it  possible  for  soils  to 
support  vegetation.  This  heat  is  derived  in  various  ways, 
as  follows :  — 

(1)  By  direct  radiation  of  rays,  both  of  light  and  of 
invisible  heat.  These  rays  when  absorbed  tend  to  raise 
the  temperature  of  the  absorbing  medium.  This  source 
of  heat  is  by  far  the  most  important  and  may  be  desig- 
nated as  the  direct  method  of  heat  induction. 

(2)  A  considerable  amount  of  heat  may  be  derived  by 
radiation  and  conduction  from  the  atmosphere  surround- 
ing the  earth.  This  heat  has  of  course  been  originally 
obtained  from  the  sun  and  is  passed  on  to  the  soil,  the 
length  of  the  waves  being  somewhat  changed  in  the  transi- 
tion.    Clouds  may  sometimes  serve  as  a  blanket  and  shut 


SOIL  HEAT  293 

in  around  the  earth  heat  that  would  otherwise  be  entirely 
lost  so  far  as  the  soil  is  concerned. 

(3)  A  certain  amount  of  heat  may  be  brought  to  the 
soil  by  precipitation.  A  warm  spring  rain,  by  falling 
on  the  earth  and  percolating  into  its  subsoil,  may  be  a 
determining  factor  in  crop  growth.  Although  the 
aggregate  amount  of  heat  added  in  this  way  is  small, 
the  opportuneness  of  its  application  is  of  no  small 
importance.  A  warm  rain  often  imparts  an  impetus- 
to  plant  growth  which  may  be  noticeable  for  many  weeks 
afterward. 

(4)  A  large  amount  of  heat  is  annually  entrapped  by 
growing  plants.  This  energy  is  stored  up  and  may  ulti- 
mately be  liberated  by  the  decay  of  the  tissue.  If  such 
oxidation  takes  place  in  the  soil,  as  it  very  largely  should 
under  good  farm  management,  a  certain  amount  of  heat 
is  liberated  in  the  soil.  How  important  this  is  it  is 
difficult  to  say,  for  such  energy  is  given  off  so  grad- 
ually as  to  be  rendered  difficult  of  measurement.  Bac- 
terial activity  is  very  closely  allied  to  the  utilization 
of  such  heat.  Except  under  exceptional  conditions,  as 
in  hotbeds  or  very  heavily  manured  lands,  such  heat 
has  no  appreciable  effect  in  altering  the  temperature  of 
a  normal  soil. 

210.  Factors  affecting  soil  temperature.  —  The  tem- 
perature that  the  soil  of  any  given  locality  may  attain 
is  dependent  on  a  certain  group  of  factors  so  closely  re- 
lated as  to  make  their  separate  consideration  sometimes 
rather  difficult.  For  convenience  these  factors  may  be 
listed  as  follows,  the  actual  temperatures  and  their 
probable  fluctuations  under  field  conditions  being  re- 
served until  the  various  intrinsic  and  external  factors  of' 
soil  heat  have  been  discussed  :  — 


294     soils:  properties  and  management 


1.  Specific  heat 

2.  Absorption 

3.  Radiation 

4.  Conductivity  and  convection 

5.  Evaporation  of  moisture 

6.  Organic  decay 

7.  Slope 

8.  Heat  supply  and  its  effects 

211.  Specific  heat.  —  The  specific  heat  of  any  material 
may  be  defined  as  its  thermal  capacity  as  compared  with 
that  of  water.  It  is  the  ratio  of  the  quantity  of  heat 
required  to  raise  the  temperature  of  a  given  weight  of 
the  substance  one  degree  Centigrade  to  the  quantity 
needed  to  change  an  equal  weight  of  water  from  19.5° 
to  20.5°  Centigrade.  A  knowledge  of  the  specific  heat 
of  soil  is  important  because  of  the  general  light  it  sheds 
on  the  warming-up  of  a  soil  in  spring  and  on  its  rate  of 
cooling  in  autumn.  The  data  from  a  number  of  investi- 
gations, in  the  order  of  their  priority,  is  here  quoted, 
the  calculations  being  based  on  dry  soil :  — 

Weight  Specific  Heat  of  Soils 


Pfaundler1  (1866) 


Liebenberg2  (1878) 


Fine  sand    .     . 

.     .1923 

Coarse  sand 

.1920 

Alluvial  soil 

.     .2507 

Diluvial  loam 

.2250 

Granite  soil 

.     .3489 

Fine  loam    .     . 

.2770 

Humous  soil     . 

.     .4143 

Humous  loam 

.3290 

Peat  .... 

.     .5069 

Granite  soil 

.3880 

1  Pfaundler,  L.  Ueber  die  Warme  Oapacitat  Verschiedener 
Bodenarten  und  deren  Einfluss  auf  die  Pflanze.  Ann.  d. 
Physik  u.  Chemie,  Band  205,  Seite  102-135.     Leipzig,  1866. 

2  Liebenberg,  R.  von.  See  Lang,  C.  Ueber  Warme  Capa- 
citat der  Bodenconstituenten.  Forsch.  a.  d.  Gebiete  d.  Agri.- 
Physik,  Band  I,  Siete  118.     1878. 


SOIL   HEAT 


295 


Lang1  (1  78) 
Coarse  sand 
Limestone  soil 
Humous  soil     . 
Garden  soil 
Peat  .... 


Patten2  (1909) 


.1980     Norfolk  sand    .     .     .1848 
.2490     Podunk   fine    sandy 

.2570        loam 1828 

.2670     Hagerstown  loam      .1914 
.4770     Leonardtown  loam    .1944# 
Galveston  clay     .     .2097 

Bouyoucos3  (1913) 

Sand 1929 

Gravel 2045 

Clay       2059 

Loam 2154 

Peat       2525 

212.  Variations  of  specific  heat.  —  These  figures  show 
a  considerable  amount  of  variation,  part  of  which  is  of 
course  due  (1)  to  inaccuracies  in  the  designation  of  the 
materials  used,  (2)  to  differences  in  methods,  and  (3) 
to  differences  in  technique:  Allowing  for  these  probable 
errors,  there  still  seem  to  be  other  factors  involved.  One 
of  these  might  be  texture,  since,  according  to  the  earlier 
investigators,  the  finer  mineral  soils  seem  to  possess  a 
higher  specific  heat.  The  data  of  Bouyoucos  and  Patten, 
however,  seem  to  controvert  this  assumption.  An  in- 
vestigation more  to  the  point  is  that  of  Ulrich.4     In  work- 

1  Lang,  C.  Ueber  Warme  Capacitat  der  Bodenconstitu- 
enten.  Forsch.  a.  d.  Gebiete  d.  Agri.-Physik,  Band  I,  Seite 
109-147.     1878. 

2  Patten,  H.  E.  Heat  Transference  in  Soils.  U.  S.  D.  A., 
Bur.  Soils,  Bui.  59,  p.  34.     1909. 

3  Bouyoucos,  G .  J.  An  Investigation  of  Soil  Temperature. 
Michigan  Agr.  Exp.  Sta.,  Tech.  Bui.  17,  p.  12.     1913. 

4  Ulrich,  R.  Untersuchungen  iiber  die  Warmekapazitat 
der  Bodenkonstituenten.  Forsch.  a.  d.  Geb.  d.  Agri.-Physik, 
Band  17,  Seite  1-31.     1894. 


296       SOILS:    PROPERTIES  AND  MANAGEMENT 

ing  with  various  grades  of  quartz  sand  he  obtained 
practically  identical  specific  heats  with  the  various  sepa- 
rates :  — 


Specific  Heat  of  Various  Grades  of  Sand   as  Found  by 

Ulrich 


Diameter  of  Sands  in  Millimeters 

Specific  Heat 

2-1 

.1912 

1-.5 

.1908 

.5-.25 

.1922 

.25-171 

.1919 

.171-.114 

.1919 

.114-.071 

.1904 

.071-.010 

.1890 

It  is  evident,  therefore,  not  only  that  texture  has  no 
very  great  direct  effect  on  specific  heat,  but  also  that 
the  controlling  factor  in  the  data  already  quoted  is  the 
composition  of  the  soil.  The  predominate  minerals 
found  in  soils  possess  a  specific  heat  of  from  .180  to  .220.1 
This  rather  narrow  range  would  normally  be  still  further 
lessened,  since  an  average  soil  is  a  complex  of  the 
different  minerals.  Humus,  then,  possessing  a  specific 
heat  of  about  .5  must,  when  added  to  any  soil,  in- 
crease markedly  its  thermal  capacity  and  would  un- 
doubtedly be  the  determining  factor  in  weight  specific 
heat  of  the  mixture. 

213.  Specific  heat  based  on  volume  of  soil.  —  Under 
normal  conditions,  however,  the  soil  contains  a  consider- 
able  amount   of  pore   space,   and   different   soils  would 

1  Ulrich,  R.  Untersuchungen  uber  die  Warmekapazitat 
der  Bodenkonstitenten.  Forsch.  a.  d.  Geb.  d.  Agri.-Physik, 
Band  17,  Seite  1-31.     1894. 


SOIL  HEAT 


29r 


therefore  show  different  weights  to  the  cubic  foot.  A 
specific  heat  comparison  based  on  weight,  therefore,  does 
not  yield  a  fair  idea  of  the  heat  capacities  of  two  soils. 
The  multiplication  of  the  weight  specific  heat  by  the 
apparent  specific  gravity  of  the  soil  in  question  will 
obviously  yield  a  volume  specific  heat,  which  is  a 
much  more  rational  basis  for  comparison.  A  quota- 
tion from  Ulrich  1  makes  clear  the  value  of  such  a  com- 
putation :  — 

Specific  Heat  of  Soil  Expressed  by  Weight  and  by  Vol- 
ume of  Soil 


Sand 
Clay 
Humus 


Apparent 
Specific 
Gravity 


1.52 

1.04 

.37 


Specific  Heat 
by  Weight 


.1909 
.2243 
.4431 


Specific  Heat 
by  Volume 


.2901 
.2333 

.1639 


It  is  evident  that  in  the  first  case  the  specific  heat  is 
governed  by  the  organic  content  of  the  soils  in  question; 
the  greater  the  amount  of  organic  material  present,  the 
higher  is  the  thermal  capacity.  Such  is  not  the  case  when 
the  specific  heat  of  the  soil  is  calculated  on  a  volume  basis. 
In  an  expression  of  the  thermal  capacity  on  this  rational 
basis,  namely,  that  of  volume,  the  apparent  specific  grav- 
ity, or  volume  weight,  is  the  dominant  factor.  The  ad- 
dition of  humus  when  this  method  of  expression  is  em- 
ployed merely  serves  to  lower  the  volume  weight,  and 


1  Ulrich,  R.  Untersuchungen  iiber  die  Warmekapazitat  der 
Bodenkonstituenten.  Forsch.  a.  d.  Geb.  d.  Agri.-Physik, 
Band  17,  Seite  1-31.     1894. 


298       SOILS:    PROPERTIES  AND  MANAGEMENT 

a  reduction  of  specific  heat  thereby  occurs.  Under  such 
conditions  more  heat  is  necessary  to  raise  the  temperature 
of  the  sand  than  is  the  case  with  the  weight  expression. 
This  is  because  of  its  high  apparent  specific  gravity.  The 
clay  shows  very  little  change,  as  its  apparent  specific 
gravity  is  about  one;  but  the  humus  exhibits  a  marked 
falling-off,  due  to  its  exceedingly  low  volume  weight. 
The  factor  that  tends  to  vary  the  specific  heat  of  dry 
soil  under  natural  conditions,  therefore,  is  the  apparent 
specific  gravity,  or  the  volume  weight.  By  deep  and 
efficient  plowing  the  farmer  may  encourage  the  warm- 
ing of  his  soil,  due  to  a  lowered  thermal  capacity.  By 
increasing  its  humus  content  he  may  attain  the 
same  result,  since  the  volume  weight  is  depressed  to 
a  markedly  greater  extent  than  the  specific  heat  is  in- 
creased by  the  addition  of  organic  matter.  In  fact,  any 
operation  on  or  any  addition  to  the  soil  that  will  vary 
its  apparent  specific  gravity  will  in  turn  affect  the  specific 
heat. 

214.  Effect  of  water  on  specific  heat.  —  One  other 
factor,  much  more  potent  than  the  two  already  men- 
tioned, is  yet  to  be  discussed.  This  factor  is  water,  so 
universally  present  in  soils  and  of  the  greatest  importance 
in  all  natural  soil  phenomena.  As  the  specific  heat  of 
water  is  very  high  compared  with  the  thermal  capacity 
of  the  soil  constituents,  any  addition  of  it  must  naturally 
raise  the  specific  heat  of  a  normal  soil.  That  moisture, 
not  apparent  specific  gravity  nor  organic  content,  is  the 
controlling  factor  is  demonstrated  from  the  following 
data,  calculated  by  Ulrich  x  on  a  volume  basis :  — 

1  Ulrich,  R.  Untersuchungen  iiber  die  Warmekapazitat  der 
Bodenkonstituenten.  Forsch.  a.  d.  Geb.  d.  Agri.-Physik,  Band 
17,  Seite  27.     1894. 


SOIL   HEAT 


299 


The  Effect  of  Soil  Moisture  on  the  Volume  Specific 
Heat  of  Soil,  the  Moisture  being  Expressed  as  a  Per- 
centage of  the  Total  Water  Capacity 


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.4767 

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.6328 

.5589 
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.7108 

.5972 
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.8458 
.9449 

It  is  at  once  evident,  from  these  data  and  the  accom- 
panying curves  (see  Fig.  46),  that  moisture,  in  its  effect 
on  the  specific  heat  of  an  average  soil,  is  so  potent  as  to 


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-Curves  showing  the  effect  of  moisture  on  the  volume  specific 
heat  of  soils  of  different  texture  and  humus  content. 


300       SOILS:    PROPERTIES  AND  MANAGEMENT 

entirely  obscure  in  most  cases  the  variations  due  directly  to 
such  factors  as  apparent  specific  gravity  and  humus  con- 
tent. Organic  matter,  because  of  its  high  water  capacity, 
usually  accentuates  the  dominance  of  moisture  in  this 
respect.  While  a  humous  soil  of  low  volume  weight  may 
warm  up  most  easily  when  dry,  its  high  water  content  may 
so  increase  its  thermal  capacity  as  to  markedly  retard 
its  temperature  changes.  This  is  exemplified  by  Petit l 
and  Bouyoucos 2  in  their  study  of  frost  penetration  in 
peat.  This  soil  was  the  last  to  freeze  in  winter  and, 
conversely,  the  last  to  thaw  in  spring.  The  advantage 
of  removing  excess  water  by  drainage  is  of  importance 
from  this  standpoint,  as  a  wet  soil  is  necessarily  a  colder 
soil  in  spring  than  one  that  is  well  drained.  This  at  least 
partially  accounts  for  the  fact  that  a  sandy  soil  is  usually 
an  early  one,  and  is  therefore  of  particular  value  in  truck- 
ing operations. 

215.  Absorptive  power  of  soils  for  heat.  —  The  greater 
proportion  of  the  heat  received  by  the  soil  is  obtained 
by  direct  radiation  from  the  sun.  This  radiant  heat  is 
propelled  by  free  wave  action  in  the  ether,  the  space 
intervening  between  the  sun  and  the  earth  being  but 
little  affected  by  the  transfer.  Were  the  total  amount 
of  heat  received  from  a  vertical  sun  by  any  unit  surface 
wholly  absorbed  by  a  layer  of  soil  twelve  inches  thick, 
the  temperature  of  the  soil  would  rise  thirty  degrees 
Fahrenheit  an  hour.     Such  is  not  the  case  under  normal 


1  Petit,  A.  Untersuchungen  iiber  den  Einfluss  des  Frostes 
auf  die  Temperaturverhaltnisse  der  Boden  von  Verschiedener 
Physikolischer  Beschaffenheit.  Forsch.  a.  d.  Geb.  d.  Agri.- 
Physik,  Band  16,  Seite  285-310.     1893. 

2  Bouyoucos,  G.  J.  An  Investigation  of  Soil  Temperature. 
Michigan  Agr.  Exp.  Sta.,  Tech.  Bui.  17,  p.  214.     1913. 


SOIL   HEAT  301 

conditions,  however,  as  the  atmosphere  continuously 
refracts,  reflects,  and  absorbs  a  certain  amount  of  this 
radiant  energy.  More  important  still  are  certain  inher- 
ent qualities  of  the  soil  itself  which  function  materially 
in  the  modification  of  the  amount,  of  heat  absorbed.  These 
intrinsic  factors  are  color,  reflection,  texture,  and  structure. 
216.  Effect  of  color  on  absorption  of  heat.  (See  Fig. 
47.)  —  In  a  natural  soil  it  is  very  difficult  to  effect  a  change 
in  soil  color  without  changing  the  texture,  structure,  and 
more  particularly  the  constitution,  of  the  particles.  In 
order  to  eliminate  these  disturbing  factors  in  a  study  of 
heat,  a  quartz  sand  colored  with  various  dyes  was  used 
by  Bouyoucos.1  The  following  data,  taken  at  Lansing, 
Michigan,  on  a  clear,  warm  day  in  August,  illustrate 
the  general  effects  of  color  on  absorption :  — 

Effect  of  Different  Colors  on  Heat  Absorption  by 
Quartz  Sand,  August  5,  1.30  p.m. 

Color  .  Temperature 

(Degrees  Centigrade) 

Black 37.6 

Blue 36.7 

Red 35.9 

Green 34.7 

Yellow 32.6 

White 31.7 

It  is  quite  evident  that  the  darker  the  soil,  the  greater 
is  its  absorptive  power.  This  is  because  of  differences 
in  reflection,  a  light-colored  soil  reflecting  more  of  the 
heat  rays  than  one  of  a  darker  color.  There  might  be  a 
question  here  as  to  the  difference  in  radiation  arising  from 

1  Bouyoucos,  G.  J.  An  Investigation  of  Soil  Temperature, 
Michigan  Agr.  Exp.  Sta.,  Tech.  Bui.  17,  p.  31.     1913, 


302       SOILS:    PROPERTIES  AND  MANAGEMENT 

color,  the  white  soils  radiating  more  heat  than  the  black 
ones.  The  following  data  from  Bouyoucos,1  substantiating 
those  of  Lang,2  are  a  conclusive  negative  answer  to  such 
a  query :  — 

Radiation  of  Different-colored  Sands,  White  being 
taken  as  1.00 

White 1.000 

Black 991 

Blue 981 

Green 981 

Red 991 

Yellow 989 

The  addition  of  organic  matter,  provided  its  decay  has 
been  of  the  proper  sort,  will  consequently  always  raise 
the  soil  temperature,  other  factors  of  course  being  equal. 
Wollny,3  in  experimentation  with  soils  covered  with  thin 
layers  of  different-colored  material,  found  marked  dif- 
ferences under  field  conditions.  The  black  soil  not  only 
exhibited  the  highest  temperature,  but  also  showed  a 
greater  amount  of  fluctuation.  The  minimum  tempera- 
tures of  the  different-colored  soils  were  almost  the  same. 
The  temperature  differences  of  course  decreased  with 
depth.  Some  typical  data  obtained  on  a  clear  day,  as 
quoted  from  Wollny's  work,  are  as  follows :  — 

1  Bouyoucos,  G.  J.  An  Investigation  of  Soil  Temperature. 
Michigan  Agr.  Exp.  Sta.,  Tech.  Bui.  17,  p.  30.     1913. 

2  Lang,  C.  Uber  Warme-absorption  und  Emission  des 
Boden.  Forsch.  a.  d.  Gebiete  d.  Agri.-Physik,  Band  1,  Seite 
379-407.     1878. 

3  Wollny,  E.  Untersuchungen  uber  den  Einfluss  der  Farbe 
des  Bodens  auf  dessen  Erwarmung.  Forsch.  a.  d.  Geb.  d. 
Agri.-Physik,  Band  I,  Seite  43-69.  1878.  Also,  Untersuch- 
ungen uber  den  Einfluss  der  Farbe  des  Bodens  auf  dessen  Erwar- 
mung. Forsch.  a.  d.  Geb.  d.  Agri.-Physik,  Band  IV,  Seite 
327-365.     1881. 


SOIL    HEAT 


303 


Temperatures  of  Different-colored  Soils  at  a  Depth  of 
4  inches,  taken  june  23,   1876,  at  munich    (in  degrees 

Centigrade) 


Time 

Air 

Black 

White 

Midnight    .... 

9.6 

13.8 

13.8 

2  A.M 

10.0 

12.4 

12.4 

4 

7.6 

10.7 

10.8 

6 

16.0 

9.6 

9.6 

8 

19.8 

10.4 

10.9 

10 

23.0 

15.7 

13.8 

Noon 

25.4 

22.1 

17.6 

2  P.M 

25.4 

26.8 

21.2 

4 

24.8 

29.4 

23.6 

6 

22.6 

28.8 

24.0 

8 

19.4 

27.2 

23.6 

10 

16.1 

24.0 

21.6 

/OT/A7E 


Fig.  47. —  Curves  showing  the  temperature  variation  of  different-colored 
soils  at  a  four  inch  depth  compared  with  air  temperature.  Munich, 
June  23,  1876. 


304       SOILS:    PROPERTIES  AND  MANAGEMENT 

Besides  the  quite  obvious  effect  of  the  dark  color  on 
the  rate  of  heat  absorption,  two  other  points  are  worthy 
of  notice.  The  first  is  the  tendency  of  the  soil  tempera- 
ture to  lag  behind  the  temperature  of  the  air,  and  the 
second  is  the  almost  equal  minimum  reached  by  the  two 
soils.  The  latter  point  would  seem  to  indicate  also  that 
color  had  little  differential  effect  on  the  heat  lost  from 
the  soil  by  radiation  into  the  air. 

217.  Effects  of  texture  and  structure  on  heat  absorp- 
tion. —  Ordinarily  the  •  texture  and  the  structure  of  a 
soil,  other  conditions  being  equal,  have  little  direct 
influence  on  rate  of  absorption.  Wollny l  found  with 
dry  and  moist  soil  that  the  coarser  the  particles,  the  higher 
is  the  temperature  during  warm  weather.  A  loose,  open 
structure  was  always  more  favorable  for  high  tempera- 
tures than  one  more  finely  pulverized.  Wollny's  tem- 
perature differences,  however,  were  very  small,  and  it  is 
probable  that  the  experimental  error,  particularly  due 
to  lack  of  moisture  control,  was  greater  than  the  observed 
differences.  Under  normal  conditions  the  practical  effects 
arising  from  the  influence  of  texture  and  structure  on 
rate  of  absorption  are  probably  entirely  eliminated  by 
other  factors.  The  importance  of  texture  and  structure, 
as  will  be  shown  later,  is  in  the  direction  of  the  control 
of  soil  heat  through  their  influence  on  soil  moisture. 
Moisture  in  turn  is  a  potent  factor  in  the  ultimate  soil 
temperature,  as  it  influences  specific  heat,  radiation,  and 
evaporation  to  such  an  extent. 

218.  Radiation  of  heat  by  soil.  —  The  principal  loss 

1  Wollny,  E.  Untersuchungen  iiber  den  Einfluss  der  Struk- 
tur  des  Bodens  auf  dessen  Feuchtigkeits-  und  Temperatur- 
verhaltnisse.  Forsch.  a.  d.  Geb.  d.  Agri.-Physik,  Band  V, 
Seite  145-209.     1882. 


SOIL  HEAT 


305 


of  heat  by  the  soil  is  through  radiation,  this  radiation 
being  controlled  by  certain  factors  of  which  moisture 
content,  soil  mulches,  artificial  coverings,  shelters,  and 
clouds  are  the  most  important.  Color  as  a  factor  in 
radiation  has  already  been  eliminated  by  the  work  of 
Bouyoucos  and  Lang.  The  effects  of  texture  and  struc- 
ture have  also  been  investigated  by  these  authors,  as 
well  as  by  other  physicists.  The  general  results  seem 
to  indicate  that  unless  a  dry  soil  is  dealt  with  these  factors 
may  be  eliminated  from  consideration  as  far  as  their 
direct  practical  effect  on  radiation  is  concerned.  Of 
course,  indirectly  through  their  influence  on  such  factors 
as  moisture,  they  are  of  extreme  importance. 

An  increase  in  the  moisture  of  a  soil  has  the  general 
effect  of  heightening  the  radiation  ratio.  This,  together 
with  the  effects  of  evaporation  and  of  increased  specific 
heat,  accounts  for  the  fact  that  an  undrained  soil  in  spring 
is  a  cold  soil.  Bouyoucos  *  found  the  following  relation- 
ships between  moist  and  dry  soils :  — 


Effect 

of  Moisture 

on  Radiation 

Soil 

Percentage  of 
Moisture 

Radiation  of 
Moist  Soil 

Radiation  of 
Dry  Soil 

Gravel 

Sand 

Clay 

Loam 

Peat       

4.7 

5.3 

17.2 

25.8 
84.9 

100 
100 
100 
100 
100 

92.4 
93.1 
91.9 
90.9 
86.1 

Mulches,  either  natural  or  artificial,  tend  to  check  the 
loss  of  soil  heat  through  their  covering  effect  and  their 


1  Bouyoucos,  G.  J.     An  Investigation  of  Soil  Temperature. 
Michigan  Agr.  Exp.  Sta.,  Tech.  Bui.  17,  p.  34.     1913. 


30(5       SOILS:    PROPERTIES  AND   MANAGEMENT 


influence  on  radiation.  As  a  mulch  is  usually  dry,  its 
radiant  power  is  lower  than  that  of  the  moist  soil  beneath. 
Shelters  decrease  radiation  by  checking  air  movement 
The  vegetation  growing  on  soil  also  lowers  radiation 
through  its  covering  effect,  although  the  temperature  of 
soils  covered  with  vegetation  is  usually  low  in  summer 
due  to  the  obstruction  of  the  sun's  rays.  Clouds,  by 
shutting  in  the  heat,  tend  to  check  radiation  and  in  many 
cases  prevent  a  frost  that  would  otherwise  occur.  The 
protecting  effect  of  snow  is  well  illustrated  from  the  fol- 
lowing data,  taken  from  Boussingoult :  — 

Effect  of  Snow  on  Soil  Temperature.1     (Temperature  in 
Degrees  Centigrade) 


Date  and  Houb 

Air 

On 

Snow 

Under 
Snow 

Feb. 

11, 

5  P.M 

+  2.5 

-    1.5 

0.0 

Feb. 

12, 

7  A.M 

-3.0 

-  12.0 

-  3.5 

Feb. 

13, 

7  A.M 

-3.8 

-    8.2 

-  2.0 

Feb. 

13, 

5.30  p.m 

+  4.5 

-     1.0 

0.0 

One  of  the  important  features  of  soil  heat  radiation  is 
its  effect  on  air  temperature.  As  the  radiant  energy  from 
the  sun  passes  through  the  atmosphere,  very  little  of  the 
heat  is  appropriated,  due  to  the  wave  lengths.  But, 
as  this  energy  is  radiated  from  the  soil,  the  heat  waves 
have  become  shortened  and  are  readily  taken  up  by  the 
atmosphere,  particularly  if  the  latter  is  moist.  How- 
ever, as  the  air  is  always  in  motion  its  heat  is  not  con- 
trolled by  the  soil  radiation  of  any  particular  locality. 


1  Warington,  R.     Lectures  on  Some  of  the  Physical  Proper- 
ties of  Soils,  p.  159.     Oxford.     1900. 


SOIL   HEAT  307 

Iii  fact,  the  soil  may  be  warmed  by  conduction  of  heat 
from  air  to  soil.  This  probably  occurs  to  some  extent 
in  spring,  when  the  air  is  growing  warmer,  due  to  low 
specific  heat  and  its  movement.  The  changes  in  air 
temperatures  are  always  more  rapid  and  usually  greater 
in  range,  due  to  the  factors  cited  above. 

219.  Conductivity  and  convection  of  heat  in  soils.  — 
While  radiation  has  to  do  with  the  transfer  of  heat  by 
ether  waves,  conductivity  is  a  term  used  in  relation  to 
molecular  transmission  of  energy  through  the  body  under 
investigation.  It  may  be  defined  as  the  amount  of  heat 
in  calories  that  will  pass  across  a  cube  of  unit  edge  (1 
centimeter),  in  unit  time  (1  second)  under  a  temperature 
gradient  of  1  degree  Centigrade.  Convection  refers  to 
the  transmission  of  heat  by  actual  apparent  and  visible 
movements  of  matter.  It  is  to  these  two  modes  of  trans- 
fer that  we  owe  the  possibility  of  the  soil's  warming  below 
a  surface  that  receives  most  of  its  heat  as  radiant  energy. 
It  must  be  remembered  that  in  studying  the  soil  we  are 
dealing  with  a  material  made  up  of  mineral  and  organic 
compounds  and  always  containing,  under  normal  condi- 
tions, a  certain  quantity  of  water.  Air  likewise  is  always 
present,  which,  while  a  poor  conductor  of  heat,  may  carry 
energy  by  convection.  Besides  these  varying  substances, 
often  in  loose  contact  and  usually  containing  air  capable 
of  considerable  movement,  there  is  bound  to  occur  a 
certain  amount  of  transfer  resistance  which  is  the  heat 
resistance  found  at  the  boundary  of  two  substances  in 
contact.  The  study  of  heat  movement  downward  through 
a  soil  is  difficult  to  analyze,  since  it  is  almost  impossible 
to  control  the  factors  concerned  while  varying  any  one. 
In  a  normal  soil  this  heat  movement  occurs  through  both 
the  agency  of  conduction  and  that  of  convection,  depend- 


308       SOILS:    PROPERTIES  AND  MANAGEMENT 

ing  on  the  texture  and  structure  of  the  soil  and  the  amount 
of  moisture  present. 

220.  Measurement  of  conductivity.  —  Ordinarily  the 
conductivity  of  a  soil  is  measured  by  applying  a  constant 
source  of  heat  as  quickly  as  possible  and  measuring  the 
change  in  temperature  by  means  of  thermometers  set 
in  the  soil  at  regular  intervals.  (See  Fig.  48.)  The  soil  in 
question  should  be  homogeneous  in  composition  and  of 
uniform  compaction,  and  should  contain  a  definite  mois- 
ture content.  It  should  of  course  be  at  a  temperature 
equilibrium  before  the  heat  is  applied.  Ordinarily  radia- 
tion and  convection  currents  are  diminished  somewhat 
by  inclosing  the  soil  in  an  insulated  compartment.  The 
study  of  heat  movement  downward  instead  of  laterally 
is  to  be  recommended,  in  order  that  unnecessary  air 
circulation  may  be  avoided  to  some  extent. 


Fig.  48. — Longitudinal  section  of  apparatus  for  the  study  of  heat  con- 
ductivity of  soil.  (C) ,  water  at  constant  temperature  ;  (t) ,  ther- 
mometer ;  (P),  copper  plates;  (F),  screw  clamp  for  pressing  soil 
firmly  against  source  of  heat ;    (r) ,  skids  for  soil  box. 


221.   Effect  of  texture  on  conductivity  of  heat.  —  The 
conductivity  of  a  soil  is  affected  by  a  number  of  factors 


SOIL  HEAT  309 

which  may  or  may  not  lend  themselves  to  modification 
in  the  field.  From  the  fact  that  type  is  of  primary  im- 
portance in  choosing  a  soil,  texture  in  its  relation  to  con- 
ductivity might  be  considered  first.  From  the  work  of 
Wagner !  and  Potts 2  it  is  clearly  established  that  the 
coarser  the  texture  of  a  soil,  the  faster  the  rate  of  conduc- 
tion of  heat  will  be,  other  factors  remaining  constant. 
Data  quoted  from  the  findings  of  Bouyoucos  3  substantiate- 
these  results :  — 

Conductivity  of  Various  Soils  as  Measured  by  the  Time 
Required  for  a  Thermometer  7  Inches  from  the  Source 
of  Heat  to  Show  a  Rise  in  Temperature 

Q  .,  Relative  Rate  of 

bo11  Conductivity 

Sand 1.00 

Loam 1.81 

Clay 1.77 

Peat 4.61 

Such  results  as  these  are  only  comparative  and  qualita- 
tive. The  difficulties  of  quantitative  determinations  are 
so  beset  by  error  that  only  one  investigator  has  as  yet 
made  any  consistent  attempt  along  this  line.  Patten,4 
who  has  prosecuted  such  an  investigation,  finds  that  such 
work  may  be  vitiated  by  thermometer  spacing,  size  of 
thermometer,   error   in   readings,   moisture   control,   and 

1  Wagner,  F.  Untersuchungen  iiber  das  Relative  Warme- 
leitungsvermogen  Verschiedner  Bodenarten.  Forsch.  a.  d. 
Geb.  d.  Agri.-Physik,  Band  VI,  Seite  1-51.     1885. 

2  Potts,  E.  Untersuchungen  Betreffend  die  Fortpflan- 
zung  der  Warme  in  Boden  durch  Leitung.  Landw.  Ver.  Stat., 
Band  XX,  Seite  273-355.     1877. 

3  Bouyoucos,  G.  J.  An  Investigation  of  Soil  Temperature. 
Michigan  Agr.  Exp.  Sta.,  Tech.  Bui.  17,  p.  20.     1913. 

4  Patten,  H.  E.  Heat  Transfer  in  Soils.  U.  S.  D.  A.,  Bur. 
Soils,  Bui.  59.     1909. 


310       SOILS:    PROPERTIES  AND  MANAGEMENT 

the  necessity  of  taking  time-temperature  curves  in  the 
unsteady  state.  His  results,  expressed  as  metric  K  (the 
heat  conductivity  coefficient  in  C.  G.  S.  units),  show  the 
same  general  comparisons  as  already  presented :  — 

Heat  Conductivity  of  Different  Soils 

K  in  C.G.S.  units 
Soil  (See  Definition  of 

Conductivity) 

Coarse  quartz 000917 

Leonardtown  loam        .      .      .      .000882 
Podunk  fine  sandy  loam     .      .      .000792 

Hagerstown  loam 000699 

Galveston  clay 000577 

Muck 000349 

222.  Effects  of  humus  and  structure  on  conductivity.  — 
A  disturbing  factor  always  present  when  soils  are  used 
in  the  determination  of  the  effect  of  texture  on  conduc- 
tivity, is  humus.  It  is  evident,  in  dry  soil  at  least,  that 
an  increase  in  the  organic  content  of  a  soil  means  a  lower- 
ing in  conductivity.  Humus,  therefore,  must  be  listed 
as  a  second  factor  tending  to  vary  the  movement  of  heat 
through  soils.  A  third  factor  is  the  structural  condition 
of  the  soil  under  examination.  Wagner  1  has  shown  in 
this  regard  that  the  more  compact  a  soil,  the  faster  is 
the  conduction  of  heat.  This  is  probably  due  to  the  more 
intimate  contact  of  the  soil  grains,  and  a  consequent 
cutting-down  of  the  insulation  factors  and  diminution 
of  the  transfer  resistance. 

223.  Influence  of  moisture  on  heat  conductivity  in  soil. 
— The  greatest  single  factor  to  be  considered  in  conduc- 

1  Wagner,  F.  Untersuchungen  iiber  das  Relative  Warme- 
leitungsvermogen  Verschiedner  Bodenarten.  Forsch.  a.  d. 
Geo!  d.  Agri.-Physik,  Band  VI,  Seite  1-5  L     1885. 


SOIL  HEAT 


311 


tivity  study,  however,  is  the  moisture  content  of  the  soil. 
The  following  curve  for  quartz  powder,  taken  from  Pat- 
ten's l  work,  illustrates  its  effect  and  shows  how  its  influ- 
ence may  heavily  override  the  factors  already  mentioned. 


J0O3 


.002 


J00/ 


J~  /o  /jr 

^o    WAITER.      &y     weight 

Fig.  49. — Effect  of  moisture  upon  the  apparent  specific  volume,  heat 
conductivity,  and  diffusivity  of  coarse  quartz  powder. 


1  Patten,   H.   E.      Heat    Transfer  in   Soils. 
Bur,  Soils,  Bui.  59,  p.  30.     1909. 


U.  S.    D.  A., 


312       SOILS:    PROPERTIES  AND  MANAGEMENT 

At  first  glance  it  appears  peculiar  that  the  heat  move- 
ment through  a  soil,  the  mineral  constituents  of  which 
possess  a  conductivity  coefficient  of  about  .010(50,  should 
be  raised  by  the  addition  of  a  liquid  possessing  a  value  of 
K  of  about  .00149,  a  conductivity  about  one-seventh  of 
the  soil  minerals.  The  explanation  of  this  as  given  by 
Patten  is  a  lowering  of  the  transfer  resistance.  He  has 
calculated  that  heat  will  pass  from  soil  to  water  approxi- 
mately one  hundred  and  fifty  times  more  easily  than 
from  soil  to  air.  This  being  true,  it  is  evident  that  as 
the  water  is  increased  in  any  soil  and  the  air  decreased, 
the  conductivity  coefficient  increases.  It  must  be  kept 
in  mind,  however,  that  as  the  moisture  increases,  the 
total  amount  of  heat  necessary  to  raise  this  soil  to  a  given 
temperature  must  also  be  increased.  The  necessity  for 
the  maintenance  of  a  medium  moisture  content  in  any 
soil  becomes  apparent,  although  the  conductivity  may 
not  thereby  be  at  its  maximum.  The  curves  in  question 
show  that  not  only  is  there  a  change  of  volume  weight, 
but  also  there  is  a  decrease  in  diffusivity  with  high  water 
percentages  —  another  reason  for  avoiding  excessive 
moisture  contents  in  a  field  soil. 

As  has  already  been  noted,  the  warming-up  of  a  soil 
becomes  less  and  less  rapid  as  the  subsoil  is  penetrated. 
This  is  not  due  to  lessened  conductivity,  but  rather  to 
a  lessened  heat  supply.  Bouyoucos f  has  shown  that 
under  natural  conditions  the  tendency  of  heat  is  to  travel 
downward  more  rapidly  than  laterally,  due  to  a  higher 
moisture  in  the  lower  depths  of  the  average  field  soil. 
The  time-temperature  curves  and  the  temperature  gradi- 

1  Bouyoucos,  G.  J.  An  Investigation  of  Soil  Temperature. 
Michigan  Agr.  Exp.  Sta.,  Tech,  Bui.  17,  p.  25.     1913. 


SOIL  HEAT 


313 


ent  for  quartz  powder  as  drawn  by  Patten  l  (see  Figs.  50 
and  51)  illustrate  the  effect  of  distance  on  temperature 
rise,  the  conductivity  coefficient  remaining  constant. 
Degrees  C. 


Fig.  50. 


so  Time  in  miff. 


•Temperature  time  curves  for  quartz  powder  at  various  dis- 
tances from  the  source  of  heat. 


From  this  brief  discussion  of  conductivity  it  may  be 
established  that  such  a  movement  is  of  importance  to 
plants  in  carrying  heat  downward  into  the  soil.  While 
it  is  affected  directly  by  tex- 
ture, structure,  and  humus 
to  a  certain  extent,  moisture 
is  the  dominant  factor.  Under 
natural  conditions  it  is  neces- 
sary to  maintain  a  medium 
moisture  content,  although 
the  conductivity  of  heat  is 
not  then  at  its  maximum. 
However,  it  must  always  be 
remembered  that  convection 
is  active  under  such  conditions  and  may  do  much  in 
facilitating  heat  distribution.  Good  tilth  and  increased 
organic   content  of  any   soil,    by   raising   the  optimum 


c. 

en 

60 
4o 
24 

, 

Distance  from  source  of ' fi'eai 

Fig.  51. — Temperature  gradi- 
ent for  air-dry  coarse  quartz 
powder. 


fatten,    H.   B.       Heat    Transfer    in    Soils,   U.   S.   D.   A., 
Bur.  Soils,  Bui.  59,  pp.  23-24.     1909. 


314       SOILS:    PROPERTIES   AND  MANAGEMENT 

moisture  content  for  plant  growth,  will  place  the  oil  in 
the  best  possible  condition,  consistent  with  plant  devel- 
opment, for  good  heat  movement. 

224.  Effect  of  evaporation  of  water  on  soil  temperature. 
—  There  is  perhaps  no  factor,  besides  the  loss  of  heat  by 
direct  radiation,  which  exerts  such  an  effect  on  soil  tem- 
perature as  does  evaporation.  The  fact  that  water  does 
not  allow  the  long  rays  received  by  direct  radiation  to  pass 
readily  through  it  accounts  for  its  rapid  vaporization. 
This  evaporation,  caused  by  an  increased  molecular 
activity,  requires  a  certain  expenditure  of  heat,  result- 
ing in  a  cooling  effect  on  the  water  and  consequently  on 
any  material  in  close  contact  with  it.  To  evaporate  a 
pound  of  water  requires  the  withdrawal  of  about  966 
heat  units.1  This  is  sufficient  to  lower  the  temperature 
of  a  cubic  foot  of  saturated  clay  soil  about  10°  Fahrenheit. 
The  difference  in  temperature  exhibited  by  wet  and  dry- 
bulb  thermometers  measures  the  cooling  effect  of 
evaporation. 

Any  condition  that  increases  the  rate  of  evaporation 
lowers  the  temperature  of  the  surface  concerned.  The 
amount  of  water  present  is  undoubtedly  the  controlling  fac- 
tor in  this  regard."  King  2  found,  in  his  study  of  a  drained 
and  an  undrained  soil  in  April,  that  the  former  maintained 
a  temperature  ranging  from  2.5°  to  12.5°  Fahrenheit 
higher  than  the  latter.     Parks  3  records  the  same  general 

1  An  English  heat  unit  is  the  amount  of  energy  necessary  to 
raise  one  pound  of  pure  water  from  32°  to  33°  F.  It  is  equal  to 
about  778  foot-pounds. 

2  King,  F.  H.  Physics  of  Agriculture,  p.  220.  Published 
by  the  author,  Madison,  Wisconsin.      1910. 

3  Parks,  J.  On  the  Influence  of  Water  on  the  Temperature 
of  Soils.  Jour.  Roy.  Agri.  Soc.  Eng.,  Vol.  5,  pp.  119-146. 
1845. 


SOIL  HEAT  315 

results  in  England.  Wollny  1  fnds  a  wet  soil  to  be  the 
cooler  in  the  daytime,  the  difference  being  roughly  pro- 
portional to  the  amount  of  water  present.  The  effect 
of  the  amount  of  water  on  the  rate  of  evaporation  is  of 
course  influenced  to  a  certain  extent  by  texture,  struc- 
ture, and  humus,  since  these  factors  exert  such  a  marked 
influence  on  water  capacity  and  capillary  movement. 

The  practical  importance  of  a  study  of  the  effect  of 
evaporation  on  soil  temperature  lies  in  the  fact  that  evap- 
oration can  be  controlled  to  a  certain  extent  under  field 
conditions.  This  is  not  so  true,  unfortunately,  of  radia- 
tion and  conduction.  Thorough  underdrainage  is  the 
dominant  operation  in  the  prevention  of  cooling  by 
evaporation.  By  this  removal  of  excess  water  the  specific 
heat  is  lowered,  radiation  is  slightly  retarded,  and  con- 
vection is  facilitated.  This  means  a  faster  warming  of 
the  soil,  tending  toward  an  optimum  temperature  rela- 
tion as  far  as  the  plant  is  concerned.  Optimum  moisture 
encourages  optimum  heat  conditions,  as  well  as  other 
favorable  relations  whether  chemical,  physical,  or  biologi- 
cal. Drainage,  lime,  humus,  and  tillage  figure  in  heat 
control  as  well  as  in  other  phases  of  soil  improvement. 

225.  Effect  of  organic  decay  on  soil  temperature.  — 
Besides  the  effect  of  organic  matter  on  color  and  its  conse- 
quent influence  on  the  absorption  of  heat,  it  may  function 
in  another  direction,  namely,  in  producing  heat  of  fer- 
mentation.    How  far  this  liberation  of  heat  under  field 


1  Wollny,  E.  Untersuchungen  fiber  den  Einfluss  des 
Wassers  auf  die  Boden.  Forseh.  a.  d.  Geb.  d.  Agri.-Physik, 
Band  IV,  Seite  147-190.  1881.  Also,  Untersuchungen  tiber 
den  Einfluss  der  Oberflachlichen  Abtrochnung  des  Boden  auf 
dessen  Temperatur-  und  Feuchligkeitsverhaltnisse.  Forseh.  a.  d. 
Geb.  d.  Agri.-Physik,  Band  III,  Seite  325-348.     1880. 


316     soils:  properties  and  management 

conditions  is  effective  in  bringing  about  any  important 
modification  of  soil  temperature  it  is  often  difficult  to 
decide.  In  greenhouses  and  hotbeds  perceptible  increases 
are  obtained  by  the  use  of  large  quantities  of  fresh  manure, 
as  high  an  increase  as  75  degrees  Centigrade  has  been 
observed  under  such  conditions.  In  the  field,  however, 
where  the  absorption  and  radiation  of  heat  are  very  large, 
where  the  organic  matter  makes  up  only  a  fraction  of  the 
soil's  components,  and  where  the  applications  of  barnyard 
manure  are  relatively  small  compared  to  the  bulk  of  the 
soil,  it  is  doubtful  whether  any  important  increase  of 
soil  heat  actually  occurs.  Georgeson,1  working  in  Japan 
with  varying  quantities  of  manure,  obtained  during  the 
first  twenty  days  an  excess  over  the  check  of  only  3.4 
degrees  Fahrenheit  from  an  application  of  eighty  tons  an 
acre.  With  twenty  tons  the  increase  was  1.7  degrees. 
Wagner2  obtained  similar  results,  finding  an  average 
excess  of  1  degree  Fahrenheit  from  the  use  of  twenty 
tons  of  barnyard  manure.  Bouyoucos  3  has  obtained  the 
latest  data  on  this  subject.  Under  controlled  laboratory 
conditions  he  found  that  unless  excessive  amounts  of 
manure  were  applied  no  appreciable  effects  were  observed. 
With  an  application  of  ten  tons  the  highest  rise  was  one- 
half  degree  Centigrade;  after  one  hundred  and  three 
days  the  manured  soil  was  only  one-fourth  degree  higher 
than  the  untreated.  Such  results  show  that  the  heat  of 
fermentation     has    little    important    practical    influence 

1  Georgeson,  C.  C.  Influence  of  Manure  on  Soil  Tempera- 
ture.     Agr.  Sci.,  Vol.  I,  pp.  25-52.     1887. 

2  Wagner,  F.  tlber  den  Einfluss  der  Dungung  mit  Organ- 
ischen  Substance  auf  die  Bodentemperatur.  Forsch.  a.  d. 
Geb.  d.  Agri.-Physik,  Band  V,  Seite  373^05.     1882. 

3  Bouyoucos,  G.  J.  An  Investigation  of  Soil  Temperature. 
Michigan  Agr.  Exp.  Sta.,  Tech.  Bui.  17,  pp.  180-190.     1913. 


SOIL   HEAT  317 

on  soil  temperature,  so  far  as  the  total  bulk  is  concerned. 
There  are  without  doubt  certain  localized  influences, 
both  chemical  and  biological,  but  how  important  they 
may  be  it  is  rather  difficult  to  say.  From  what  is  known 
at  the  present  time  it  seems  that  organic  matter  exerts 
its  greatest  temperature  effects  through  the  darkening  of 
the  color  and  the  increase  in  moisture  capacity  of  the 
soil. 

226.  Relation  of  slope  to  soil  temperature.  —  The  rela- 
tion of  exposure  to  soil  heat  is  the  last  phase  to  be  con- 
sidered, with  the  exception  of  meteorological  factors, 
which  are  external  in  their  relationships  rather  than  intrin- 
sic as  have  been  most  of  the  phases  already  discussed. 
The  slope  of  a  surface  varies  the  amount  of  heat 
absorbed  from  the  sun,  without  affecting,  of  course,  the 
absorptive  power  of  the  surface  involved.  The  greater 
the  inclination  of  a  soil  from  a  right-angle  interception 
of  the  heat  rays,  the  less  rapid  will  be  its  rise  of  tempera- 
ture in  a  given  unit  of  time,  the  source  of  heat  remaining 
constant.  This  is  because  the  greater  the  inclination, 
the  greater  is  the  amount  of  surface  a  given  amount  of 
heat  must  serve.  It  is  evident  that  a  less  amount  of 
heat  will  reach  each  unit  of  soil  surface,  and  a  conse- 
quent slower  rise  in  temperature  of  the  soil  so  situated 
will  result.  Under  normal  conditions,  therefore,  any 
inclination  that  will  cause  a  surface  to  approach  a  right- 
angle  interception  of  the  sun's  rays  will  not  only  increase 
its  rate  of  temperature  rise  but  at  the  same  time  will 
increase  its  average  seasonal  temperature.  In  the  North 
Temperate  Zone  this  of  course  is  a  southerly  inclination. 
The  following  diagram,  illustrating  the  conditions  on  the 
42d  parallel  at  noon  on  June  21,  makes  clear  this  rela- 
tionship: — 


318       SOILS:    PROPERTIES   AND  MANAGEMENT 


Fig.  52. — Diagram  showing  the  proportional  amount  of  heat  received  to 
the  unit  area  by  different  slopes  on  June  21,  at  the  42d  parallel  north. 

It  is  seen  that  in  this  case  a  southerly  slope  of  20° 
receives  to  a  unit  area  the  greatest  amount  of  heat,  the 
level  soils  and  the  soils  having  northerly  inclination  of 
20°  differing  in  the  order  named.  The  following  table 
shows  the  proportional  amount  of  heat  received  by  each" 
one  of  these  soils  per  unit  area  at  midday  with  such  an 
inclination  of  the  sun's  rays  :  — 


Proportional  Amount  of  Heat  Received  per  Unit  Area 
by  Different  Slopes  on  June  21,  at  the  42d  Parallel 
North  Latitude 

20°  Southerly  slope  =  106 

Level  =  100 

20°  Northerly  slope  =    81 


SOIL  HEAT  319 

These  figures  show  not  only  that  the  slope  itself  is  impor- 
tant, but  also  that  the  direction  of  the  inclination  must 
play  a  part  in  the  selection  of  land  with  its  probable 
temperature  relationships  borne  in  mind.  The  investi- 
gations of  Wollny,1  which  have  since  been  corroborated 
by  King  2  and  others,  may  be  cited  at  this  point  as  typi- 
cal:  — 

Average    Temperature    at   6   Inches    of  a  Humous  Sandy 
Loam  from  April  to  October,  1877,  Munich,  Germany 

Temperature 
in  Degrees 
Centigrade 

South 14.46 

Southeast      ....  .     .     .  •  14.42 

Southwest 14.42 

East 13.99 

West 13.98 

Northwest 13.64 

Northeast 13.56 

North 13.52 

1  Wollny,  E.  Untersuchungen  iiber  den  Einfluss  der 
Exposition  auf  die  Erwarmung  des  Bodens.  Forsch.  a.  d. 
Geb.  d.  Agri.-Physik,  Band  I,  Seite  263-294,  1878;  Unter- 
suchungen iiber  die  Feuchtigkeits-  und  Temperaturverhalt- 
nisse  des  Bodens  bei  Verschiedener  Neigung  des  Terrains  gegen 
den  Horizont.  Forsch.  a.  d.  Geb.  d.  Agri.-Physik,  Band  IX, 
Seite  1-70,  1886;  Untersuchungen  iiber  die  Feuchtigkeits- 
und  Temperaturverhaltnisse  des  Bodens  bei  Verschiedener 
Neigung  des  Terrains  gegen  die  Hummelsrichtung  und  gegen 
den  Horizont.  Forsch.  a.  d.  Geb.  d.  Agri.-Physik,  Band  X, 
Seite  1-54,  1887;  Untersuchungen  iiber  die  Temperaturver- 
haltnisse des  Bodens  bei  Verschiedener  Neigung  des  Terrains 
gegen  die  Hummelsrichtung  und  gegenden  Horizont.  Forsch. 
a.  d.  Geb.  d.  Agri.-Physik,  Band  X,  Seite  345-364.     1887. 

2  King,  F.  H.  Physics  of  Agriculture,  p.  218.  Published 
by  the  author,  Madison,  Wisconsin.     1910. 


320       SOILS:    PROPERTIES  AND  MANAGEMENT 


Wollny  found  also  that  the  soil  temperature  on  the 
southward  slopes  varied  according  to  the  time  of  year. 
For  example,  the  southeasterly  inclination  was  highest 
in  the  early  season,  the  southerly  slope  during  mid-season, 
and  the  southwesterly  slope  during  the  fall.  A  southeast- 
erly slope  is  usually  preferred  by  gardeners.  Orchardists 
also  pay  strict  attention  to  aspect,  as  it  often  is  a  factor 
in  susceptibility  to  sun  scald  and  other  diseases. 

King,  in  comparing  a  red  clay  with  a  southerly  slope 
of  18°  to  that  on  a  level  on  July  21,  obtained  the  follow- 
ing results :  — 


Temperature 

in  Degrees  Fahrenheit  of  Red  Clay  as 
Influenced  by  Slope 

First  Foot 

Second  Foot 

Third  Foot 

Southerly  slope 
Level 

.     .     . 

70.3 

67.2 

3.1 

68.1 

65.4 

2.7 

66.4 
03.0 

~2S 

It  is  apparent  immediately  that  the  influence  of  slope 
is  not  confined  to  the  surface,  but,  owing  to  conduction 
and  convection,  is  felt  to  a  considerable  depth.  Slope, 
therefore,  together  with  moisture  control,  becomes  a 
dominant  factor  in  the  heat  relations  of  a  soil.  This  is 
particularly  true  with  specialized  crops,  with  which  the 
early  wrarming  of  the  soil  is  important.  A  normally  early 
soil  may  become  late  because  of  exposure,  or  a  naturally 
late  soil  may  become  earlier  due  to  an  inclination  south- 
ward. Slope  many  times  is  a  dominant  factor  in  the 
adaptation  of  crop  to  soil. 

227.  Heat  supply  and  its  effects.  —  The  direct  heat 
supply  is  without  doubt  the  controlling  factor  in  soil 


SOIL  HEAT 


321 


temperature,  influenced,  of  course,  by  the  conditions 
already  discussed.  The  effect  of  this  heat  supply  is  re- 
flected in  the  seasonal,  monthly,  and  daily  soil  tempera- 
tures at  the  surface  and  at  varying  depths  below.  The 
following  data  illustrate  the  differences  that  may  ordi- 
narily be  expected  to  take  place  from  season  to  season  on 
an  average  soil :  — 

Average  Temperature  Readings  Taken  at  Breslau,  Ger- 
many.1 Average  of  Ten  Years,  1901-1910  (in  Degrees 
Fahrenheit) 


Air 

1  Inch 
Deep 

8 
Inches 
Deep 

16 
Inches 
Deep 

28 
Inches 
Deep 

40 
Inches 
Deep 

52 
Inches 
Deep 

Winter      .     .     . 

29.4 

28.2 

33.3 

34.9 

37.1 

38.7 

40.6 

Spring       .     .     . 

45.5 

44.9 

45.3 

45.4 

44.5 

43.7 

43.5 

Summer    .     .     . 

63.3 

62.8 

63.4 

63.4 

61.6 

59.3 

57.5 

Autumn    .     .     . 

44.8 

43.7 

48.6 

50.5 

52.1 

52.6 

53.3 

Average  Temperature  Readings  Taken  at  Lincoln,  Ne- 
braska.2 Average  of  Twelve  Years,  1890-1902  (in 
Degrees  Fahrenheit). 


Winter 
Spring 
Summer 
Autumn 


Air 


25.9 
49.9 
73.8 
53.9 


1  Inch 
Deep 


28.8 
54.8 
83.0 
56.4 


3 

Inches 
Deep 


28.8 
53.6 
80.9 
57.6 


Inches 
Deep 


29.5 
51.6 
79.1 
57.1 


12 
Inches 
Deep 


32.2 

48.5 
73.8 
57.5 


24 
Inches 
Deep 


36.3 
45.7 
69.0 
59.3 


36 
Inches 
Deep 


39.1 
44.3 
66.2 
60.3 


1  Schulze,  B.,  and  Burmester,  H.  Beobochtungen  tiber 
Temperaturverhaltnisse  der  Bodenoberflache  und  verschiedener 
Bodentiefen.  Internat.  Mitt,  fur  Bodenkunde,  Band  II,  Heft 
2-3,  Seite  133-148.     1912. 

2  Swezey,  G.  D.  Soil  Temperatures  at  Lincoln,  Nebraska. 
Nebraska  Agr.  Exp.  Sta.,  16th  Ann.  Rept.r  pp.  95-102.     1903. 


322       SOILS:    PROPERTIES  AND  MANAGEMENT 

These  average  readings,  taken  at  different  points,  arc 
supported  by  the  data  of  other  observers.1  It  is  apparent 
that  seasonal  variation  of  soil,  temperature  is  considerable, 
even  at  the  lower  depths.  The  surface  layers  of  soil  seem 
to  vary  nearly  in  accord  with  the  air  temperature,  and 
therefore  exhibit  a  greater  fluctuation  than  the  subsoil. 
In  general  the  surface  soil  is  warmer  in  spring  and  sum- 
mer than  the  lower  layers,  but  cooler  in  fall  and  winter. 
The  following  data  taken  at  Lincoln,  Nebraska,  may  be 
of  interest :  — 


Average  Monthly  Temperature  Readings  2  taken  at  Lin- 
coln, Nebraska.     Average  of  Twelve  Years. 


IInch 
Deep 

3 

6 

9 

12 

24 

36 

Air 

Inches 

Inches 

Inches 

Inches 

Inches 

Inches 

Deep 

Deep 

Deep 

Deep 

Deep 

Deep 

January  .     . 

25.2 

27.3 

27.8 

28.6 

30.0 

31.2 

35.4 

38.5 

February 

24.2 

27.7 

27.3 

27.8 

28.3 

30.2 

33.5 

35.5 

March      .     . 

35.8 

38.2 

37.2 

36.6 

35.6 

35.4 

35.4 

35.8 

April  .     .     . 

52.1 

57.5 

56.0 

53.3 

50.6 

49.3 

45.6 

43.8 

May    .     .     . 

61.9 

68.7 

67.5 

65.1 

63.3 

60.7 

56.2 

53.3 

June    .     .     . 

71.0 

78.1 

78.0 

75.7 

73.8 

69.9 

64.6 

61.3 

July     .     .     . 

76.0 

85.1 

83.6 

81.6 

79.3 

75.7 

70.2 

67.4 

August     . 

74.5 

82.9 

81.3 

80.1 

78.5 

75.7 

72.2 

69.8 

September    . 

67.6 

73.8 

73.4 

72.0 

70.7 

69.2 

68.7 

67.6 

October    .     . 

55.5 

56.7 

58.4 

57.8 

58.3 

57.8 

60.0 

61.3 

November    . 

38.7 

38.7 

40.9 

41.5 

43.3 

44.7 

49.2 

52.2 

December     . 

28.3 

31.6 

31.4 

32.0 

33.4 

35.2 

40.1 

43.3 

Average  .     . 

50.9 

55.5 

55.3 

54.6 

53.8 

52.9 

52.6 

•7J.5 

Range      .     . 

51.8 

57.8 

56.3 

53.8 

51.0 

45.5 

38.7 

34.3 

1  Ebermayer,  E.  Untersuchungen  iiber  das  Verhalten 
Verse hiedener  Bodenarten  gegen  Warme.  Forsch.  a.  d.  Geb. 
d.  Agri-Physik,  Band  14,  Seite  195-253.     1891. 

2  Swezey,  G.  D.  Soil  Temperatures  at  Lincoln,  Nebraska. 
Nebraska  Agr.  Exp.  Sta.,  16th  Ann.  Rept.,  pp.  95-102.     1903. 


SOIL   HEAT 


323 


The  upper  soil  layers  vary  in  accordance  with  the  air 
temperature,  the  maximum  and  the  minimum  occurring 
in  the  same  month.  A  lagging  (see  Fig.  53)  is  apparent 
in  the  subsoil,  due  to  the  slow  response  of  this  area  to  the 
heat  penetrating  from  above.  These  figures  also  show 
the  surface  soil  to  be  warmer  in  spring  and  summer,  and 
cooler  in  winter  and  fall,  than  the  lower  depths.  The 
surface  soil  not  only  never  falls  as  low  in  temperature  as 
the  air,  but  reaches  a  higher  point  in  summer.  This  is 
shown  in  the  range  of  the  air  and  soil  temperatures.  The 
range  for  the  air  is  51.8°,  while  that  for  the  soil  is  57.8°, 
56.3°,  53.8°,  51.0°,  45.5°,  38.7°,  and  34.3°,  respectively, 
for  the  depths  ranging  from  1  inch  to  36  inches.  While 
this  range  of  soil  temperature  is  greater  in  the  aggregate 
than  that  of  the  air,  the  changes  are  much  slower  and 
often  extend  over  a  number  of  days,  while  the  air  may  vary 
many  degrees  in  an  hour. 


8o"/= 

70 

4 

s' 

rr^^. 

\ 

60 

/ 

s 

V 

SO 

: 

* 

\> 

\v\ 

4-0 

/     t 

36* 

sn 

■ -j 

p. 

O1-  N 

/2' 
3' 

20 

— •-». 

/ 

4/K 

o'/lf/      F£B     /*mg     /iPG      rtjY   <juM£     <suiy     job     <i£pr    ocro      /rov       0£C 

Fig.  53. — Curves  showing  the  average  monthly  temperature  readings  at 
various  soil  depths.     Average  of  twelve  years,  Lincoln,  Nebraska. 


324      SOILS;   PROPERTIES  and  management 

The  daily  and  hourly  temperature  of  the  air  and  the 

soil  may  be  fairly  constant  or  rather  variable,  according 
to  conditions.  On  days  of  sunshine,  however,  consis- 
tent, changes  may  be  expected.     The  air  temperature  rises 

from  morning  until  about  two  o'clock,  when  the  maxi- 
mum is  reached.  It  then  falls  rapidly.  The  soil,  how- 
ever, does  not  reach  it  maximum  temperature  until 
later  in  the  afternoon,  due  to  the  lagging  so  apparent    in 

soil   temperature   changes.    This   lagging   is   greater   in 

the  lower  layers  than  at  the  surface.  The  following  data,1 
iaken  on  a  bright  day  on  May  L'ti  in  Germany,  illustrate 
the  ordinary  differences  that  may  be  expected  in  soil 
and    air    temperat  nres  :  — 

HoUBLI  Ti;\ii'i;i(\Tn(KS  takia   in  Ckumany  ON  May  '-'<'..    I 
ON  A  Loam  Son-  at  l-l\<  ii  Di 


Hour 

Am 

Bare  Soil 

Midnight       

2  A.M 

4 

6 

8 

10 

Noon 

2    l'.  M 

4 

('» 

8 

10 

W  t 

54.3 
52.7 
67.6 
76.4 
82.0 
83.5 
85.6 
84.2 
7S.1 
58.7 
65.1 

60.4 

58.5 
57.0 
68,4 
63.3 

69.8 
74.8 
77.0 
77.7 
73.9 
69.8 

1  Wollny,  E.  tJnterauchungen  iiber  don  Kinfluss  der 
Pflanzendeoke  und  di-v  Beschattung  auf  <li<-  PhyrikoUsohen 
Eigenjohaften  <l<-s  Bodens.  Foriofeu  a.  d,  Geb.  'I.  A^ri.-Physik, 
Band  <>,  Seite  l«>7  -266.     1885. 


SOIL  HEAT 


325 


&o*F 

76 

vsoc^r 

6Q 

\ 

AIR, 

A 

r          * 

!            4 

t-            < 

6           4 

s         > 

0             / 

»-            * 

1        41       f       9       *»  r**e 

Fio.  54. — Curves  showing   the  hourly   toinpornhnvs  of  hare  soil  at  a 
depth  of  four  inches  and  oi  {he  air  :'hovo  th<>  soil. 


The  temperature  of  the  soil  at  the  surface  in;iy  often 
exceed  tliat  of  the  air,  and  the  amount,  of  daily  fluctua- 
tion may  be  greater;  but  for  the  lower  depths  the  tem- 
perature curve  flattens  out.  The  subsoil  shows  but 
little  daily,  and  eves  monthly,  variation,  and  is  affected 
only  by  seasonal  changes. 

228.  Control  of  soil  temperature. — The  means  of 
practical  control  and  modification  of  soil  temperature 
are  those  commonly  in  vogue  in  good  soil  management. 
The  most  important  factor  is,  of  course,  soil  moisture. 
Good  drainage,  proper  tilth  developed  by  deep  plowing, 
plenty  of  lime,  and  sufficient,  organic  matter,  favor  opti- 
mum moisture  conditions.  Such  moisture  regulation 
means  a  lowered  specific  heat  and  good  conductivity. 
The  use  of  a  soil  mulch  or  an  artificial  covering  not  only 
will  check  evaporation  but  will  markedly  retard  loss  of 

heat  by  radiation.     Any  farmer  who  so  controls  his  soil 


326       SOILS:    PROPERTIES  AND  MAN  AG  EM  I 

moisture  that  optimum  conditions  as  far  as  the  plant  is 
concerned  may  be  obtained,  should  have  no  fear  of  a 
poor  utilization  of  heat. 

The  increase  of  soil  humus,  of  course,  may  act  directly 
in  heat  control  by  darkening  the  color  and  increasing 
absorption.  A  soil  mulch,  being  dry,  not  only  checks 
evaporation  but  lowers  radiation  while  increasing  absorp- 
tion. Any  methods  of  handling  the  land  which  tend  to 
better  the  physical  condition  of  the  soil  and  increase  its 
tilth  tend  also  toward  a  proper  heat  control  at  the  same 
time.  The  whole  question  may  be  summarized  by  say- 
ing that  if  a  farmer  adopts  a  proper  system  of  moisture 
control  and  at  the  same  time  employs  methods  that  tend 
always  toward  a  better  physical  condition  of  the  soil, 
the  problem  of  control  of  soil  heat  will  be  automatically 
solved.  He  will  then  have  brought  about  the  best  condi- 
tions for  heat  absorption  and  will  have  facilitated  conduc- 
tion and  convection,  while  at  the  same  time  retarding 
losses  by  evaporation  and  radiation. 


CHAPTER   XV 

AVAILABILITY   OF  PLANT    NUTRIENTS    AS 
DETERMINED    BY   CHEMICAL    ANALYSIS 

Fortunately  for  mankind,  only  an  exceedingly  minute 
proportion  of  the  soil  is  at  any  one  time  soluble  in  water 
or  in  the  aqueous  solutions  with  which  it  is  in  contact. 
It  is  this  great  degree  of  insolubility  that  gives  the  soil 
its  permanence,  for  in  humid  regions,  without  this  property, 
it  would  be  rapidly  carried  away  in  the  drainage  water. 
The  portion  of  the  soil  that  is  soluble  in  the  various  natural 
solvents  with  which  it  comes  in  contact  furnishes  mineral- 
food  materials  for  plants.  The  great  mass  of  soil,  which 
is  relatively  insoluble,  is  constantly  subjected  to  natural 
processes  which  very  slowly  bring  its  constituents  into 
solution.  The  agents  that  are  concerned  in  the  decom- 
position of  rock  also  act  on  the  soil  to  bring  about  its 
further  disintegration,  and  thereby  render  it  more  soluble ; 
while  added  to  these  are  the  operations  of  tillage,  which 
contribute  to  the  same  end. 

Only  the  surfaces  of  the  soil  particles  come  into  contact 
with  the  decomposing  agents,  and  hence  it  is  the  surface 
matter  of  the  particles  that  gradually  goes  into  solution. 
The  factors  that  determine  how  rapidly  solution  shall 
proceed  are :  (1)  the  amount  of  surface  exposed,  which, 
as  has  been  seen,  varies  with  the  size  of  the  particles ;  (2) 
the  composition  of  the  particles ;  (3)  the  strength  of  the 
decomposing  and  solvent  agencies.     Were  it  not  for  this 

327 


328       SOILS:    PROPERTIES  AND  MANAGEMENT 

process,  there  would  soon  be  no  mineral  food  available 
to  plants,  as  drainage  water  and  the  growth  of  crops  take 
up  relatively  large  quantities  of  these  substances  each 
year;  but  in  spite  of  this  loss  the  soil  is  able  to  provide 
at  least  some  plant-food  material  for  each  crop,  when 
called  upon  by  the  plant. 

229.  Solubility  of  the  soil  in  various  solvents.  —  For 
purposes  of  analyses  that  are  intended  to  show  the  amounts 
of  mineral  plant-food  materials  in  a  soil,  any  one  of  sev- 
eral different  solvents  may  be  used.  These  solvents  differ 
in  strength,  and  consequently  the  percentages  of  the 
various  constituents  obtained  from  samples  of  the  same 
soil  are  different  for  each  solvent.  A  chemical  analysis 
of  a  soil  is  a  determination  of  the  quantities  of  the  con- 
stituents that  have  been  dissolved  in  the  solvent  used. 
Therefore  it  will  readily  be  seen  that  the  interpretation 
of  a  chemical  analysis  must  depend  largely  on  the  nature 
of  the  solvent,  and,  unless  the  solvent  is  equivalent  in 
its  action  to  some  process  or  processes  in  nature,  the  results 
must  be  entirely  arbitrary. 

The  methods  that  have  been  used  for  obtaining  solu- 
tions of  the  soil  for  analysis  may  be  grouped  as  follows  :  — 

1.  Complete  solution  of  the  soil. 

2.  Partial  solution  with  strong  acids. 

3.  Partial  solution  with  weak  acids. 

4.  Extraction  with  water. 

230.  Complete  solution  of  the  soil.  —  By  the  use  of 
hydrofluoric  and  sulfuric  acids  and  by  fusion  with  alkalies, 
the  entire  soil  mass  may  be  decomposed  and  all  its  inor- 
ganic constituents  determined.1     Such  an  analysis  shows 

1  Wiley,  Harvey  W.  Principles  and  Practices  of  Agri-* 
cultural  Chemical  Analysis,  Vol.  1,  pp.  398-399.     1906. 


AVAILABILITY  OF  PLANT  NUTRIENTS 


329 


the  total  quantity  of  the  plant-food  materials  except 
nitrogen,  which  is  never  determined  in  any  of  the  acid 
solutions  but  by  a  separate  process.1  A  deficiency  of 
any  particular  substance  may  be  discovered  in  this  way, 
but  nothing  can  be  learned  as  to  the  ability  of  the  plant 
to  obtain  nutriment  from  the  soil.  A  rock  may  show  as 
much  mineral  plant-food  material  as  a  rich  soil.  This 
method  of  analysis  is  used  only  to  ascertain  the  ultimate 
limitations  of  a  soil  or  its  possible  deficiency  in  any  essen- 
tial constituent.  Results  of  such  analyses  are  to  be  found 
in  paragraphs  46,  48,  52,  53  of  this  text. 

231.  Partial  solution  with  strong  acids.  —  While  sul- 
furic, nitric,  and  hydrochloric  acids  have  all  been  used  as 
solvents,2  the  one  most  commonly  employed  is  hydrochloric 
acid  of  1.115  specific  gravity.3  It  has  been  used  to  such 
an  extent  that  it  may  be  considered  the  standard  solvent, 
and  a  statement  of  a  chemical  analysis  of  a  soil  in  this 
country  may  be  considered  as  based  on  this  solvent  unless 
otherwise  stated. 


1  Official  and  Provisional  Methods  of  Analysis.  U.  S.  D.  A., 
Bur.  Chem.,  Bui.  107  (revised),  p.  19.     1908. 

2  Analyses  using  concentrated  mineral  acids  on  the  same  soil. 
From  Snyder,  Harry.  Soils.  Minnesota  Agr.  Exp.  Sta.,  Bui. 
41,  p.  66.     1895. 


Total  insoluble  percentage 
Potash  percentage        .     . 
Lime  percentage      .     .     . 
Magnesia  percentage 
Phosphoric  acid  percentage 
Sulfuric  acid  percentage  . 


Hydro- 
chloric 


81.20 
0.42 
0.55 
0.40 
0.23 
0.08 


Nitric 


83.45 
0.30 
0.30 
0.32 
0.23 
0.08 


Sulfuric 


80.45 
0.52 
0.53 
0.52 
0.26 
0.10 


3  Official  and  Provisional  Methods  of  Analysis. 
Bur.  Chem.,  Bui.  107  (revised),  pp.  14-18.     1908. 


U.  S.  D.  A. 


330       SOILS:    PROPERTIES  AND  MANAGEMENT 

An  analysis  by  this  method  is  supposed  to  show  the 
proportion  of  plant-food  materials  in  a  soil  that  are  in  a 
condition  to  be  ultimately  used  by  plants  at  the  time 
when  the  analysis  is  made,  and  the  plant-food  materials 
that  are  not  dissolved  by  treatment  with  hydrochloric 
acid  are  assumed  to  be  in  a  condition  in  which  plants 
cannot  use  them.  The  difficulty  with  this  assumption  is 
that,  while  treatment  with  hydrochloric  acid  of  a  given 
strength  marks  a  definite  point  in  the  solubility  of  the 
compounds  in  the  soil,  it  does  not  bear  a  uniform  rela- 
tion to  the  natural  processes  by  which  these  compounds 
become  available  to  the  plant. 

In  the  case  of  most  soils  a  large  proportion  is  not  de- 
composed by  treatment  with  strong  hydrochloric  acid, 
and  the  portion  that  is  dissolved  may  contain  a  larger  or 
a  smaller  quantity  of  the  agriculturally  important  ele- 
ments, depending  on  the  character  of  the  soil.  Thus  if 
calcium  is  present  as  a  phosphate,  a  larger  proportion 
will  be  dissolved  by  the  acid  than  if  it  is  in  the  form  of 
silicate.  The  form  in  which  potassium  occurs  also  in- 
fluences greatly  the  amount  shown  by  analysis. 

Snyder  *  has  analyzed  a  number  of  soils  by  means  of 
digestion  with  strong  hydrochloric  acid,  and  has  then 
decomposed  the  acid-insoluble  residue  by  fusion  and 
determined  its  composition.  Veitch  2  has  analyzed  soils 
by  the  hydrochloric  acid  method  and  by  means  of  com- 
plete solution.  A  few  examples  are  given  below  to  show 
how  soils  may  vary  in  the  solubility  of  their  constituents 
in  strong  hydrochloric  acid  :  — 

1  Snyder,  Harry.  Soils.  Minnesota  Agr.  Exp.  Sta.,  Bui.  41, 
p.  35.     1895. 

2  Veitch,  F.  P.  The  Chemical  Composition  of  Maryland 
Soils.     Maryland  Agr.  Exp.  Sta.,  Bui.  70,  p.  103.     1901. 


AVAILABILITY  OF  PLANT  NUTRIENTS 


331 


Percentage  of  Constituents  not  Soluble  in  HCl, 
1.115   SP.    GR. 


Soil  prom  Minnesota 

Son,  prom  Maryland 

Fair 
Haven 

Holden 

Experi- 
ment 
Station 

Colum- 
bia 

Chesa- 
peake 

Hudson 
River 
Shale 

Potash  .     .     . 

94 

81 

83    • 

95 

67 

73 

Lime 

25 

61 

41 

90 

82 

37 

Magnesia   . 

58 

76 

36 

34 

29 

28 

Phosphoric 

anhydride    . 

40 

45 

18 

66 

15 

0 

Sulfuric  anhy- 

dride      .     . 

74 

90 

20 

— 

— 

— 

232.  Significance  of  a  strong  hydrochloric  acid  analysis. 

—  This  method  of  analysis  was  originally  thought  to 
give  some  indication  of  both  the  permanent  fertility  and 
the  immediate  manurial  needs  of  a  soil ;  but  for  each 
question  the  accuracy  of  the  deductions  is  limited  by  a 
number  of  conditions  that  make  it  impossible  invariably 
to  predict  from  an  analysis  how  productive  a  soil  may  be 
or  what  particular  manure  may  be  profitably  applied. 
It  is  very  apparent  that  the  chemical  composition  of  a 
soil  is  only  one  of  the  many  factors  affecting  its  produc- 
tiveness. Unfortunately,  not  all  the  factors  are  under- 
stood, and  consequently  these  unknown  ones  cannot  be 
determined  either  qualitatively  or  quantitatively.  If 
it  ever  becomes  possible  to  determine  quantitatively  all 
the  factors  entering  into  soil  productiveness  in  the  field 
condition,  the  problem  will  be  solved. 

233.  Relation  of  texture  to  solubility.  —  The  ratio  of 
sand  to  clay  in  a  soil,  and  the  distribution  of  the  fer- 
tilizing materials  in  these  constituents,  will  affect  the  mini- 


332       SOILS:    PROPERTIES  AND  MANAGEMENT 

mum  quantity  of  any  constituent  required  to  produce 
a  good  crop.  Hilgard  has  shown  that  the  addition  of 
four  or  five  volumes  of  quartz  sand  to  one  volume  of  a 
heavy,  but  highly  productive,  black  clay  soil  greatly 
increased  the  productiveness,  while  diluting  the  potash 
content  of  the  mixture  to  0.12  per  cent  and  the  phosphoric 
acid  to  0.03  per  cent.  It  is  evident  that  in  this  soil  the 
plant-food  materials  were  in  a  condition  to  be  easily 
taken  up  by  the  plant  when  the  physical  condition  of  the 
soil  was  suitable. 

If  these  small  quantities  of  food  elements  had  been 
distributed  in  the  sand  particles  as  well  as  in  the  original 
clay,  the  result  would  doubtless  have  been  different. 
Suppose,  for  example,  that  fifty  per  cent  of  the  potash 
and  phosphoric  acid  bad  been  in  the  sand  particles  and 
the  remainder  in  the  clay ;  in  that  case  the  former,  in  a 
soil  exposing  much  the  less  surface  to  dissolving  liquids, 
would  be  proportionately  less  soluble,  and  as  the  minimum 
quantity  is  approached,  as  showrn  by  the  more  dilute  soil's 
yielding  less  than  the  other,  the  effect  would  doubtless 
have  been  to  decrease  the  production.  In  some  soils, 
particularly  those  of  arid  regions,  the  larger  particles 
may  carry  much  of  the  mineral  nutrients,  in  wThich  case 
it  is  quite  evident  that  a  higher  percentage  of  fertility 
is  required  than  in  soils  carrying  the  plant-food  material 
largely  in  the  small  particles. 

234.  Nature  of  the  subsoil.  —  The  nature  and  com- 
position of  the  subsoil  is  naturally  a  factor  in  determining 
soil  productiveness,  and  must  be  considered  as  well  as 
the  top  soil.  An  impervious  subsoil,  or  a  very  loose 
sandy  one,  will  confine  the  productive  zone  largely  to 
the  topsoil  and  hence  require  a  greater  proportionate 
amount  of  fertility  in  that  part  of  the  soil. 


AVAILABILITY  OF  PLANT  NUTRIENTS  333 

235.  Calcium  carbonate.  —  A  determination  of  the 
amount  of  calcium  present  as  a  carbonate  is  important 
as  an  aid  to  the  interpretation  of  an  analysis  of  the  soil. 
Lime  not  so  combined  is  generally  in  the  form  of  a  sili- 
cate, or  possibly  a  phosphate.  If  there  is  a  large  quantity 
of  calcium  carbonate  in  a  soil,  the  potash,  phosphoric 
acid,  and  nitrogen  are  likely  to  be  more  readily  soluble, 
and  smaller  quantities  are  sufficient  for  crop  growth,  than 
if  the  calcium  is  not  found  in  this  form.  The  effect  of 
the  carbonate  of  lime  on  the  nitrogen  1  compounds  is  to 
furnish  a  base  for  the  acids  produced  in  the  formation  of 
nitrates,  and  its  presence  promotes  this  process.  It 
probably  replaces  potassium  in  certain  compounds  where 
otherwise  it  would  be  secured  with  more  difficulty.  It 
insures  the  presence  of  some  phosphates  of  lime,  in  which 
form  phosphorus  is  more  soluble  than  when  combined 
with  iron.  The  form  of  the  manures  to  be  used  on  the 
soil  will  also  depend  in  large  measure  on  the  presence 
or  the  absence  of  calcium  carbonate.  For  example, 
where  calcium  carbonate  is  deficient,  steamed  bone  or 
Thomas  slag  are  likely  to  be  more  profitable  than  super- 
phosphate, and  nitrate  of  soda  than  sulfate  of  ammonium. 
Finally,  the  absence  of  calcium  carbonate  indicates  the 
need  of  liming,  and  if  the  analyses  show  a  considerable 
quantity  of  potash  and  phosphoric  acid,  but  practice 
shows  these  materials  to  be  somewhat  deficient,  it  is 
probable  that  liming  will  be  very  beneficial,  and  that 
manures  carrying  these  substances  will  not  be  so  essential 
as  the  chemical  analysis  would  indicate.  It  must  be 
stated,  however,  that  there  are  cases  for  which  these  de- 
ductions do  not  hold,  owing  to  the  intervention  of  other 
factors. 

1  Not  determined  in  the  hydrochloric  acid  extract. 


334       SOILS:    PROPERTIES  AND  MANAGEMENT 

236.  Deficiency  of  ingredients  and  manurial  needs.  — 
Many  standards  have  been  set  for  the  minimum  quantity 
of  each  of  the  important  soil  constituents  that  must  be 
present  in  order  to  insure  a  productive  soil.  Experi- 
ence has  shown,  however,  that  no  definite  standards  hold 
for  all  soils.  By  comparing  analyses  of  soils  of  known 
productivity  with  that  of  a  soil  under  Investigation  it  is 
an  easy  matter  to  ascertain  whether  the  soil  contains  a 
large  quantity  of  each  agriculturally  important  ingredi- 
ent; but  when  the  quantity  of  any  constituent  is  low, 
it  becomes  a  difficult  matter  to  tell  how  this  will  affect 
the  agricultural  value  of  the  soil.  Some  soils  will  be 
productive  with  0.05  per  cent  of  phosphoric  anhydride, 
while  others  are  unproductive  when  all  the  plant  nutrients 
are  present  in  ample  quantity. 

The  fact  that  the  degree  of  productiveness  of  a  soil 
cannot  always  be  gauged  by  its  analysis  gives  rise  to  a 
similar  uncertainty  with  regard  to  its  manurial  needs. 
A  soil  may  contain  potassium  in  very  large  quantities, 
sufficient  to  produce  crops  for  hundreds  of  years,  as  indi- 
cated by  a  strong  hydrochloric  acid  analysis,  and  yet  a 
potassium  salt  may  be  used  with  profit.  On  the  other 
hand,  it  is  evident  that  as  the  content  of  any  constituent 
becomes  less,  the  probable  need  for  its  application  be- 
comes greater,  and  a  knowledge  of  the  composition  of 
the  soil  thus  suggests  a  practice  without  assuring  its 
success.  An  analysis  of  the  hydrochloric,  acid  extract, 
therefore,  cannot  be  taken  as  an  infallible  guide  to  the 
fertilizer  needs  of  a  soil,  and  of  itself  should  not  be  relied 
upon;  but  in  connection  with  other  knowledge,  particu- 
larly that  derived  from  fertilizer  tests,  it  may  be  useful. 

237.  Partial  solution  with  weak  acids.  —  The  difficulty 
in  judging  of  the  properties  of  a  soil  from  the  results  of 


AVAILABILITY   OF  PLANT  NUTRIENTS  335 

a  strong  hydrochloric  acid  analysis  has  led  to  the  use  of 
weak  acids  for  obtaining  the  solution.  These  weak  acids 
dissolve  much  less  of  the  soil  constituents  than  do  the 
strong  acids,  and  the  portion  so  dissolved  is  supposed  to 
represent  more  nearly  the  amount  that  the  plant  can  make 
use  of.  Both  dilute  organic  acids  and  dilute  mineral 
acids  have  been  used.  Among  the  former  are  citric, 
acetic,  oxalic,  and  tartaric  acids.  The  assumption  on 
which  the  use  of  the  organic  acids  is  based  is  that  they 
correspond  to  the  solvent  agents  in  the  soil  combined  with 
the  solvent  action  that  the  plant  is  supposed  to  possess, 
and  thus  dissolve  from  the  soil  the  quantities  of  nutrients 
that  the  plant  could  take  up  if  it  came  in  contact  with 
all  the  soil  particles  to  a  depth  represented  by  the  sample 
analyzed. 

238.  Advantages  in  the  use  of  dilute  acids.  —  The  ac- 
tion of  each  of  these  dilute  acids  on  the  same  soil  does 
not  give  equal  quantities  of  the  various  constituents  in 
solution.  The  dilute  acids  naturally  dissolve  a  much 
smaller  amount  of  material  from  the  soil  than  does  strong 
hydrochloric  acid.  The  dilute  acids  permit  the  detection 
of  smaller  quantities  of  easily  soluble  phosphoric  acid  and 
potash  than  does  the  latter,  larger  quantities  of  soil  being 
used.  For  example,  a  chemical  analysis  of  the  strong 
hydrochloric  acid  solution  is  very  likely  not  to  show  any 
increase  in  the  phosphorus  or  potassium  in  a  soil  that  may 
have  been  abundantly  manured  with  these  fertilizers 
and  its  productiveness  greatly  increased  thereby.  This 
is  because  the  amount  of  plant-food  material  added  is  so 
small  in  comparison  with  the  weight  of  the  area  of  soil 
nine  inches  deep  over  which  it  is  spread  that  the  increase 
in  percentage  may  well  come  within  the  limits  of  analytical 
error.     An  acre  of  soil   nine  inches  deep  weighs  about 


336      SOILS:    PROPERTIES  AND  MANAGEMENT 

2,500,000  pounds.  If  to  this  there  is  added  a  dressing 
of  2500  pounds  of  phosphoric  acid  fertilizer  containing 
400  pounds  of  phosphoric  acid,  it  would  increase  the  per- 
centage of  that  constituent  in  the  soil  only  0.016  per  cent 
—  a  difference  that  could  not  be  detected  by  the  analysis 
of  the  hydrochloric  acid  solution. 

239.  The  one-per-cent  citric  acid  method.  —  This 
method  was  proposed  by  Dyer1  and  was  shown  by  him 
to  give  results  with  Ejothamrted  soils  that  permitted  of 
an  accurate  estimation  of  their  relative  productivity. 
Dyer  adopted  the  one-per-cent  strength  as  the  result  of 
an  investigation  in  which  he  determined  the  acidity  of 
the  juices  in  the  roots  of  over  one  hundred  species  or 
varieties  of  plants  representing  twenty  different  natural 
orders.  The  average  acidity  of  the  juices  of  the  twenty 
orders,  calculated  to  crystallized  citric  acid,  was  0.91 
per  cent,  which  led  Dyer  to  adopt  a  strength  of  1  per  cent. 
It  must  be  said,  however,  that  the  different  varieties 
varied  greatly  in  this  respect,  some  having  ten  times  as 
much  acidity  as  others.  The  implication  is  that  plants  pro- 
duce a  solvent  action  on  a  soil  in  proportion  to  the  acidity 
of  their  juices,  but  an  examination  of  Dyer's  figures  does 
not  show  that  the  size  of  the  crop  ordinarily  produced  by 
the  plants  tested  would  in  many  cases  correspond  to  the 
acidity  of  these  juices.  Thus,  of  the  Cruciferse  the  horse- 
radish has  several  times  the  acidity  of  the  Swedish  turnip 
or  of  the  field  cabbage,  although  the  crop  produced  by  the 
former  is  much  less  than  that  of  the  latter. 

240.  Usefulness  of  the  citric  acid  method.  —  As  shown 
by  Dyer,   the  use   of  a   one-per-cent   solution   of  citric 

1  Dyer,  Bernard.  On  the  Analytical  Determination  of  Prob- 
ably Available  "Mineral"  Plant  Food  in  Soils.  Jour.  Chem. 
Soc.,  Vol.  LXV,  pp.  115-167.     1894. 


AVAILABILITY  OF  PLANT  NUTRIENTS         337 

acid  is  well  adapted  to  show  the  amount  of  easily- 
soluble  phosphoric  acid  and  potash  in  certain  soils,  but 
for  other  soils  it  has  failed  to  give  satisfaction  in 
the  hands  of  a  number  of  analysts.  It  is  doubtless 
best  suited  to  soils  rich  in  calcium  and  low  in  iron  and 
aluminium. 

The  reason  urged  by  Dyer  for  the  superiority  of  the 
citric  acid  method  over  the  hydrochloric  acid  extraction 
is  that  soils,  shown  by  experience,  to  need  phosphoric 
manures,  yielded  a  relatively  much  greater  quantity  of 
phosphorus  to  citric  acid  than  to  hydrochloric  acid  when 
compared  with  soils  not  needing  this  element. 

The  application  of  both  the  hydrochloric  and  citric  acid 
methods  to  a  soil,  when  used  to  supplement  each  other, 
may  add  greatly  to  a  knowledge  of  the  potential  and 
present  productiveness  of  the  soil. 

According  to  Dyer,1  for  cereals  and  for  most  other 
crops  there  should  be  present  in  a  soil  at  least  .01  per  cent 
of  phosphoric  acid,  soluble  in  one-per-cent  citric  acid. 
A  soil  containing  less  than  this  quantity  is  deficient  in 
phosphoric  acid,  unless  this  acid  exists  largely  in  the  form 
of  ferric  or  aluminium  phosphate,  which  is  not  readily 
soluble  in  citric  acid  but  is  fairly  available  to  the  plant. 
Sod  land  contains  organic  compounds  of  phosphorus  that 
are  readily  available  to  the  plant;  hence  such  soil,  to 
indicate  sufficiency,  should  show  by  analysis  more  than 
0.01  per  cent  of  phosphoric  acid.  The  quantity  of  potash 
soluble  in  the  same  solvent  should  also  be  not  less  than 
0.01  per  cent  in  arable  land. 

1  Dyer,  Bernard.  A  Chemical  Study  of  the  Phosphoric 
Acid  and  Potash  Contents  of  the  Wheat  Soils  of  Broadbalk 
Field,  Rothamsted.  Philosoph.  Trans.  Royal  Soc.  London, 
Series  B,  Vol.  194,  pp.  235-290.     1901. 


338       SOILS:    PROPERTIES  AND  MANAGEMENT 

241.  Dilute  mineral  acids.  —  Of  the  mineral  acids 
in  a  diluted  form  used  for  extracting  soils,  those  that 
have  received  the  most  attention  are  one-fifth  normal 
nitric l  or  hydrochloric  acid  and  one  two-hundredth 
normal  hydrochloric  acid.2  The  methods  employing 
these  solvents  are  admittedly  empirical.  There  is  no 
natural  relation  between  these  solvents  and  the  processes 
by  which  the  plant  obtains  its  nutriment  from  the  soil. 

The  solvent  that  has  received  the  most  attention  is 
one-fifth  normal  nitric  acid.  In  ease  of  manipulation 
this  is  preferable  to  the  one-per-cent  citric  acid,  which  is 
rather  tedious  to  work  with.  It  has  been  used  nearly 
as  extensively  in  this  country  as  the  latter  has  in  Great 
Britain.  Its  use  has  been  confined  largely  to  the  deter- 
mination of  the  readily  available  phosphorus  and  potas- 
sium in  the  soil,  as  has  the  citric  acid  method.  It  is 
obvious  that  some  minerals  are  more  readily  soluble  than 
are  others,  and  for  that  reason  the  method  will  distinguish 
between  phosphorus  and  potassium  in  different  forms. 
The  calcium  phosphates  are  supposed  to  be  entirely  soluble 
in  this  solvent.  According  to  Fraps 3  it  dissolves  iron 
and  aluminium  phosphates  to  only  a  slight  extent,  thus 
distinguishing  between  these  forms  of  phosphorus.  Fraps 
finds  also  that  no  potassium  is  removed  from  orthoclase 
and  microcline,  that  less  than  ten  per  cent  is  dissolved 


1  Official  and  Provisional  Methods  of  Analysis.  U.  S.  D.  A., 
Bur.  Chem.,  Bui.  107  (revised),  p.  18.     1908. 

2  Wiley,  H.  W.  Principles  and  Practice  of  Agricultural 
Analysis,  pp.  394-396.     Easton,  Pennsylvania.     1906. 

3  Fraps,  G.  S.  Active  Phosphoric  Acid  and  Its  Relation 
to  the  Needs  of  the  Soil  for  Phosphoric  Acid  in  Pot  Experi- 
ments. Texas  Agr.  Exp.  Sta.,  Bui.  126,  pp.  7-72.  1909. 
Also,  The  Active  Potash  of  the  Soil  and  Its  Relation  to  Pot 
Experiments.     Texas  Agr.  Exp.  Sta.,  Bui.  145,  pp.  5-39.     1912. 


AVAILABILITY  OF  PLANT  NUTRIENTS         339 

from  glauconite  and  biotite,  and  that  from  fifteen  to  sixty 
per  cent  is  dissolved  from  muscovite,  nephelite,  leucite, 
apophyllite  and  phillipsite. 

There  are  several  factors,  however,  that  make  the  use 
of  one-fifth  normal  nitric  acid  an  uncertain  guide  to  the 
available  phosphorus  and  potassium  in  the  soiL  When 
a  soil  is  treated  with  the  acid  some  of  it  is  neutralized  by 
the  reactions  that  result  and  thus  its  strength  is  lessened. 
This  may  have  no  relation  to  the  quantities  of  phosphorus 
or  potassium  dissolved.  Some  analysts  correct  for  the 
neutralization  and  some  do  not.  Again,  as  with  strong 
hydrochloric  acid,  the  degree  of  solubility  of  the  soil  con- 
stituents in  the  nitric  acid  may  not  correspond  with  the 
ability  of  the  plant  to  obtain  these  substances.  With 
this,  as  with  the  other  methods  discussed,  the  objection 
holds  that  the  result  cannot  be  taken  as  an  infallible  guide 
to  the  productiveness  of  a  soil,  or  to  its  fertilizer  needs; 
but  each  of  the  methods  affords  some  information  in 
regard  to  a  soil,  and  is  thus  of  value. 

242.  Extraction  with  an  aqueous  solution  of  carbon 
dioxide.  —  As  carbon  dioxide  is  a  universal  constituent 
of  the  water  of  the  soil,  and  without  doubt  a  potent  factor 
in  the  decomposition  of  the  mineral  matter,  it  has  been 
proposed  to  use  a  solution  of  carbon  dioxide  as  a  solvent 
in  soil  analysis.  The  amounts  of  soil  constituents  taken 
up  by  this  solvent  are  much  less  than  are  taken  up  by 
any  of  the  others  heretofore  mentioned,  but  all  mineral 
substances  used  by  plants  are  soluble  in  it  to  some  extent. 
The  amount  of  phosphorus  is  so  small  as  to  make  its 
detection  by  the  gravimetric  method  difficult.  Like 
other  methods  employing  very  weak  solvents,  this  method 
is  open  to  the  objection  that  the  extraction  fails  to  remove 
a  considerable  portion  of  the  dissolved  matter  that  is 


340       SOILS:    PROPERTIES  AND  MANAGEMENT 

retained  by  adsorption,  and  as  this  varies  with  soils  of 
different  texture  a  fair  comparison  of  such  soils  is  impos- 
sible. 

243.  Extraction  with  pure  water.  —  When  soil  is  di- 
gested with  distilled  water,  all  the  mineral  substances 
used  by  plants  are  dissolved  from  it,  but  in  very  small 
quantities.  It  has  been  proposed  to  use  this  extract  for 
soil  analysis  on  the  ground  that  it  involves  no  artificial 
solvent  the  presence  or  amount  of  which  in  the  soil  is 
doubtful,  but  shows  those  substances  that  are  undoubtedly 
in  a  condition  to  be  used  by  plants.  By  determining  the 
water  content  of  the  soil  and  using  a  known  quantity  of 
water  for  the  extraction,  the  percentage  of  the  various 
constituents  in  the  soil  water  or  in  the  dry  soil  may  be 
calculated. 

The  substances  dissolved  from  the  soil  by  extraction 
with  distilled  water  are  probably  only  those  contained 
in  the  soil-water  solution,  including  a  part  of  the  solutes 
held  by  adsorption.  The  aqueous  extract  does  not  con- 
tain the  entire  quantity  of  the  nutritive  salts  in  solution 
in  the  soil  water,  and  hence  is  not  a  measure  of  the 
fertility  held  in  that  form.  An  undetermined  quantity 
of  nutrients  is  retained  in  the  water,  in  the  very  small 
spaces  and  on  the  surface  of  the  soil  particles.  It  is, 
however,  a  fair  comparative  measure  of  the  content  of 
available  nutrients. 

244.  Influence  of  absorption. — The  quantity  of  ex- 
tracted material  depends  on  the  absorptive  properties  of 
the  soil  and  on  the  amount  of  water  used  in  the  extrac- 
tion, or  on  the  number  of  extractions.  Analyses  of  the 
aqueous  extract  of  a  clay  and  of  a  sandy  soil  on  the  Cornell 
University  farm  serve  to  illustrate  the  greater  retentive 
power  of  the  former  for  nitrates.     Sodium  nitrate  was 


AVAILABILITY  OF  PLANT  NUTRIENTS 


341 


applied  to  a  clay  soil  and  to  a  sandy  loam  soil  at  the  rate 
of  640  pounds  to  the  acre.  Analyses  of  aqueous  extracts 
some  ninety  days  later  showed  the  following :  — 


Kind  of  Soil 

Fertilizer 

Nitrates  in  Soil 
(Parts  per  million) 

Clay 

Clay 

Sandy  loam 

Sandy  loam    .     .     .     . 

Sodium  nitrate 
No  fertilizer 
Sodium  nitrate 
No  fertilizer 

7.8 

1.8 

150.0 

29.7 

There  was  apparently  a  much  greater  retention  of 
nitrate  by  the  clay  soil,  as  shown  by  a  comparison  of  the 
fertilized  and  the  unfertilized  plats  on  both  soils. 

Schulze  x  extracted  a  rich  soil  by  slowly  leaching  1000 
grams  with  pure  water,  so  that  one  liter  passed  through 
in  twenty-four  hours.  The  extract  for  each  twenty-four 
hours  was  analyzed  every  day  for  a  period  of  six  days. 
The  total  amounts  dissolved  during  each  period  were  as 
follows :  — 


Successive  Extractions 

Total  Matter 
Dissolved 

(Grams) 

Volatile 
(Grams) 

Inorganic 
(Grams) 

First 

0.535 

0.340 

0.195 

Second   

0.120 

0.057 

0.063 

Third 

0.261 

0.101 

0.160 

Fourth 

0.203 

0.083 

0.120 

Fifth 

0.260 

0.082 

0.178 

Sixth 

0.200 

0.077 

0.123 

1  Schulze,  F.  Ueber  den  Phosphorsaure-Gehalt  des  Wasser- 
Auszugs  der  Ackererde.  Landw.  Vers.  Stat.,  Band  6,  Seite 
409-412.     1864. 


342       SOILS:    PROPERTIES  AND  MANAGEMENT 

It  will  be  noticed  that  the  dissolved  matter,  both  or- 
ganic and  inorganic,  fell  off  markedly  after  the  first  ex- 
traction, which  was  larger  because  of  the  matter  in  solu- 
tion in  the  soil  water.  Later  extractions  were  doubtless 
supplied  largely  from  the  substances  held  by  adsorption, 
which  gradually  diffuse  into  the  water  extract  as  the 
tendency  to  maintain  equilibrium  of  the  solution  overcomes 
the  adsorptive  action.  With  the  removal  of  the  adsorbed 
substances,  the  equilibrium  between  the  soil  particles 
and  the  surrounding  solution  is  disturbed,  solvent  action 
is  increased,  and  more  material  gradually  passes  from  the 
soil  into  the  solution.  In  this  way  the  uniform  and  con- 
tinuous body  of  extractives  is  maintained. 

245.  Other  factors  influencing  extraction.  —  For  pur- 
poses of  soil  analysis,  the  quantity  of  water  used  for  extrac- 
tion must  be  placed  at  some  arbitrary  figure,  and  this 
is  open  to  the  objection  that  it  does  not  represent  accu- 
rately the  soil-water  solution.  Analyses  of  soils  of  different 
types  are  not  comparable,  and  the  water  extract  cannot 
be  considered  as  measuring  the  concentration,  or  even 
the  composition,  of  the  solution  existing  between  the 
root  hair  and  the  soil  particles.  However,  for  studying 
some  of  the  changes  which  go  on  in  the  soil  and  which  are 
detectable  in  the  soil-water  solution,  the  practice  may  be 
followed  to  advantage. 

246.  The  soil  solution  in  situ.  —  It  has  already  been 
pointed  out  that  the  interstitial  spaces  of  any  arable  soil 
contain  more  or  less  water  all  the  time;  that  there  is  a 
constant  tendency  for  this  water  to  assume  the  capillary 
condition  owing  to  the  gravitational  movement  of  free 
water ;  and  that  the  normal  evaporation  of  moisture  from 
the  soil  tends  to  reduce  the  capillary  film  to  the  condition 
of   hygroscopic    water   (par.   132).       As   the   movement 


AVAILABILITY  OF  PLANT  NUTRIENTS         343 

of  free  water  is  comparatively  rapid  and  that  of  capil- 
lary water  relatively  slow,  the  soil  moisture  supply  is 
usually  somewhere  between  the  point  of  lento-capillarity 
and  free  water.  In  this  condition  each  particle  or  aggre- 
gation of  particles  is  enveloped  in  a  thin  moisture  film, 
and  this  film  water  is  constantly  in  motion  although  the 
movement  is  rather  slow. 

Soils  are  more  or  less  soluble  in  pure  water ;  and  in  soil 
water,  charged  as  it  always  is  with  carbon  dioxide,  they 
are  still  more  readily  soluble.  Consequently  the  moisture 
films  constantly  tend  to  approach  a  state  of  equilibrium 
with  respect  to  the  water-soluble  matter  in  the  soil  parti- 
cles. If  plants  are  entirely  dependent  for  their  mineral 
nutrients  on  the  supply  in  the  soil-water  solution,  the 
strength  of  this  solution  becomes  an  important  matter. 
The  supply  of  mineral  nutrients  for  higher  plants  will  be 
discussed  later  (par.  339).  Even  if  the  plant  itself 
has  no  influence  on  the  supply  of  mineral  nutrients  that  go 
into  solution,  the  quantity  of  food  that  it  finds  in  the 
soil  solution  already  prepared  for  its  use  must  constitute 
an  important  factor  in  its  growth. 

Unfortunately  there  is  no  adequate  method  of  ascer- 
taining the  strength  of  the  solution.  Attempts  have 
been  made  to  remove  this  solution  from  the  soil,  but  it 
is  altogether  unlikely  that  the  analyses  of  the  liquid 
obtained  represent  the  composition  of  the  soil  solution, 
because  of  the  very  small  quantity  of  the  liquid  available 
for  analysis  and  also  because  of  the  uncertainty  that  the 
sample  obtained  was  representative  of  the  soil  solution. 

247.  Devices  for  obtaining  a  soil  solution.  —  An  at- 
tempt by  Briggs  and  McLane  1  to  sample  the  soil  solution 

1  Briggs,  Lyman  J.,  and  McLane,  John  W.  The  Moisture 
Equivalent  of  Soils.    U.  S.  D.  A.,  Bur.  Soils,  Bui.  45,  pp.  6-8.  1907. 


344       SOILS:    PROPERTIES  AND  MANAGEMENT 

involved  the  use  of  centrifugal  motion,  which  developed 
a  force  of  two  or  three  thousand  times  that  of  gravita- 
tion. When  the  soil  contained  a  rather  large  quantity 
of  capillary  water,  a  small  amount  of  it  could  be  removed 
in  this  way. 

Another  device,  by  Briggs  and  McCall,1  consists  of  a 
close-grained,  unglazed,  porcelain  tube,  closed  at  one  end 
and  provided  at  the  other  with  a  tubulure,  by  which  it 
can  be  connected  with  an  exhausted  receiver.  This 
tube  is  moistened  and  buried  in  the  soil.  If  the  moisture 
content  of  the  soil  is  sufficient  to  reduce  the  pressure  of 
the  capillary  water  surface  in  the  soil  to  less  than  the  dif- 
ference between  the  pressure  inside  and  outside  of  the 
tube,  there  will  be  a  movement  of  water  inward.  This 
water  may  be  collected  and  analyzed. 

More  recently  Van  Suchtelen  has  used  another  method 
to  obtain  the  soil  solution.2  He  replaces  the  soil  water 
by  means  of  paraffin  in  a  liquid  state,  at  the  same  time 
subjecting  the  soil  to  suction  on  a  filter.  The  displaced 
water  is  considered  to  represent  the  soil  solution. 

248.  Composition  and  concentration  of  the  soil  solu- 
tion. —  It  has  generally  been  held  that  because  some  soils 
are  more  productive  than  others,  and  because  fertilizers 
containing  soluble  salts  frequently  increase  the  yields  of 
crops,  the  soil  solution  in  the  better-yielding  soils  is  more 
concentrated,  at  least  as  regards  plant  nutrients,  than  is 
that  in  the  poorer  soils.     The  argument  is,   of  course, 

Griggs;  L.  J.,  and  McCall,  A.  G.  An  Artificial  Root  for 
Inducing  Capillary  Movement  of  Soil  Moisture.  Science, 
N.  S.,  Vol.  20,  pp.  566-569.     1904. 

2  Van  Suchtelen,  F.  H.  H.  Methode  zur  Gewinnung  der 
Naturlichen  Bodenlosung.  Jour.  f.  Landw.,  Band  60,  Seite 
369-370.     1912. 


AVAILABILITY  OF  PLANT  NUTRIENTS         345 

based  on' the  assumption  that,  other  things  being  equal, 
plant  growth  is  a  function  of  the  concentration  of  the 
plant  nutrients  in  the  soil  solution.  According  to  this 
conception,  increased  or  decreased  soil  fertility  is  reflected 
in  the  composition  and  concentration  of  the  soil  solution, 
and  this  in  turn  in  crop  yields.  The  soil  solution  is  there- 
fore a  variable  quantity,  and,  to  some  extent  at  least,  within 
the  control  of  man.  An  elaborate  explanation  for  the 
responsiveness  of  the  soil  solution  has  been  worked  out 
by  Van  Bemmelen  and  his  school. 

249.  Variability  in  composition  and  concentration  of 
the  soil  solution.  —  The  process  of  rock  weathering  has, 
according  to  Van  Bemmelen,1  Biltz,2  and  others,  resulted 
in  deep-seated  chemical  changes  in  some  of  the  mineral 
constituents  of  the  soil,  whereby  there  are  formed  com- 
plex colloidal  silicates  which,  in  the  form  of  gels,  cover 
the  surfaces  of  the  soil  particles.  These  colloidal  com- 
plexes may  contain  iron,  aluminium,  calcium,  magnesium, 
potassium,  phosphorus,  and  other  substances,  which  are 
absorbed  from  the  different  electrolytes  as  ions  or  as 
salts  and  depend  in  quantity  on  the  concentration  of  the 
solution  from  which  they  are  absorbed.  They  therefore 
act  like  solid  solutions,  whose  composition  changes 
with  every  change  in  the  concentration  of  the  liquid  solu- 
tion that  comes  in  contact  with  them.  This  relation  of 
the  colloidal  complexes  to  the  soil  water  with  which  they 
come  in  contact  is  essentially  different  from  that  of  the 

1  Van  Bemmelen,  J.  M.  Beitrage  zur  Kenntniss  der 
Verwitterungsprodukte  der  Silikate  in  Ton,  Vulkanischen,  und 
Laterit-Boden.  Zeit.  f.  Anorganische  Chemie,  Band  42, 
Seite  265-324.     1904. 

2  Biltz,  W.  Ueber  die  Gegenseitige  Beeinflussung  Col- 
loidal Geloster  Stoffe.  Ber.  deutsch.  chem.  Gesell.,  Band 
37,  Seite  1095-1116.     1904. 


346     SOILS:  properties  and  management 

pure  minerals,  as  they  are  not  true  chemical  combina- 
tions. The  organic  matter  in  the  soil  adds  another  class 
of  colloidal  matter ;  so  that,  in  the  opinion  of  Van  Bem- 
melen,1  the  colloidal  silicates  and  the  colloidal  humus  form, 
in  various  proportions,  a  mass  of  colloidal  complexes  that 
control  the  composition  of  the  soil  solution.  The  col- 
loidal condition  of  this  material  is  readily  decomposable 
under  variations  in  temperature  and  concentration  of 
solutions,  and  would  doubtless  be  in  a  state  of  constant 
transition  in  the  soil. 

This  conception  of  the  soil  surface  would  account  for 
changes  in  the  concentration  of  the  soil  solution  due  to 
the  application  of  soluble  fertilizers,  and  would  also 
explain  the  continued  effect  of  such  fertilizers  on  the 
theory  that  they  are  absorbed  by  the  colloidal  complexes 
and  redissolved  as  the  soil  solution  tends  to  become  more 
dilute. 

A  somewhat  different  view  has  been  taken  by  Whitney 
and  Cameron,  who  hold  that  the  composition  and  con- 
centration of  the  solution  in  all  soils  is  practically  the 
same.  Their  conception,  according  to  a  recent  paper  by 
Cameron,2  appears  to  differ  from  that  of  Van  Bemmelen 
in  assuming  that  the  soil  water  is  in  contact  with  the  soil 
particles  for  such  a  short  time  that  the  quantity  of  matter 
that  goes  into  solution  is  too  slight  to  bear  any  relation 
to  the  total  quantity  of  soluble  matter  in  the  soil.  The 
soil  solution  does  not  come  into  equilibrium  with  the 
soil  mass,  nor  even  approximate  such  a  condition.     The 

1  Van  Bemmelen,  J.  M.  Die  Zusammensetzung  der  Acker- 
erde.     Landw.  Vers.  Stat.,  Band  37,  Seite  347-373.     1890. 

2  Cameron,  F.  K.  Concentration  of  the  Soil  Solution. 
Original  Communications,  Eighth  International  Congress  of 
Applied  Chem.,  Vol.  15,  pp.  43-48.     1912. 


AVAILABILITY  OF  PLANT  NUTRIENTS         347 

solution  being  similar  in  all  soils,  it  follows  that  the  rela- 
tive productiveness  of  different  soils  bears  no  relation  to 
the  supply  of  soluble  nutrients,  but  must  be  due  to  other 
factors.  Hence  soluble  fertilizers  increase  plant  growth, 
not  by  supplying  a  greater  quantity  of  plant  nutrients, 
but  through  other  effects  on  the  soil  —  as,  for  instance, 
their  favorable  influence  on  tilth,  or  through  the  de- 
struction of  toxic  matter. 

250.  Discussion  of  the  theories  regarding  soil  solu- 
tions. —  The  difficulty  in  securing  a  true  sample  of  the 
soil  solution  as  it  exists  in  situ  complicates  any  attempt 
to  ascertain  how  these  theories  comport  with  the  actual 
condition  of  the  soil  solution.  A  number  of  attempts 
have  been  made  to  throw  light  on  this  subject,  but  none 
of  the  data  obtained  is  of  a  nature  to  definitely  prove  the 
correctness  of  either  theory.  The  evidence,  so  far  as  it 
goes,  indicates  that  the  water  extract  of  soils  differs  in 
concentration  in  different  soils,  and  is  increased,  under 
some  conditions,  by  large  and  continued  applications 
of  soluble  fertilizers.  There  can  be  no  doubt,  more- 
over, that  plant  growth  in  properly  balanced  nutrient 
solutions  increases  with  the  concentration  of  the  solu- 
tion up  to  several  thousand  parts  to  the  million,  as  has 
been  demonstrated  by  many  experiments. 

One  rather  convincing  experiment  may  be  quoted. 
Hall,  Brenchley,  and  Underwood !  analyzed  the  water 
extract  from  certain  plats  on  the  Rothamsted  Experi- 
ment Station  farm,  the  fertilizer  treatment  and  the 
yields  of  which  had  been  recorded  for  a  long  term  of  years. 

iHall,  A.  D.,  Brenchley,  W.  E.,  and  Underwood,  T.  M. 
The  Soil  Solution  and  the. Mineral  Constituents  of  the  Soil. 
Philosoph.  Trans.  Royal  Soc.  London,  Series  B,  Vol.  204,  pp. 
179-200.     1913. 


348       SOILS:    PROPERTIES  AND   MANAGEMENT 

Complete  analyses  of  the  soil  from  the  several  plats  were 
also  made :  — 

Yields  of  Crops,  and  Composition  of  Soil  and  Water 
Extract  of  Soil,  on  Rothamsted  Experiment  Station 
Farm 


Unmanured  .  .  . 
N  +  P206  .  .  • 
N  +  K20  .  .  . 
Complete  fertilizer 
Farm  manure     .     . 


Yield 

to  THE 

Acre 

(Pounds) 


1,276 
3,972 

2,985 
5,087 
6,184 


Complete  Analysis 


PjOs 
(percentage) 


0.099 
0.173 
0.102 
0.182 
0.176 


K20 

(percentage) 


0.183 
0.248 
0.257 
0.326 
0.167 


Water  Extract 


PjOs 
(p.  p.  m.) 


0.525 
3.900 
0.808 
4.025 
4.463 


KjO 
(p.  p.  m.) 


3.40 

3.88 

30.33 

24.03 

26.45 


A  similarly  treated  set  of  plats,  which  had  been  planted 
to  another  crop  and  analyzed  as  were  these,  gave  similar 
results.  It  is  a  very  striking  example  of  the  effect  of 
long-continued  treatment  of  the  soil  with  a  certain  fertilizer 
on  the  composition  of  the  water  extract.  The  subject, 
however,  must  be  investigated  further,  as  it  is  of  funda- 
mental importance  to  a  knowledge  of  the  properties  of 
soils. 


CHAPTER  XVI 
THE  ABSORPTIVE   PROPERTIES  OF  SOILS 

If  the  brown  water  extract  from  manure  is  filtered 
through  a  clay  soil  not  containing  soluble  alkalies,  the 
filtrate  will  be  nearly  colorless.  Many  solutions  of  dye- 
stuffs  are  affected  in  the  same  way.  Solutions  of  alkali 
or  alkaline  earth  salts  are  more  or  less  modified  by  this 
operation,  the  bases  being  retained  by  the  soil  to  a  greater 
extent  than  are  the  acids.  Thus,  when  a  solution  of 
the  nitrate,  sulfate,  or  chloride  of  any  one  of  these  bases 
is  filtered  through  the  soil,  a  part  of  the  base  is  absorbed 
by  the  soil,  while  most  of  the  acid  comes  through  in  the 
filtrate.  If  these  bases  are  in  the  form  of  phosphates 
or  silicates,  not  only  the  base  is  absorbed,  but  the  acid 
as  well. 

251.  Substitution  of  bases.  —  Associated  with  the 
absorption  of  the  base  from  solution,  there  is  liberation 
of  some  other  base  from  the  soil,  which  combines  with 
the  acid  in  the  solution  and  appears  in  the  filtrate  as  a 
salt  of  that  acid. 

When  absorption  takes  place  from  solution,  the  base 
is  never  entirely  removed,  no  matter  how  dilute  the  solu- 
tion may  be.  A  dilute  solution  of  potassium  chloride 
filtered  through  a  soil  will  produce  a  filtrate  containing 
some  calcium,  magnesium,  or  sodium  chloride,  or  all 
these  salts,  and  some  potassium  chloride.  The  more 
dilute  the  solution,  the  larger  will  be  the  proportion  re- 

349 


350       SOILS:    PROPERTIES  AND  MANAGEMENT 


tained,  but  the  less  the  total  quantity  absorbed.  Peters  1 
treated  100  grams  of  soil  with  250  cubic  centimeters  of  a 
solution  of  potassium  salts,  and  found  that  the  potassium 
of  different  salts  was  retained  in  different  proportions, 
and  that  the  stronger  solutions  lost  relatively  less  than 
the  weaker,  while  more  potassium  was  removed  from  the 
stronger  solutions. 


Strength  op  Solution 

-&  Normal 

2]0  Normal 

Grams  KjO  absorbed 

Grams  K2O  absorbed 

KCl 

K2S04         

K2C04        

0.3124 
0.3362 
0.5747 

0.1990 
0.2098 
0.3134 

The  same  bases  are  not  always  absorbed  in  the  same 
proportion  by  different  soils ;  one  soil  may  have  a  greater 
absorptive  power  for  potassium,  while  another  may  re- 
tain relatively  more  ammonia.  They  seem  to  be  inter- 
changeable, as  any  absorbed  base  may  be  released  by 
another  in  solution.  The  absorptive  power  of  a  soil  for 
certain  bases  is  reflected  in  the  composition  of  the  drainage 
water  from  the  soil.  The  composition  of  the  drainage 
water  varies  with  different  soils,  and  a  soluble  fertilizer 
applied  to  one  soil  will  have  a  different  effect  on  the  com- 
position of  the  drainage  water  than  if  applied  to  a  different 
soil.  This  is  well  illustrated  from  lysimeter  experiments 
by    Gerlach2    at    Bromberg.     Several    soils    were    used, 

Meters,  E.  Ueber  die  Absorption  von  Kali  durch  Acker- 
erde.     Landw.  Vers.  Stat.,  Band  2,  Seite  113-151.     1860. 

2  Gerlach,  Dr.  tlber  die  durch  sickerwasser  dem  Boden 
Entzogenen  Menge  Wasser  und  Nahrstoffe.  111.  Landw.  Zei- 
tung,  30  Jahrgange,  Heft  95,  Seite  871-881.     1910. 


THE  ABSORPTIVE  PROPERTIES   OF  SOILS       351 

one  of  each  being  fertilized  and  one  unfertilized.  The 
lysimeters  were  1.2  meters  deep  and  contained  4  cubic 
meters  of  soil.  The  drainage  water  was  caught  and 
analyzed  for  four  years.  The  first  year  there  was  no 
crop,  the  second  year  potatoes  were  grown,  the  third 
oats,  and  the  fourth  rye.  The  following  results  were 
shown :  — 


Average   Composition   of   Drainage   Water  in  Parts 

Million 

PER 

Soil 

Treatment 

Total 
Nitrogen 

Nitric 
Nitrogen 

Organic 
Nitrogen 

K20 

CaO 

Moor  soil   . 

Loamy  sand 
low  in  humus 

Sandy  loam 
high  in  humus 

J  Fertilized 
( Untreated 

j  Fertilized 
[  Untreated 

j  Fertilized 
\  Untreated 

32.7 
65.0 

25.5 
20.9 

67.8 
69.5 

30.0 
60.3 

25.1 
20.4 

64.6 
66.1 

2.7 
4.7 

0.4 
0.5 

3.1 
3.4 

32.2 
26.2 

25.1 

8.5 

70.2 
47.4 

405 
507 

92 
90 

399 
414 

Absorption  will  not  proceed  to  an  unlimited  extent. 
A  soil  will  cease  to  absorb  any  particular  substance  after 
a  certain  quantity  has  been  taken  up.  This  quantity 
will  vary  with  every  soil.  Clay  and  loam  soils  have 
greater  absorptive  power  than  sandy  soils.  This  differ- 
ence, both  as  to  amount  and  as  to  rate  of  absorption,  is 
well  shown  by  the  following  curves  adapted  from  Schreiner 
and  Failyer.1 

1  Schreiner,  O.,  and  Failyer,  G.  H.  The  Absorption  of 
Phosphates  and  Potassium  by  Soils.  U.  S.  D.  A.,  Bur.  Soils, 
Bui.  32.  1906.  See  also  Cameron,  F.  K.,  and  Bell,  J.  M. 
The  Mineral  Constituents  of  the  Soil  Solution.  U.  S.  D.  A., 
Bur.  Soils,  Bui.  30,  pp.  42-66.  1905.  Patten,  H.  E.,  and  Wag- 
gaman,  W.  H.  Absorption  by  Soils.  U.  S.  D.  A.,  Bur.  Soils, 
Bui.  52.     1908. 


352       SOILS:    PROPERTIES  AND  MANAGEMENT 


500O/>ytnm. 

4000 

y/CLAY 

zooo 

CLAY 

LOAM 

Z0O0 

~      3A//DY 

SO/L 

/OOO     f/S' 

Y*> 

/ 

0                      /C 

00 

26 

OO 

•30 

OO 

40t 

70 

so 00  cc. 

Fig.  55. — Curves  showing  the  absorption  of  P04  in  parts  to  a  million 
by  various  soils  from  a  solution  of  monocalcium  phosphate,  contain- 
ing 200  parts  to  a  million  of  P04.  The  volume  of  the  percolate  is 
used  as  the  abscissas. 


Note.  —  The  law  which  appears  to  govern  absorption  of  phos- 
phates and  potash  by  the  soil  may  be  expressed  mathematically 
as  follows:  — 


dy 
dv 


K{A  -  y) 


in  which  K  is  a  constant,  A  the  maximum  quantity  possible 
for  the  soil  to  absorb,  and  y  the  quantity  actually  fixed  when  v, 
volume  of  the  solution,  has  percolated  through. 

A  short  discussion  of  the  mathematics  of  this  law  may  be 
found  in  the  following  publication :  Schreiner,  O.,  and  Failyer, 
?t  ?•  ^The  AbsorPtion  of  Phosphates  and  Potassium  by  Soils! 
U.  S.  D.  A.,  Bur.  Soils,  Bui.  32,  pp.  23-24,  37-39.     1906 


THE  ABSORPTIVE  PROPERTIES   OF  SOILS      353 


800 

CLAY 

600 

440 

CLAY  LOAM 

zoo    / 

SANDYSO/L 

/& 

s 

Fig.  56. — Curves  showing  the  absorption  of  K  in  parts  to  a  million  by- 
various  soils  from  a  solution  containing  200  parts  to  a  million  of  K. 
The  volume  of  the  percolate  is  used  as  the  abscissas. 

252.  Time  required  for  absorption.  —  The  amount  of 
absorption  depends  on  the  time  of  contact  between  the 
soil  and  the  solution.  While  a  large  part  of  the  dissolved 
base  is  taken  up  in  a  short  time  after  being  placed  in 
contact  with  the  soil,  the  maximum  absorption  is  effected 
only  after  a  considerable  period.  Ammonia,  according 
to  Way,  reaches  its  maximum  absorption  in  half  an  hour ; 
while  Henneberg  and  Stohmann  1  found  that  phosphorus 
required  twenty-four  hours  to  reach  the  same  degree  of 
absorption. 


1  Henneberg,  W.,  and  Stohmann,  F.     Ueber  das  Verhalten  der 
Ackererde  gegen  Losungen  von  Ammoniak  und  Ammoniaksalzen. 
Jour,  f .  Landw.,  Neue  Folge,  Band  3  (Der  ganze  Reihe  siebenter 
Jahrgang),  Seite  25-47.     1859. 
2a 


354       SOILS:    PROPERTIES  AND  MANAGEMENT 

This,  however,  has  no  significance  so  far  as  danger 
from  loss  of  a  soluble  fertilizer  constituent  is  concerned, 
since  water,  even  after  a  heavy  rain,  would  not  pass  so 
quickly  through  the  soil  that  absorption  would  not  take 
place,  except  possibly  in  the  case  of  soil  of  a  very  coarse 
texture.  The  depth  through  which  the  substance  is 
distributed  in  the  soil  may,  however,  be  influenced  by 
the  time  required  for  its  absorption.  Ordinarily  ferti- 
lizers do  not  penetrate  very  far  into  the  soil.  Demolon 
and  Bronet l  have  investigated  the  rate  and  distance  of 
penetration  of  certain  soluble  salts  in  soils,  and  find  that 
a  total  rainfall  of  ten  inches  is  not  sufficient  to  carry  down 
sodium  nitrate  in  a  sandy  soil  to  a  depth  of  eight  inches. 

253.  Insolubility  of  certain  absorbed  substances.  — 
Although  bases  once  absorbed  may  be  easily  displaced 
by  other  bases,  it  is  difficult  to  dissolve  them  from  the 
soil  with  pure  water.  Peters2  treated  100  grams  of  soil 
with  250  cubic  centimeters  of  water  containing  potassium 
chloride,  of  which  0.2114  gram  of  K20  was  absorbed. 
The  soil  was  then  leached  with  distilled  water,  using  125 
cubic  centimeters  of  water  daily  for  ten  days.  At  the 
end  of  that  time  0.0875  gram  of  K20  had  been  removed, 
or  at  the  rate  of  28,100  parts  of  water  to  one  part  of 
K20  dissolved  from  the  soil.  Henneberg  and  Stoh- 
mann3   found   that   it   required    10,000   parts   of   water 


1  Demolon,  A.,  and  Bronet,  G.  Sur  la  Penetration  des 
Engrais  Solubles  dans  les  Sols.  Ann.  Agron.,  Tome  28,  pp. 
401-418.     1911. 

2  Peters,  E.  Ueber  die  Absorption  von  Kali  durch  Acker- 
erde.     Landw.  Vers.  Stat.,  Band  2,  Seite  113-151.     1860. 

3  Henneberg,  W.,  and  Stohmann,  F.  Ueber  das  Verhalten 
der  Ackererde  gegen  Losungen  von  Ammoniak  und  Ammoniak- 
salzen.  Jour.  f.  Landw.,  Neue  Folge,  Band  3  (Der  ganze  Reihe 
siebenter  Jahrgang),  Seite  25-47.     1859. 


THE  ABSORPTIVE  PROPERTIES   OF  SOILS       355 

to  dissolve  one    part    of   absorbed    ammonia    from   the 
soil. 

254.  Influence  of  size  of  particles.  —  The  surface  area 
of  the  soil  particles  determines  to  some  extent  the  amount 
of  substance  absorbed.  For  this  and  other  reasons,  a 
fine-grained  soil  absorbs  a  greater  quantity  of  material 
than  a  coarse-grained  soil.  In  fact,  it  was  early  shown  by 
Way  *  that  the  phenomenon  of  absorption  is  largely  a 
function  of  the  silt,  clay,  and  humus  of  the  soil. 

255.  Causes  of  absorption.  —  A  number  of  causes 
have  been  assigned  for  the  absorption  of  substances  by 
soils,  and  there  can  be  no  doubt  that  the  phenomenon  is 
not  due  to  any  one  process.  Several  distinct  causes  are 
now  very  generally  recognized,  while  others  that  have 
been  suggested  may  have  a  part  in  the  result. 

256.  Zeojites.  —  As  the  result  of  his  extended  researches 
on  absorption  of  soils,  Way  concluded  that  the  property 
of  absorption,  or  fixation  of  bases,  rests  largely  with  the 
hydrated  silicates  of  aluminium,  containing  calcium  or 
magnesium  and  one  of  the  alkali  metals,  these  compounds 
being  known  as  zeolites.  He  prepared  artificially  a 
hydrated  silicate  of  aluminium  and  sodium,  and  found 
that  by  treating  this  with  a  solution  of  a  calcium  salt 
he  could  replace  most  of  the  sodium,  obtaining  thereby 
a  silicate  of  aluminium,  calcium,  and  part  of  the  sodium 
that  was  originally  contained  in  the  silicate.  The  re- 
mainder of  the  sodium  could  be  replaced  by  potassium 
from  solution  and,  likewise,  by  magnesium  and  ammonium. 

1  Way,  J.  T.  On  the  Power  of  Soils  to  Absorb  Manure. 
Jour.  Royal  Agr.  Soc.  England,  Vol.  11,  pp.  313-379.  1850. 
Also,  On  the  Power  of  Soils  to  Absorb  Manure.  Jour.  Royal  Agr. 
Soc.  England,  Vol.  13,  pp.  123-143.  1852.  Also,  On  the 
Influence  of  Lime  on  the  "  Absorptive  Properties  "  of  Soils.  Jour- 
Royal  Agr.  Soc.  England,  Vol.  15,  pp.  491-515.     1854. 


356       SOILS:    PROPERTIES  AND  MANAGEMEST 


Way  found  further  that  exposure  to  a  strong  heat  de- 
stroyed the  absorptive  properties  of  these  substances,  as 
did  also  treatment  with  strong  hydrochloric  acid.  In  all 
these  respects  the  absorptive  properties  of  the  soil  and 
of  the  zeolites  coincide. 

257.  Chabazite.  —  Eichhorn *  experimented  with  the 
natural  zeolite  chabazite,  and  found  that  he  could  produce 
substitutions  by  means  of  the  proper  salt  solutions.  In 
column  I  of  the  table  below  is  given  the  composition  of 
chabazite  used  for  the  experiment ;  in  column  II  is  stated 
its  composition  after  treatment  with  a  solution  of  sodium 
chloride;  and  in  column  III  the  composition  after  the 
zeolite  is  further  treated  with  a  solution  of  ammonium 
chloride :  — 

Composition  op  Chabazite  originally  and  after  Treat- 
ment with  Sodium  Chloride  and  afterwards  with 
Ammonium  Chloride 


Column 

I 

II 

in 

Si02         

47.4 

48.3 

51.3 

A1203 

20.7 

21.0 

22.2 

CaO 

10.4 

6.7 

4.2 

K20 

0.7 

0.6 

{   0.6 

Na20 

0.4 

5.4 

HoO 

20.2 

18.3 

14.9 

(NH4)20      .... 

0.0 

0.0 

6.9 

The  substitutions  were  evidently  made  at  the  expense 
of  calcium  in  the  compound,  both  when  treated  with 
sodium  and  when  treated  with  ammonium  salts  in  chemi- 
cally equivalent  quantities.     These  and   subsequent  ex- 

1  Eichhorn,  H.  Ueber  die  Einwirkung  Verdiinuter  Salz- 
losungen  auf  Ackererde.  Landw.  Centrlb.  f.  Deutschland, 
6  Jahrgang,  Band  2,  Seite  169-175.     1858. 


THE  ABSORPTIVE  PROPERTIES   OF  SOILS       357 

periments  by  numerous  investigators  have  been  rather 
widely  accepted  as  indicating  that  the  zeolites  are  at 
least  partly  responsible  for  the  absorptive  properties  of 
soils.  It  has  been  shown  further  that  the  absorptive 
power  of  a  soil  is  more  or  less  proportional  to  the  quantities 
of  acid-soluble  silicates  it  contains.  The  zeolites  being 
rather  easily  soluble  in  strong  mineral  acids,  it  is  held 
that  the  bases  so  combined  are  more  readily  available 
to  plants  than  in  most  combinations  found  in  the  soil, 
and  yet  are  not  easily  leached  out  of  it. 

258.  Presence  of  zeolites  questioned.  —  On  the  other 
hand,  zeolites  have  never  been  definitely  proved  to  be 
present  in  soils.  Merrill x  has  attempted  to  show  that 
they  cannot  be  of  wide  occurrence  in  soils,  but  neither 
their  absence  nor  their  presence  has  been  demonstrated. 
Since  the  time  when  Way  first  published  his  researches 
in  1850,  the  zeolite  constituents  of  the  soil  have  generally 
been  held  to  be  largely  responsible  for  its  absorptive 
power  for  bases. 

259.  Absorption  of  phosphoric  acid.  —  It  has  already 
been  said  that  although  hydrochloric,  sulfuric,  and  nitric 
acids  are  not  absorbed  by  soils,  except  in  small  quantities, 
phosphoric  acid  is  absorbed  and  retained  in  an  almost 
insoluble  condition  so  far  as  extraction  with  water  is 
concerned.  That  this  absorption  cannot  be  due  to 
zeolites  is  generally  conceded,  and  has  recently  been 
demonstrated,  for  permutite  at  least,  by  Rostworowski 
and  Wiegner,2  who  in  a  carefully'  conducted  experiment 


1  Merrill,  G.  P.  Rocks,  Rock  Weathering,  and  Soils,  pp. 
362-367.     New    York.     1906. 

2  Rostworowski,  S.,  and  Wiegner,  G.  Die  Absorption  de,r 
Phosphorsaure  durch  "Zeolithe"  Permutite.  Jour.  f.  Landw., 
Band  60,  Seite  223-235.     1912. 


358       SOILS:    PROPERTIES  AND  MANAGEMENT 

with  this  zeolite  —  which  is  an  amorphous  gel  containing 
potassium,  calcium,  aluminium  and  silicic  acid  —  found 
that  there  was  no  absorption  of  phosphoric  acid  from  a 
neutralized  solution  of  monocalcium  phosphate  or  from 
a  solution  of  dicalcium  phosphate  at  various  degrees  of 
concentration. 

260.  Formation  of  insoluble  phosphates.  —  The  reten- 
tion of  soluble  phosphoric  acid  in  soils  may  be  easily  ac- 
counted for  by  the  fact  that  there  are  present  in  all  soils 
hydrated  ferric  oxide  and  hydrated  silicates  of  alumina, 
and  frequently  calcium  carbonate,  with  which  substances 
phosphoric  acid  in  solution  would  naturally  form  com- 
pounds insoluble  in  water.  Iron  and  aluminium  phos- 
phates are  practically  insoluble  in  water  containing  carbon 
dioxide  or  weak  organic  acids  such  as  might  be  present 
in  soil  water.  Calcium  carbonate  forms  with  a  soluble 
phosphate  fertilizer  some  dicalcium  phosphate,  the 
solubility  of  which  in  soil  water  is  much  greater  than 
the  iron  and  aluminium  phosphates.  This  is  one  of  the 
advantages  of  keeping  a  soil  well  supplied  with  lime  if 
a  superphosphate  fertilizer  is  to  be  used.  Even  the 
tricalcic  phosphate,  although  less  soluble  than  the  dicalcic, 
is  more  readily  soluble  than  the  iron  and  aluminium 
phosphates.  As  lime  has  a  tendency  to  move  downward 
in  soil,  and  as  phosphoric  acid  is  retained  in  the  plowed 
depth  when  added  as  a  fertilizer,  it  is  important  that  the 
applications  of  lime  be  sufficiently  frequent  to  keep  this 
part  of  the  soil  in  a  condition  to  form  the  lime  phosphates. 

Cameron  l  has  suggested  that  the  absorption  of  phos- 
phoric acid  is  probably  due  to  the  formation  with  lime 
or  ferric  oxide  of  a  solid  solution,  which  might  account 

1  Cameron,  F.  K.  The  Soil  Solution,  p.  59.  Easton,  Penn- 
sylvania.    1911. 


THE  ABSORPTIVE  PROPERTIES   OF  SOILS       359 

for  the  availability  of  phosphorus  in  soils  to  which  a 
superphosphate  fertilizer  had  been  applied  many  months 
previously.  It  might  explain  also  the  availability  of 
a  superphosphate  on  soils  devoid  of  calcium  carbonate. 
Although  such  availability  is  always  less  than  where 
this  carbonate  exists,  it  is  greater  than  would  be  ac- 
counted for  by  the  solubility  of  ordinary  iron  phosphate. 

261.  Adsorption.  —  There  is  a  physical  fixation,  termed 
adsorption,  due  to  the  concentration  of  the  soil  solution 
in  immediate  contact  with  the  surface  of  the  particles. 
The  phenomenon  is  familiarly  exemplified  in  the  clarify- 
ing effect  of  the  charcoal  filter.  This  process  results  in 
the  retention,  in  fine-grained  soils,  of  considerable  soluble 
material  that  would  otherwise  be  washed  out.  In  the 
case  of  nitrates,  which  are  not  retained  by  the  zeolites, 
adsorption  is  an  important  factor  (par.  244).  If  a 
solution  of  a  known  quantity  of  nitrate  of  soda  is  added 
to  a  clay  soil,  and  an  attempt  is  then  made  to  extract 
the  nitrate  from  the  soil  with  distilled  water,  it  will  be 
found  impossible  to  recover  a  very  appreciable  propor- 
tion of  the  amount  added.  While  adsorption  probably 
does  not  account  for  all  the  nitrates  retained,  there  can 
be  no  doubt  that  it  plays  an  important  part.  Nutritive 
salts  held  in  this  way  are  readily  available  to  the  plant, 
whose  root  hairs  come  in  contact  with  the  soil  particles. 
It  is  not  impossible  that  other  fertilizer  constituents  are 
held  by  the  soil  in  this  manner. 

262.  Absorption  by  colloids.  —  According  to  Van  Bem- 
melen,1  who  has  made  a  very  exhaustive  study  of  this 

1  Van  Bemmelen,  J.  M.  Die  Absorptionsverbindungen  und 
das  Absorptions  vermogen  der  Ackererde.  Landw.  Vers.  Stat., 
Band  35,  Seite  69-136.  1888.  Also,  Die  Absorption,  Seite 
548.     Dresden,    1910. 


360       SOILS:    PROPERTIES  AND  MANAGEMENT 

subject,  absorption  by  soils  is,  without  doubt,  largely 
due  to  the  presence  of  colloidal  matter  which  exerts  an 
absorbent  action  for  water,  gases,  solutes,  and  solids  in 
suspension.  The  colloidal  matters  in  soils  that  contrib- 
ute to  their  absorptive  properties  are  the  following :  — 

(1)  remains  of  plant  and  animal  tissues ; 

(2)  humous  substances ; 

(3)  colloidal  iron  oxide ; 

(4)  colloidal  silicic  acid  ; 

(5)  amorphous  colloidal  silicates  that  have  been  formed 

through  weathering. 

Van  Bemmelen  also  credits  crystalline  silicates  with 
absorbent  properties,  although  he  does  not  consider  that 
their  action  is  very  important.  Absorption  is  brought 
about  also  by  true  chemical  combination  of  soil  com- 
pounds with  substances  in  solution,  by  which  certain  of 
the  cations  or  anions  in  solution  are  chemically  combined 
and  remain  in  the  soil  in  a  very  difficultly  soluble  condition. 
263.  Absorptive  properties  of  colloidal  matter.  — 
Among  the  products  of  rock  weathering  there  have  been 
found  in  soils  amorphous  substances  that  are  of  the  nature 
of  colloidal  gels.  These,  with  the  other  colloidal  matter, 
are  contained  in  the  very  small  particles  that  remain  for 
a  long  time  in  suspension  when  soil  is  stirred  up  in  water. 
These  colloids  are  coagulated  by  many  acids,  and  by 
some  bases  and  salts.  This  is  especially  true  of  the 
material  that  is  dialyzable.  Some  of  these  again  go  into 
solution  on  being  treated  with  water,  while  others  remain 
insoluble  until  they  undergo  molecular  change.  Many 
colloids  form  hydrogels  with  soil  water.  These  hydrogels 
are  not  ordinary  chemical  compounds.  Gels  dry  very 
slowly.     They  adsorb  water  in  varying  quantities,   not 


THE  ABSORPTIVE  PROPERTIES   OF  SOILS      361 

in  certain  definite  proportion  as  do  crystalloids  in  the 
process  of  crystallization.  The  more  water  adsorbed  by 
colloids,  the  less  firmly  is  it  held  in  combination.  There- 
fore it  is  easier  to  evaporate  the  water  when  a  large  quan- 
tity has  been  taken  up,  and  as  the  amount  decreases  it 
becomes  more  difficult  to  drive  it  off. 

Another  property  of  colloidal  matter  is  that  when  it 
is  separated  from  solution  it  carries  down  with  it  other 
substances  in  the  solution  from  which  it  is  precipitated. 
If,  on  the  other  hand,  the  colloidal  matter  has  been  pre- 
cipitated in  a  pure  state,  it  absorbs  substances  from 
solutions  with  which  it  remains  in  contact  for  some  time. 
The  substances  taken  up  in  this  way  are  not  chemically 
combined,  but  substances  that  unite  chemically  may  be 
absorbed. 

The  combinations  produced  by  absorption  are  weak 
and  it  is  possible  to  leach  out  the  combined  substances, 
which  are  generally  held  in  the  water  of  the  gels.  The 
following  example  of  one  kind  of  absorption  is  given 
by  Van  Bemmelen : x  ten  grams  of  a  hydrogel  having  the 
composition  Si02  .  4.2  H20,  shaken  with  100  cubic  cen- 
timeter solution  of  20  molecular  equivalent  KC1,  will 
absorb  0.8  to  1.1  molecular  equivalent  of  the  dissolved 
substance.  The  absorption  in  this  case  was  as  if  the 
solution  had  been  diluted  with  4.2  to  5.8  cubic  centimeters 
of  water.  As  the  amount  of  gel  water  in  10  grams  of 
hydrogel  of  Si02  is  about  5  cubic  centimeters,  the  as- 
sumption may  be  made  that  the  dissolved  substance  is 
taken  up  in  equal  concentration  by  the  gel  water.  Ten 
grams  of  hydrogel  of  Si02  shaken  with  100  cubic  centi- 

1  Van  Bemmelen,  J.  M.  Die  Absorptionsverbindungen  und 
das  Absorptionsvermogen  der  Aekererde.  Landw.  Vers.  Stat., 
Band  35.  Seite  75.     1888. 


362       SOILS:    PROPERTIES  AND  MANAGEMENT 

meter  solution  of  50  molecular  equivalent  KC1  —  that 
is,  2\  times  the  concentration  of  the  former  solution  - 
absorbs  2\  times  as  much,  or  2.1  to  2.5  molecular  equiva- 
lent. This  applies  also  to  concentrations  five  times 
stronger  than  the  first  mentioned  above,  but  beyond  that 
the  relation  is  not  so  simple.  It  serves,  however,  to 
illustrate  the  manner  in  which  the  absorption  takes  place 
from  dilute  solutions. 

264.  Selective  absorption.  —  A  selective  absorption  is 
very  common,  especially  from  solutions  of  salts  having 
weak  acids,  a  greater  fixation  of  the  bases  taking  place 
than  of  the  acids.  Dissociation  of  the  salts  takes  place 
in  the  solution,  the  bases  being  absorbed,  in  consequence 
of  which  further  dissociation  occurs;  and  this  proceeds 
until  an  equilibrium  is  established  between  the  absorbing 
and  combining  power  of  the  colloidal  material  and  the 
reverse  action  of  the  water  and  resulting  acids.  In  this 
way  the  absorptive  power  decreases  as  the  amount  ab- 
sorbed becomes  greater. 

The  colloidal  silicates  possess  the  property  of  absorbing 
a  certain  base  when  presented  to  it  in  solution,  and  con- 
tributing in  return  a  chemically  equivalent  quantity  of 
some  other  base.  Potassium  is  most  firmly  combined 
in  the  soil  and  most  strongly  withdrawn  from  solution, 
with  an  exchange  of  a  chemically  equivalent  quantity  of 
calcium,  sodium,  and  magnesium,  which  passes  into  the 
solution.  If  a  soil  is  treated  with  a  solution  of  potassium, 
magnesium,  sodium,  or  calcium  salts  of  equal  concentra- 
tion, the  concentration  of  the  solution  in  the  end  is  less 
for  the  potassium  than  for  the  magnesium,  and  less  for 
the  magnesium  than  for  the  sodium  and  the  calcium, 
because  the  potassium  is  most  strongly  bound  in  the 
colloidal  material,  while  the  calcium  and  sodium  are  least 


THE  ABSORPTIVE  PROPERTIES   OF  SOILS       363 

so.  In  other  words,  the  action  of  a  calcium  salt  in  solu- 
tion on  the  absorbed  potassium  combination  is  less  than 
the  action  of  a  dissolved  potassium  salt  on  the  absorbed 
calcium  combination.  Thus  it  comes  about  that  under 
similar  conditions  of  temperature,  volume,  and  concen- 
tration of  the  solution,  the  quantity  of  calcium  or  of 
sodium  or  of  magnesium  that  goes  into  solution  when 
colloidal  silicates  are  treated  with  a  solution  of  a  potassium 
salt  is  greater  than  the  quantity  of  potassium  that  would 
go  into  solution  if  the  same  silicates  were  treated  with  a 
solution  containing  the  salts  of  any  of  these  other  bases. 

265.  Absorptive  power  of  colloidal  silicates.  —  The 
quantity  of  a  substance  that  a  certain  weight  of  a  colloidal 
silicate  can  absorb  increases  with  the  strength  of  the 
solution  of  the  substance  presented  for  absorption,  be- 
cause the  final  solution  can  remain  stronger  and  conse- 
quently its  solvent  power  for  that  particular  substance 
is  less.  The  point  of  equilibrium  between  the  fixing 
power  of  the  colloid  and  the  solvent  action  of  the  solvent 
therefore  varies  with  the  strength  of  the  solution. 

The  nature  of  the  acid  with  which  a  base  is  combined 
likewise  has  an  influence  on  the  quantity  of  the  base 
absorbed.  A  base  combined  with  a  weak  acid  is  ab- 
sorbed in  greater  amount  than  the  same  base  combined 
with  a  strong  acid.  This  is  presumably  because  the 
stronger  acid  remaining  in  solution  has  a  greater  solvent 
action. 

266.  Absorption  by  colloids  versus  absorption  by 
zeolites.  —  The  early  conception  of  the  phenomenon  of 
fixation  in  soils  was  naturally  a  chemical  one  and  was 
founded  on  the  chemical  knowledge  of  that  day.  The 
fact  that  the  substitution  of  bases  in  the  solutions  passed 
through  the  soil  was  in  chemically  equivalent  quantities, 


364       SOILS:    PROPERTIES  AND  MANAGEMENT 

placed  it  in  line  with  what  was  known  regarding  chemical 
reactions.  Zeolites  were  found  to  possess  absorbent 
properties  of  a  similar  nature  toward  salts  in  solution, 
characteristic  of  which  is  the  substitution  of  bases  and 
the  appearance  in  solution  of  the  released  base  in  com- 
bination with  the  acid  of  the  original  salt.  It  was  a 
natural  conclusion  that  true  mineral  zeolites  exist  in 
soil  and  that  the  absorptive  properties  of  soil  are  due  to 
their  action. 

Many  years  later,  when  the  principles  of  physical 
chemistry  had  been  applied  to  the  study  of  colloids,  it 
was  shown  that  absorptive  properties  are  possessed  by 
certain  colloids  similar  to  those  characteristic  of  soils. 
Zeolites  have  never  actually  been  isolated  from  any  soil. 
This  fact  has  always  occasioned  some  doubt  as  to  the 
hypothesis  to  which  their  properties  have  given  rise. 
Colloids,  on  the  other  hand,  are  well  known  to  occur  in  soils, 
but  the  exact  nature  of  soil  colloidal  matter  is  not  well 
understood  ;  consequently  there  is  considerable  indefinite- 
ness  about  the  extent  of  their  absorptive  function,  and 
even  Van  Bemmelen  grants  the  crystalloids  a  part  in  this 
phenomenon. 

The  zeolite  hypothesis  furnished  an  explanation  for 
the  form  in  which  the  available  plant-food  materials  of 
the  soil  are  held.  On  it  is  largely  based  the  idea  that 
the  solution  of  a  soil  in  strong  hydrochloric  acid  repre- 
sents  the  nutrients  that  are  available  to  plants.  The 
silicates  that  go  into  solution  are  held  to  be  the  zeolitic 
silicic  acid  and  the  bases  with  which  it  is  united.  The 
fact  that  such  treatment  largely  destroys  the  absorptive 
properties  of  a  soil  is  taken  as  a  proof  of  this.  It  would, 
however,  answer  equally  well  as  an  argument  in  favor 
of  colloidal  absorption,  as  the  colloidal  condition  of  the 


THE  ABSORPTIVE  PROPERTIES  OF  SOILS      365 

silicates  would  be  destroyed  by  the  same  treatment. 
On  the  whole,  the  evidence  appears  to  be  in  favor  of 
the  dominance  of  colloidal  absorption  rather  than 
crystalloidal  absorption  by  soils,  with  its  important 
function  in  conserving  soluble  fertilizers  and  retaining 
a  supply  of  plant  nutrients  in  a  more  or  less  readily 
available  condition. 

267.  Absorption  by  organic  matter.  —  The  partially 
decomposed  organic  matter  in  soils,  especially  that  part 
which  has  undergone  such  transformations  as  to  form 
humus  (par.  90),  has  an  absorptive  power.  Soils  rich 
in  humus,  without  doubt,  owe  much  of  their  fertility 
to  the  retention  by  that  constituent  of  a  large  supply  of 
readily  available  plant-food  material.  Many  prairie  soils 
that  have  been  reduced  in  productiveness  under  culti- 
vation respond  to  the  application  of  organic  matter  in  a 
remarkable  manner.  Humus  in  these  soils  seems  to  be  the 
chief  conserver  of  readily  available  plant-food  materials. 

Van  Bemmelen,1  who  has  studied  these  compounds, 
states  that  soils  hold  colloidal  humous  compounds  con- 
taining ammonia,  potassium,  sodium,  and  other  sub- 
stances, as  well  as  iron  oxide.  A  part  is  soluble,  or  forms 
soluble  compounds  with  alkalies,  but  the  principal  part 
is  insoluble.  Some  of  these  latter  compounds  are  of  a 
colloidal  nature  and  of  changing  composition.  The  soluble 
matter  is  easily  precipitated  by  a  salt  solution  and  carries 
down  with  it  bases  from  the  solution.  Absorption  of  bases 
also  takes  place  from  solution,  with  substitution  of  one 
base  for  another.  Potassium  is  more  strongly  held  in 
combination  than  is  calcium  or  magnesium.  Bases  are 
removed,  however,  only  from  salts  of  the  weaker  acids. 

1  Van  Bemmelen,  J.  M.  Die  Absorption,  Seite  135-141. 
Dresden,  1910. 


s 


366       SOILS:    PROPERTIES  AND  MANAGEMENT 

268.  Absorption  of  water  vapor  and  of  gases  by  soils.  — 
Hygroscopic  water  in  soils  has  already  been  discussed  (pars. 
133,  134,  135).  It  need  merely  be  remarked  here  that 
there  is  a  close  relation  between  the  absorptive  power  of  a 
soil  for  water  vapor  and  for  bases.  Soils  having  a  high 
content  of  humus  and  composed  of  very  fine  material  are 
likely  to  have  great  absorptive  properties  for  both  vapors 
and  solutes. 

In  a  similar  way  soils  absorb  gases.  The  deodorizing 
property  of  soil  is  well  known.  Decomposing  organic 
matter  is  rendered  inoffensive  by  covering  it  with  soil. 
Gases  produced  in  the  processes  of  decomposition  are 
largely  absorbed  by  the  soil.  The  fertility  of  the  soil 
may  be  increased  by  the  absorption  of  certain  gases. 

269.  Absorption  of  ammonia.  —  Ammonia,  which  exists 
in  minute  quantities  in  the  air,  is  absorbed  by  soils,  and 
also  when  given  off  by  decomposing  organic  matter  in  the 
soil.  As  all  nitrogeneous  organic  matter  may  eventually 
form  ammonia  when  decomposed,  the  ability  of  the  soil 
to  absorb  it  is  very  important.  Quartz  alone  will  absorb 
only  a  very  small  quantity  of  ammonia,  while  a  clay  soil 
will  hold  practically  all  that  is  likely  to  be  produced  by 
the  decomposition  of  the  organic  matter  incorporated  in  it. 

270.  Absorption  of  carbon  dioxide.  —  Carbon  dioxide 
is  absorbed  by  soils  to  a  very  considerable  extent,  and 
this  also  adds  to  the  productiveness  of  soils,  since  it  aids 
in  their  decomposition.  The  supply  of  carbon  dioxide 
comes  from  decomposing  organic  matter  and  from  plant 
roots.  As  will  be  explained  later,  the  soil  air  always 
contains  a  considerable  supply  of  this  gas,  and  its  con- 
densation and  absorption  is  constantly  going  on.  It 
forms  soluble  bicarbonates  with  the  alkalies  and  bases 
of  soils,  producing  a  readily  available  plant-food  material. 


THE  ABSORPTIVE  PROPERTIES   OF  SOILS      367 

271.  Absorption  of  nitrogen  and  oxygen.  —  Nitrogen 
is  absorbed  by  soils  to  a  greater  degree  than  is  oxygen. 
The  latter  probably  is  of  greater  importance  to  soil  fer- 
tility, as  its  absorption  is  accompanied  by  oxidation  of 
other  absorbed  gases.  Because  of  their  absorptive  prop- 
erties and  their  great  surface  area,  soils  have  strong 
oxidizing  power. 

The  absorption  of  gases  by  soils  is  largely  an  absorp- 
tion phenomenon,  the  gases  being  condensed  on  the 
surface  of  the  particles.  •  Von  Dobeneck x  has  shown 
that  the  absorption  is  greater,  the  finer  the  particles 
of  soil ;  but  this  increase  is  not  directly  proportional 
to  the  increase  in  surface,  large  particles  apparently 
having  a  greater  adsorptive  power  than  their  surface 
area  would  indicate. 

272.  Relation  of  temperature  to  gas  absorption.  — 
The  temperature  of  the  soil  influences  its  absorptive 
properties  for  vapors.  As  the  temperature  increases  the 
absorption  becomes  less.  Hilgard 2  does  not  find  this 
to  be  the  case  (par.  136).  He  exposed  soils  to  a  moisture- 
saturated  atmosphere  and  found  that  they  absorbed 
more  moisture  at  high  than  at  low  temperatures.  In 
his  conclusions,  however,  he  is  doubtless  in  error.  All 
the  work  previous  to  his  gave  a  directly  contrary  result, 
and  a  more  recent  investigation  by  Patten  and  Gallagher 3 
confirmed  the  work  of  the  earlier  investigators. 

^on  Dobeneck,  A.  F.  Untersuchungen  uber  das  Adsorp- 
tionsvermogen  und  die  Hygroskopizitat  der  Bodenkonstit- 
uenten.  Forsch.  a.  d.  Agri.-Physik.,  Band  15,  Seite  163-228. 
1892. 

2  Hilgard,  E.  W.     Soils,  pp.  196-198.     New  York,  1906. 

3  Patten,  H.  E.,  and  Gallagher,  F.  E.  Absorption  of  Gases 
and  Vapors  by  Soils.  U.  S.  D.  A.,  Bur.  Soils,  Bui.  51,  pp.  31- 
35.     1908. 


368     SOILS:  properties  and  management 

273.  Relation  of  absorptive  capacity  to  productiveness. 
—  The  absorptive  capacity  of  a  soil  is  not  so  much  a 
measure  of  its  immediate  as  of  its  permanent  productive- 
ness. It  is  well  known  that  a  very  sandy  soil  responds 
quickly  to  the  application  of  soluble  manures,  but  that 
the  effect  is  confined  mainly  to  one  season ;  while  a  clay 
soil,  although  not  so  quickly  responsive  to  fertilization, 
shows  the  effect  of  the  application  much  more  markedly 
the  second  or  the  third  year  than  does  the  sandy  soil. 
Adsorption,  which  is  largely  shown  in  sandy  soil,  holds 
the  nutritive  material  in  a  very  readily  available  con- 
dition, while  absorption  by  amorphous  compounds 
renders  these  substances  somewhat  less  readily  available. 
There  are  also  other  reasons  why  the  sandy  soil  is  more 
responsive.  King,1  in  working  with  eight  types  of  soil 
from  different  parts  of  the  United  States,  found  that 
those  soils  removing  the  most  potassium  from  solution 
gave  the  largest  yield  of  crops.  It  would  not  be  per- 
missible, however,  to  adopt  this  test  as  a  method  for 
determining  productiveness  in  soil. 

274.  Absorption  as  related  to  drainage.  —  The  drainage 
water  from  cultivated  fields  in  humid  regions,  and  to  a 
less  extent  in  semiarid  and  arid  regions,  except  where 
irrigation  is  practiced,  carries  off  very  considerable  quan- 
tities of  plant-food  material.  The  loss  of  this  material 
is  due  to  the  operation  of  the  various  natural  disintegrating 
agents  on  the  soil  mass,  and  to  the  application  of  fertilizing 
materials  in  a  soluble  form.  The  various  absorptive  prop- 
erties stand  between  the  natural  solubility  of  the  soil  and 
the  tendency  to  loss  in  drainage,  and  hold,  in  a  condition 

1  King,  F.  H.  Influence  of  Farm  Yard  Manure  upon  Yield 
and  upon  the  Water-Soluble  Salts  of  Soils,  p.  25.  Madison, 
Wisconsin.     1904. 


TEE  ABSORPTIVE  PROPERTIES   OF  SOILS      369 

in  which  they  may  readily  be  used  by  the  plant,  these 
materials  which  would  otherwise  be  lost. 

275.  Substances  usually  carried  in  drainage  water.  — 
However,  some  material  is  always  lost  in  drainage  water, 
of  which,  among  the  bases  of  the  soil,  those  most  likely  to 
be  found  are  calcium,  sodium,  magnesium,  and  potassium ; 
and  of  the  acids,  carbonic,  nitric,  sulfuric,  and  hydrochloric. 
Nitric  acid  and  lime  undergo  the  most  serious  losses. 
The  former  may  be  curtailed  to  a  great  extent  by  keeping 
crops  growing  on  the  soil  during  all  the  time  that  nitri- 
fication is  going  on,  and  if  the  crop  does  not  mature,  or 
if  for  any  other  reason  it  is  not  desired  to  harvest  the 
crop,  it  should  be  plowed  under,  to  return  the  nitrogen  in 
the  form  of  organic  matter.  A  crop  used  for  this  purpose 
is  called  a  catch  crop.  Rye  is  used  rather  commonly  as 
a  catch  crop,  as  it  continues  growth  until  late  in  the  fall 
and  resumes  growth  early  in  the  spring,  conserving  ni- 
trates whenever  nitrification  is  likely  to  occur,  and 
it  may  then  be  plowed  under  to  prepare  the  land  for 
another  crop.  Rye  also  has  the  advantage  of  small 
cost  for  seed. 

The  loss  of  calcium  cannot  well  be  prevented,  and 
the  use  of  commercial  fertilizers  always  greatly  increases 
such  loss.  The  only  remedy  is  the  application  of  some 
form  of  calcium  to  the  soil. 

276.  Drainage  records  at  Rothamsted.  —  Drainage 
water  from  a  series  of  plats  at  the  Rothamsted  Experi- 
ment Station,  which  have  been  manured  in  various 
ways  and  planted  to  wheat  each  year  since  1852,  have 
been  analyzed  at  certain  times,  and  the  results  of  these 
analyses,  as  compiled  by  Hall,1  give  some  idea  of  the  loss 

1  Hall,  A.   D.     The  Book  of  the  Rothamsted  experiments, 
pp.  237-239.     New  York,  1905. 
2b 


370       SOILS:    PROPERTIES  AND  MANAGEMENT 

of  salts  from  cultivated  soils.  The  drainage  water  was 
obtained  from  the  tile  drains,  a  line  of  which  extended 
under  each  plat  from  one  end  to  the  other  and  opened 
into  a  ditch,  so  that  the  water  could  be  collected  when 
desired,    The  analyses  are  shown  in  the  table  on  page 

371. 

Ammoniacal  nitrogen  in  the  drainage  water  is  very  small 
in  quantity,  but  nitrate  nitrogen  is  present  in  quantities 
sufficient  to  make  the  loss  of  some  concern.  The  use  of 
sodium  nitrate  occasioned  the  greatest  loss  of  nitrogen, 
while  ammonium  salts  and  farm  manure  contributed 
nearly  as  much.  From  forty  to  fifty  pounds  of  nitrogen  to 
the  acre  may  be  lost  annually  in  this  way ;  this  amount 
would  have  a  commercial  value  of  eight  or  nine  dollars. 

277.  Drainage  records  at  Bromberg.  —  It  is  not 
always  the  case  that  a  manured  soil  loses  more  fertilizing 
material  than  an  unfertilized  one.  Gerlach l  reports 
experiments  in  soil  tanks  at  the  Bromberg  Institute  of 
Agriculture,  as  the  result  of  which  five  soils,  when  ration- 
ally fertilized,  yielded  larger  crops  and  lost  in  the  main 
less  nitrogen  and  lime  in  the  drainage  water  than  the 
same  soils  unmanured.  The  loss  of  potash  was  slightly 
greater  from  the  manured  than  from  the  unmanured  soils. 
Apparently  the  stimulation  that  the  plants  received 
from  the  fertilizer  enabled  them  to  make  such  a  good 
growth  that  they  absorbed  more  soluble  nitrogen  and 
lime  in  excess  of  the  unfertilized  plants  than  was  added 
in  the  fertilizer,  and  nearly  as  much  potash. 

1  Gerlach,  M.  Ueber  die  durch  Sickerwasser  dem  Boden 
Entzogenen  Menge  Wasser  und  Nahrstoffe.  Illus.  Landw. 
Zeitung,  30  Jahrgang,  Heft  95,  Seite  871-881.  1910.  Also, 
Untersuchungen  iiber  die  Menge  und  Zusammensetzung  der 
Sickerwasser.  Mitt.  K.  W.  Inst.  f.  Landw.  in  Bromberg,  Band 
3,  Seite  351-381.     1910. 


THE  ABSORPTIVE  PROPERTIES   OF  SOILS      371 


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372      SOILS:    PROPERTIES  AND  MANAGEMENT 

278.  Losses  of  nitrogen  and  calcium.  —  The  most 
serious  losses  of  plant  nutrients  in  the  drainage  water  of 
soils  are  those  of  nitrogen  and  calcium,  and  both  are  to 
an  extent  unavoidable.  Potassium  and  phosphorus, 
which  must  also  be  purchased  in  manures,  are  lost  only 
at  the  rate  of  a  few  pounds  to  the  acre.  Nitrogen  and 
calcium  may  be  conserved  by  maintaining  a  crop  on  the 
soil  continually.  A  large  removal  of  nitrogen  in  the 
drainage  water  is  usually  accompanied  by  a  large  re- 
moval of  calcium;  for  nitrogen  is  leached  from  the  soil 
mainly  in  the  form  of  nitric  acid,  which  of  course  com- 
bines with  a  base,  and  calcium  being  the  base  finally 
liberated  it  is  carried  off  in  drainage  water.  While  most 
of  the  calcium  in  drainage  water  is  in  the  form  of  bicar- 
bonate, the  quantity  is  greatly  increased  by  nitric  acid. 

The  relation  of  nitric  acid  to  calcium  in  drainage  water 
is  shown  by  experiments  with  soil  in  large  tanks  from  which 
drainage  water  was  collected.  Plants  were  grown  in  the 
soil  of  certain  tanks,  while  others  had  none,  other  conditions 
being  similar.  Analyses  of  the  drainage  water  at  Ithaca, 
New  York,  as  reported  by  Lyon  and  Bizzell l  show  a  greatly 
increased  loss  of  calcium  from  the  implanted  tanks,  from 
which  the  loss  of  nitrate  nitrogen  was  also  much  greater :  — 

Nitrogen  and  Calcium  Removed  in  Drainage  Water  be- 
tween May  23,  1910,  and  May  1,  1911.  Calculated  to 
Pounds  to  the  Acre 


Crop  Grown 

Nitrate  Nitrogen 

Calcium 

None 

Maize 

Oats 

119.6 
10.8 
12.5 

406.7 
158.0 
173.4 

1  Lyon,  T.  L.,  and  Bizzell,  J.  A.  Composition  of  the  Drain- 
age Water  of  a  Soil  with  and  without  Vegetation.  Jour.  Indus, 
and  Eng.  Chem.,  Vol.  3,  pp.  742-743.     1911. 


THE  ABSORPTIVE  PROPERTIES  OF  SOILS      373 

Where  crops  were  present  to  absorb  the  nitric  acid, 
calcium  was  greatly  conserved.  The  quantities  of  ma- 
terial carried  off  in  drainage  water .  was  doubtless  ab- 
normally high  in  this  case,  as  the  soil  had  recently  been 
placed  in  the  tanks. 

279.  Composition  of  surface  water.  —  Another  method 
proposed  for  obtaining  these  data  is  to  analyze  and 
measure  the  water  draining  from  a  known  area  of  land. 
Norton  x  has  done  this  in  the  valley  of  Richland  Creek, 
Arkansas,  and  has  calculated  the  loss  of  a  number  of  the 
soil  constituents.  A  comparison  of  the  figures  obtained 
by  Norton  with  those  obtained  by  Lyon  and  Bizzell  in 
the  experiments  just  quoted  will  give  some  idea  of  the 
quantities  of  mineral  matter  removed  from  soils  by 
drainage  water.  The  Arkansas  soil  had  presumably 
received  little  manure.  The  soil  in  the  Cornell  Uni- 
versity tanks  had  previously  received  fifteen  tons  of 
stable  manure.  The  Arkansas  drainage  doubtless  in- 
cluded some  surface  water  that  had  never  passed  through 
the  soil  and  was  therefore  poor  in  mineral  matter;  the 
large  quantity  of  volatile  matter  indicates  its  surface 
nature,  as  water  that  passes  through  a  soil  contains  little 
organic  matter. 

There  is  little  similarity  in  the  results  of  these  analyses. 
They  serve,  however,  to  bring  out  the  differences  between 
the  composition  of  the  run-off  and  the  drainage  water  of 
soils,  in  so  far  as  that  may  be  judged  from  widely  dif- 
ferent soils  and  climatic  conditions,  including  the 
rainfall. 

1  Norton,  J.  H.  Quantity  and  Composition  of  Drainage 
Water  and  a  Comparison  of  Temperature,  Evaporation,  and 
Rainfall.  Journal  Am.  Chem.  Soc,  Vol.  30,  pp.  1186-1190. 
1908. 


374       SOILS:    PROPERTIES  AND  MANAGEMENT 


Substances  Removed  in  Drainage  Water  from  One  Acre 
of  Land.    Pounds  in  One  Year 


Norton 

Lyon  and  Bizzell 

Planted  Soil 

Bare  Soil 

Total  solids     .     . 
Organic  matter    . 
Nitrogen 

794.0 

134.0 

4.0 

5.0 

0.1 

81.0 

800 

0 

11 

6 
Trace 
158 

2584 

0 

119 

Potash    .... 

11 

Phosphoric  acid  . 
Lime 

Trace 
407 

It  will  be  seen  that  the  total  solids  in  the  drainage 
water  from  the  Arkansas  land  and  from  the  planted 
tank  were  not  greatly  different  in  amount,  but  that 
some  of  the  constituents  differed  greatly.  This  was 
notably  the  case  with  organic  matter  and  with  lime. 
The  former  was  doubtless  carried  largely  in  the  run-off 
and  not  in  the  leachings.  The  latter  was  probably  more 
abundant  in  the  glaciated  soil  used  in  the  tank  than 
in  the  residual  soil  of  Arkansas. 


CHAPTER  XVII 
ACID,  OR  SOUR,  SOILS 

Some  soils  are  known  as  acid,  or  sour,  soils.  The 
property  of  acidity  is  of  practical  significance  because 
some  plants  do  not  grow  so  well  on  sour  soils  as  they  do 
on  soils  that  are  neutral  or  alkaline ;  on  the  other  hand, 
some  crops  prefer  an  acid  soil.  Sour  soils  are  rarely  met 
with  in  arid  regions,  but  in  humid  sections  of  the  United 
States  they  are  commonly  found. 

280.  Nature  of  soil  acidity.  —  Soils  may  be  acid,  or 
sour,  so  far  as  their  relation  to  plant  growth  is  concerned, 
(1)  when  free  acids  are  present,  (2)  when  no  soluble  free 
acid  exists,  but  when  there  is  a  deficiency  of  basic  material 
in  the  soil.  Decomposition  of  organic  matter  in  certain 
soils  under  an  inadequate  supply  of  oxygen  often  results 
in  the  formation  of  considerable  quantities  of  organic 
acids,  as  has  already  been  explained  (par.  93). 

281.  Positive  acidity.  —  The  formation  of  organic 
acids  under  conditions  of  insufficient  oxygen  supply  is 
frequently  seen  in  muck  and  other  soils  high  in  organic 
matter  that  are  saturated  with  water  and  that  are  also 
deficient  in  lime.  In  such  cases  an  acid  condition  is  very 
likely  to  be  found,  but  when  the  land  is  drained  the 
acidity  usually  disappears  because  of  the  better  aeration 
resulting.  When  a  large  quantity  of  green  vegetation 
is  plowed  under,  as  is  done  in  green-manuring  land,  a 
sour  condition  sometimes  appears  after  the  material  has 

375 


376    -  SOILS:    PROPERTIES  AND  MANAGEMENT 

had  time  partially  to  decay.  The  acidity  of  soil  that 
arises  from  the  presence  of  free  acids  has  been  termed 
positive  acidity. 

It  is  to  be  presumed  that  soils  in  which  free  acids  exist 
are  rather  deficient  in  basic  material,  and  that  the  bases 
are  held  so  firmly  combined  that  some  of  the  relatively 
weak  organic  acid  present  is  not  capable  of  forming  salts 
with  them.  Plummer  l  has  shown  that  dihydroxystearic 
acid  when  added  to  an  acid  soil  had  a  distinctly  toxic 
effect  on  wheat  plants,  but  when  added  to  the  same  soil 
previously  treated  with  lime  there  was  no  toxic  effect, 
indicating  that  this  substance  retained  its  acid  properties 
in  the  unlimed  soil. 

282.  Negative  acidity.  —  A  soil  deficient  in  basic 
material  but  containing  no  soluble  free  acids  may  be 
sour  as  regards  its  relation  to  plant  growth.  At  least 
such  a  soil  may  be  greatly  benefited  by  liming,  although 
it  shows  no  acidity  to  most  of  the  ordinary  indicators  of 
acidity  when  these  are  used  in  the  customary  way.  This 
condition  has  been  termed  negative  acidity  and  is  really 
not  acidity  according  to  a  correct  use  of  the  word.  Such 
acidity  does  not  have  a  direct  effect  on  the  plant,  but 
an  indirect  one  arising  from  a  lack  of  bases.  Soils  that 
are  acid  in  this  sense  always  have  a  large  capacity  for 
absorbing  lime  or  other  bases,  before  exhibiting  an  al- 
kaline reaction.  Calcium  being,  as  has  already  been 
seen  (par.  264),  the  base  most  liberally  released  to  solution, 
there  is  a  tendency  toward  the  formation  of  calcium 
carbonate  in  any  soil  dependent  on  the  equilibrium  be- 

1  Plummer,  J.  K.  The  Isolation  of  Dihydroxystearic  Acid 
from  Volusia  Silt  Loam.  Thesis  presented  in  partial  fulfillment 
of  the  requirements  for  the  degree  of  Master  of  Science.  Cornell 
University  Library  (not  published).     1911. 


ACID,    OR   SOUR,    SOILS  377 

tween  the  basic  material  and  the  absorptive  substances 
in  the  soil.  Thus,  a  soil  containing  large  quantities  of 
clay,  and  other  absorbent  substances  requires  more  basic 
material  for  the  formation  of  calcium  carbonate  than 
does  a  soil  having  less  absorptive  material.  Further- 
more, with  the  same  original  content  of  basic  material, 
the  former  soil  requires  a  greater  addition  of  lime  to 
overcome  its  sourness  than  does  the  latter.  For  this 
reason  a  heavy  soil  usually  requires  a  larger  dressing  of 
lime  to  correct  its  acidity  than  does  a  light  one. 

Even  if  a  soil  does  not  have  its  absorptive  capacity  for 
bases  satisfied,  there  is  some  formation  of  calcium  car- 
bonate constantly  taking  place,  as  is  evidenced  by  the 
removal  of  the  bicarbonate  of  calcium  in  the  drainage 
water  of  soils  that  are  distinctly  acid.  The  benefits  that 
soils  derive  from  the  presence  of  calcium  carbonate  will 
be  mentioned  later  (pars.  454-457).  It  need  only  be  said 
here  that  its  presence  in  insufficient  quantity  constitutes 
a  form  of  so-called  acidity,  or  sourness,  in  soils.  The 
formation  of  calcium  carbonate  in  a  given  soil  increases 
with  the  mass  of  base.  The  effect  of  an  application 
of  lime,  therefore,  is  to  increase  the  quantity  of  car- 
bonate formed,  even  when  the  absorptive  capacity  of 
the  soil  is  not  satisfied.  This  is  why  even  relatively 
small  applications  of  lime  are  beneficial  to  soils  having 
great  absorptive  capacity. 

283.  Production  of  sour  soils.  —  Soils  in  a  humid 
region  tend  to  become  acid.  This  may  be  due  to  any  one 
or  more  of  several  causes :  (1)  removal  of  calcium  and 
other  bases  in  drainage  water;  (2)  removal  of  bases  by 
plants;  (3)  formation  of  salts  of  the  bases  with  organic 
matter  incorporated  with  soil;  (4)  accumulation  of  acid 
residues  of  fertilizers. 


378       SOILS:    PROPERTIES  AND  MANAGEMENT 

284.  Removal  of  bases  by  drainage  as  a  cause  for 
acidity.  —  The  most  potent  cause  of  acid  soils  is  doubt- 
less the  removal  of  bases  in  drainage  water.  The  quan- 
tities of  basic  material  that  may  be  lost  from  an  acre  of 
soil  are  shown  elsewhere  (pars.  278,  279).  These  bases  are 
removed  largely  as  bicarbonates,  being  obtained  from 
the  hydrated  aluminium  silicates  and  other  colloidal 
matter.  When  the  soil  is  uncropped  a  considerable  loss 
of  lime  occurs  in  the  form  of  nitrate.  As  the  decom- 
position of  the  organic  matter  of  the  soil  always  results 
in  the  formation  of  carbon  dioxide  and  nitric  acid,  and 
as  decomposition  is  continually  going  on  except  when  the 
temperature  of  the  soil  becomes  too  low  to  admit  of  it, 
the  drain  of  bases  from  the  soil  is  almost  continuous. 
Formation  of  carbon  dioxide  and  of  nitric  acid  occurs 
largely  in  the  surface  soil ;  consequently  the  removal 
of  bases  begins  there.  The  result  is  that  soils  are 
likely  to  contain  less  calcium  in  the  surface  layers  than 
at  lower  depths.  Ames  and  Gaither  1  have  shown  from 
a  large  number  of  analyses  of  Ohio  soils  that  those  con- 
taining calcium  carbonate  in  appreciable  quantities  have 
more  calcium  in  the  subsoil  than  in  the  surface  six 
inches.  In  other  soils  this  was  not  uniformly  the  case. 
Leaching  is,  of  course,  greater  in  amount  where  con- 
siderable quantities  of  calcium  carbonate  are  present 
than  where  it  is  lacking. 

285.  Removal  of  bases  by  plants.  —  Plants  always 
remove  more  bases  than  acids  from  soils  in  the  process 
of  their  growth.  The  table  in  paragraph  339  showing  the 
composition  of  the  ash  of  some  crops  indicates  that  the 
calcium,     potassium,     and     magnesium     removed     from 

1Ames,  J.  W.,  and  Gaither,  E.  W.  Soil  Investigations. 
Ohio  Agr.  Exp.  Sta.,  Bui.  261.     1913. 


ACID,    OR   SOUR,    SOILS  379 

the  soil  in  this  way  is  very  considerable.  When  the 
vegetation  on  the  land  is  returned  to  it  after  life 
ceases  and  its  organic  material  is  again  incorporated 
with  the  soil,  there  is  no  loss  in  this  way,  but  in  ordi- 
nary agricultural  practices  most  of  the  above-ground 
portion  of  the  crops  is  removed  from  the  land.  The 
manure  of  growing  animals  returns  to  the  soil  only  a 
small  proportion  of  the  calcium  that  was  originally  in 
the  plants. 

Breazeale  and  LeClerc  !  found  that  the  selective  action 
of  plants  in  absorbing  more  bases  than  acids  from  a 
nutrient  solution  caused  the  solution  to  become  toxic  to 
wheat  seedlings  because  of  its  acidity. 

286.  Effect  of  green  manures  on  acidity.  —  Although 
the  return  of  vegetation  to  the  land  on  which  it  grew 
does  not  result  in  any  actual  loss  of  basic  material  to 
the  soil,  it  generally  results  in  the  formation  and  libera- 
tion of  organic  acids  that  unite  with  the  basic  material 
and  thus  render  it  neutral.  In  soils  deficient  in  lime 
the  incorporation  of  green-manure  crops  has  been  con- 
sidered to  temporarily  produce  an  acid  condition. 
Coville 2  determined  the  acidity  of  some  green-ma- 
nure crops,  on  the  basis  of  which  he  has  estimated 
the  acidity,  in  terms  of  ground  limestone  required  to 
neutralize  it,  when  the  lime  contained  in  the  crop  is  de- 
ducted from  the  total  lime  required.  This  is  given  in  the 
table  on  the  next  page. 


1  Breazeale,  J.  F.,  and  LeClerc,  J.  A.  The  Growth  of  Wheat 
Seedlings  as  Affected  by  Acid  or  Alkaline  Conditions.  U.  S. 
D.  A.,  Bur.  Chem.,  Bui.  149.     1912. 

2  Coville,  J.  V.  The  Agricultural  Utilization  of  Acid  Lands 
by  Means  of  Acid-Tolerant  Crops.  U.  S.  D.  A.,  Bui.  No.  6, 
p.  5.     1913. 


380       SOILS:    PROPERTIES  AND  MANAGEMENT 


Weight,  Lime  Content,   and  Acidity  of  Green  Manures 
to  the  Acre 


Crop 

i  Weight 
(tons) 

Lime 
Content 
(pounds) 

Acidity,  ex- 
pressed AS    Ll.MK 
Requirement 
(pounds) 

Alfalfa 

Red  clover       .... 

Cowpea       ...... 

Rye         ...... 

Broom  sedge   .... 

2J 

2 

2* 
2 

1 

139 

131 

92 

11 

4 

267 
142 
200 

178 
89 

As  decomposition  proceeds  the  acids  are  oxidized,  and 
finally  basic  material  is  held  largely  in  combination  with 
so-called  humus  of  the  soil.  This  is  doubtless  in  the 
form  of  a  colloidal  complex,  not  a  definite  chemical  com- 
pound. Analyses  by  Snyder 1  of  purified  humous  ash 
from  eight  productive  prairie  soils  have  been  averaged 
and  are  presented  in  tabular  form  in  paragraph  97. 

The  quantity  of  basic  material  ordinarily  held  by  the 
organic  matter  of  the  soil  is  small  compared  with  the 
total  soil  content.  The  bases  contained  in  humus  are 
principally  potassium  and  sodium  —  not  calcium,  as 
might  be  expected  in  the  salt  of  an  organic  acid  formed 
in  the  soil.  Humus  in  the  soil  tends  to  overcome  acidity 
and  functions  as  an  alkali.  In  respect  to  its  composition 
and  properties,  much  of  it  resembles  a  colloidal  com- 
plex rather  than  a  chemical  combination  of  soil  bases 
with  organic  acids. 

It  has  often  been  observed  that  land  from  which  forest 
has  been  cleared  will  yield  good  crops  of  red  clover  for 


1  Snyder,    Harry. 
41.     1895. 


Soils.     Minnesota  Agr.    Exp.    Sta.,   Bui. 


ACID,    OR   SOUR,    SOILS  381 

several  decades,  after  which  it  becomes  more  and  more 
difficult  to  obtain  a  crop  until  the  attempt  must  finally 
be  abandoned.  The  change  from  forest  to  tillage  has 
opened  the  way  for  an  acid  condition  of  soil,  through  the 
loss  of  bases  carried  off  in  the  crops  and  the  destruction 
of  humus  by  tillage.  The  dissipation  of  humus  is  doubt- 
less the  more  serious  source  of  loss.  Instances  may  be 
cited  in  which  a  farm  has  been  so  managed  as  to  main- 
tain the  humus  supply  and  the  ability  of  the  soil  to  pro- 
duce red  clover,  although  surrounding  farms,  on  which 
humus  has  been  depleted,  have  completely  failed  to  grow 
this  crop. 

Apparently  humus  holds  the  basic  constituents  of  the 
soil  in  a  form  in  which  they  function  as  rather  easily 
soluble  salts,  instead  of  locking  them  up  as  insoluble 
silicates.  A  given  quantity  of  base  in  a  soil  is  therefore 
more*  effective  in  preventing  acidity  by  combining  with 
weak  acids,  and  possibly  in  forming  carbonates,  if  the  soil 
is  well  supplied  with  humus  than  if  it  is  lacking  in  that 
constituent. 

287.  Effect  of  fertilizers  on  soil  acidity.  —  That  the 
continued  use  of  ammonium  sulfate  on  land  may  result 
in  producing  a  sour  condition  has  been  shown  by  a  num- 
ber of  investigators.  The  absorption  and  nitrification 
of  the  ammonia  of  that  salt,  and  its  final  utilization  by 
plants,  leaves  sulfuric  acid,  which  combines  with  calcium 
and  escapes  in  the  drainage  water.  This  may  occur  even 
when  this  fertilizer  is  used  in  quantities  not  excessive, 
but  continued  for  many  years,  as  has  been  shown  by 
Gardner  and  Brown  '  at  the  Pennsylvania  Experiment 

1  Gardner,  F.  D.,  and  Brown,  B.  E.  The  Lime  Require- 
ment of  the  General  Fertilizer  Plats  as  Determined  Periodically. 
Rept.  Pennsylvania  Agr.  Exp.  Sta.,  1910-1911,  pp.  25-60. 


382       SOILS:    PROPERTIES  AND  MANAGEMENT 

Station.  Other  fertilizers  leaving  an  acid  radicle  in  the 
soil  also  act  in  this  way.  It  is  conceivable  that  potassium 
chloride  and  potassium  sulfate  might  have  a  tendency 
to  produce  an  acid  condition,  but  the  bases  in  these  salts 
do  not  disappear  from  the  soil  so  quickly  as  would  am- 
monia, and  consequently  their  action  is  slower. 

The  use  of  free  sulfur  on  the  land  as  a  means  of  com- 
bating certain  fungous  diseases  may  lead  to  the  formation 
of  a  sour  soil  through  the  oxidation  of  the  sulfur  with 
formation  of  sulfuric  acid.  Lint l  has  found  that  a 
soil  in  which  sulfur  was  used  at  the  rate  of  600  pounds 
to  the  acre  for  prevention  of  potato  scab,  changed  in  its 
lime  requirement  from  2431  pounds  to  4177  pounds  as  a 
result  of  the  one  treatment. 

288.  Acidity  in  relation  to  climate  and  to  formation  of 
soil.  —  In  an  arid  or  a  semiarid  climate  soils  are  not  likely 
to  become  sour.  The  great  source  of  lime  removal, 
leaching,  operates  to  only  a  slight  extent,  or  not  at  all, 
in  a  dry  climate.  The  removal  of  bases  in  crops  is  ap- 
parently offset  by  the  upward  movement  of  bicarbonates 
in  the  capillary  water.  Experience  shows  that  acidity  is 
not  a  problem  in  soils  of  dry  countries. 

Soils  that  are  derived  from  limestone  or  that  have  been 
mixed  with  limestone  soils  in  the  process  of  their  forma- 
tion are,  under  similar  climatic  conditions,  less  likely 
to  become  acid  than  are  soils  that  originally  contained 
less  lime.  The  fact  that  a  soil  is  derived  from  limestone, 
however,  does  not  insure  that  it  may  not  be  benefited  by 
an  application  of  lime. 

289.  Weeds  that  flourish  on  sour  soils.  —  The  acidity 
or  the  basicity  of  soils  influences  very  greatly  the  growth 

1  Lint,  H.  Clay.  The  Influence  of  Sulfur  on  Soil  Acidity. 
Jour.  Indus,  and  Eng.  Chem.,  Vol.  6,  pp.  747-748.     1914. 


ACID,   OR   SOUR,   SOILS  383 

of  vegetation  and  determines  to  a  large  degree  its  nature. 
The  flora  undergoes  a  considerable  variation  as  a  soil 
changes  from  a  basic  to  a  sour  condition.  This  is  because 
some  plants  are  injured  to  a  greater  extent  than  are  others 
by  the  conditions  that  accompany  an  acid  reaction  of  the 
soil.  Some  higher  plants  really  grow  better  on  a  sour 
soil  than  they  do  on  an  alkaline  one,  but  these  form  only 
a  minority  of  the  plants  of  agricultural  importance. 
Weeds  that  abound  and  appear  to  flourish  on  acid  soils 
may  do  so  either  because  they  grow  better  on  sour  soil 
than  on  basic,  or  because  other  vegetation  growing  on 
the  soil  does  not  thrive  and  therefore  the  dominant  weeds 
of  the  region  have  less  competition  than  they  otherwise 
would  have.  There  are  certain  weeds  that  may  be  taken 
to  indicate  a  sour  soil  when  present  in  large  numbers. 
Some  of  these  are  found  in  one  part  of  the  country  and 
some  in  another :  — 

Weeds  that  Flourish  on  Sour  Soils 
Common  name  Botanical  name 


Sheep  sorrel *  . 
Paintbrush  .  . 
Daisy  .  .  . 
Horsetail  rush  2 
Corn  spurry  2  . 
Wood  horsetail 2 
Plantain 1  .  . 
Goose  grass 3 


Rumex  acetosella 
Hieracium  aurantiacum 
Bellis  perennis 
Equisetum  afvense 
Spergula  arvensis 
Equisetum  sylvaticum 
Plantago  major 
Polygonum  aviculare 


1  Knisely,  A.  L.     Acid  Soils.     Oregon  Agr.  Exp.  Sta.,  Bui. 
90,  p.  23.     1906. 

2  Whitson,  A.  R.,  and  Weir,  W.  W.     Soil  Acidity  and  Liming. 
Wisconsin  Agr.  Exp.  Sta.,  Bui.  230,  pp.  7-11.     1913. 

3  Voelcker,   J.   A.     The  Woburn   Field  Experiments.     Jour. 
Royal  Agr.  Soc.  England,  Vol.  69,  pp.  337-357.     1908. 


384       SOILS:    PROPERTIES  AND  MANAGEMENT 

290.  Crops  adapted  to  sour  soils.  —  There  are  some 
useful  agricultural  plants  that  grow  better  on  sour  soils 
than  on  alkaline  soils,  while  other  plants  are  apparently 
indifferent  to  the  condition  of  the  soil  in  this  respect.  As 
acid  soils  are  of  very  common  occurrence,  and  as  the 
correction  of  this  difficulty  may  not  always  be  financially 
profitable  or  otherwise  desirable,  it  is  important  to  know 
what  plants  will  thrive  and  how  agricultural  practice 
may  be  maintained  on  such  soils.  A  list  of  these  plants, 
based  on  different  authorities,  is  herewith  given :  — 

Crops  Adapted  to  Sour  Soils 

Blueberry  !  Hairy  vetch  1 

Cranberry  2  Crimson  clover  * 

Strawberry  1  Potato  2 

Blackberry 2  Sweet  potato  1 

Raspberry  2  Rye  2 

Blackcap  2  Millet 2 

Watermelon  2  Buckwheat ' 

Turnip  l  Carrot 1 

Red  top  2  Lupine  2 

Rhode  Island  bent-grass  2  Serradella  2 

Cowpea1  Radish  2 

Soybean 1       '  Velvet  bean 2 
Castor  bean 2 

The  very  considerable  number  of  these  plants,  and 
especially  the  inclusion  among  them  of  legumes  that 
may  be  grown  for  soil  improvement,  suggest  the  possi- 

1  Coville,  F.  W.  The  Agricultural  Utilization  of  Acid  Lands 
by  Means  of  Acid-Tolerant  Crops.  U.  S.  D.  A.,  Bui.  No.  6 
pp.    7-12.     1913. 

2  Wheeler,  H.  J.  The  Liming  of  Soils.  U.  S.  D.  A.,  Farmers' 
Bui.  77.     1905. 


ACID,    OR   SOUR,    SOILS 


385 


bility  of  a  successful  agricultural  practice  on  acid  soils 
where  the  important  money  crop  to  be  grown,  or  some 
other  condition,  would  make  it  undesirable  to  correct 
the  soil  acidity.  There  are  certain  crops,  such  as  blue- 
berries and  cranberries,  that  require  an  acid  soil;  there 
are  others,  such  as  potatoes,  that  may  suffer  less  from 
disease  if  the  soil  is  sour.  These  crops  are  sometimes  the 
ones  that  are  of  greatest  financial  importance  in  a  region, 
and  it  therefore  becomes  desirable  to  maintain  an  acid 
condition  of  soil. 

291.  Crops  that  are  injured  by  acid  soils.  —  There 
are  many  plants  that  are  injured  by  a  sour  condition  of 
the  soil,  and  these  include  some  of  the  most  important 
farm  crops.  It  should  therefore  be  borne  in  mind  that 
for  most  farm  practice  an  acid  soil  is  very  undesirable. 
One  notable  reason  for  this  is  that  such  crops  as  red 
clover  and  alfalfa,  which  are  of  great  value  both  as  a 
means  of  improving  soil  and  for  hay,  can  be  grown  only 
with  great  uncertainty- or  not  at  all  on  acid  soils. 


Crops  that  are  Injured  by 

Sour 

Soils  x 

Alfajfa                                 Salsify 

Cauliflower 

Red  clover                          Squash 

Cabbage 

Saltbush                              Spinach 

Cucumber 

Timothy                              Red  beet 

Lettuce 

Kentucky  blue-grass          Sorghum 

Onion 

Maize                                  Barley 

Okra 

Oats                                     Sugar  beet 

Peanut 

Pepper                                 Currant 

Tobacco 

Parsnip                               Mangel-wurzel 

Kohlrabi 

Pumpkin                            Celery 

Eggplant 

1  Wheeler,  H.  J.     The  Liming  of  Soils. 

U.  S; 

.  D.  A.,  Farmers' 

Bui.  77  (revised).     1905. 

2c 

386       SOILS:    PROPERTIES  AND  MANAGEMENT 

While  soils  may  be  either  sour  or  alkaline,  there  are 
also  degrees  of  sourness.  Thus  a  soil  may  be  so  sour  as 
to  completely  prevent  the  growth  of  one  kind  of  plant 
and  yet  produce  excellent  crops  of  another  plant  which 
would  have  perished  if  the  soil  had  been  more  acid.  For 
example,  red  clover  will  grow  fairly  well  on  soil  that  is 
too  sour  to  raise  alfalfa. 

292.  Qualitative  tests  for  acidity.  —  A  simple  test  to 
indicate  an  acid  condition  of  soil  is  not  so  easy  of  execu- 
tion nor  so  infallible  in  its  prediction  as  might  be  desired. 
The  object  of  such  a  test  is  to  ascertain  whether  a  soil  is 
not  well  adapted  to  the  growth  of  certain  plants  and 
whether  the  application  of  lime  would  benefit  it  in  this 
respect.  A  number  of  tests  have  been  proposed  which 
will  be  outlined  and  briefly  discussed. 

293.  Litmus  paper  test.  —  Blue  litmus  paper  is  brought 
into  contact  with  the  wet  soil.  A  rapid  and  decided 
change  to  red  is  taken  to  indicate  an  acid  condition  of 
the  soil.  Carbonic  acid,  which  is  always  present  in  soils, 
is  supposed  to  give  only  a  faint  pink  color  to  the  litmus 
paper.  Various  ways  of  bringing  the  paper  into  contact 
with  the  soil  have  been  recommended,  among  others  the 
interposing  of  filter  paper  between  the  soil  and  the  litmus 
paper.1  It  is  also  generally  pointed  out  that  the  acid 
perspiration  on  the  fingers  may  lead  to  delusion. 

A  criticism  of  the  test  has  been  made  by  Cameron,2 
who  states  that  the  absorbent  action  of  soils  for  bases  is 
greater  than  is  that  of  paper,  while  for  acids  the  reverse 

1  Kellerman,  K.  F.,  and  Robinson,  T.  R.  Legume  Inoculation 
and  the  Litmus  Reaction  of  Soils.  U.  S.  D.  A.,  Bur.  Plant  Indus., 
Circ.  71,  pp.  3-11.     1910. 

2  Cameron,  F.  K.  The  Soil  Solution,  pp.  65-66.  Easton, 
Pennsylvania.     1911. 


ACID,    OR    SOUR,   SOILS  387 

is  the  case.  Consequently  the  base  that  had  produced 
the  blue  color  is  absorbed  from  the  litmus,  leaving  the 
acid  compound,  which  is  red.  Cameron  concludes  that 
the  test  is  unreliable,  and  proposes  to  extract  the  soil 
with  water,  boil  it  in  order  to  expel  carbon  dioxide,  and 
then  test  the  reaction  of  the  solution. 

Much  litmus  paper  that  is  sold  is  of  very  poor  quality ; 
but  when  good  paper  is  used  and  the  test  is  carefully 
made,  the  general  experience  has  been  that  it  is  a  fairly 
good,  although  not  an  infallible,  guide  to  the  need  of  a 
soil  for  lime.  Red  coloration  due  to  absorptive  action  is 
probably  an  advantage  rather  than  a  source  of  error  in 
the  test,  as  a  soil  strongly  absorptive  of  bases  is  likely 
to  need  lime.  This  coloration  does  not  necessarily  in- 
dicate the  presence  of  free  acid,  but  merely  need  of  lime. 

294.  Ammonia  test.  —  In  this  test  the  soil  is  stirred 
with  a  dilute  solution  of  ammonia  hydroxide.  After 
settling,  if  the  supernatant  liquid  on  standing  takes  on 
a  dark  chocolate  or  a  black  color  it  is  said  to  be  acid. 
This  method,  which  has  been  proposed  by  Miintz,1  is 
not  of  general  application  and  would  not  always  be  re- 
liable in  the  case  of  soils  of  arid  regions.  The  depth  of 
color  is  not  a  guide  to  the  degree  of  acidity,  since  many 
acid  soils  are  low  in  organic  matter. 

295.  Zinc  sulfide  test.  —  A  test  recently  proposed  by 
Truog  2  consists  in  mixing  the  soil  to  be  tested  with  a 
small  quantity  of  calcium  chloride  and  a  very  little  zinc 
sulfide.     Water  is  added  and  the  mixture  is  heated  to 

1  Wheeler,  H.  J.,  Hartwell,  B.  L.,  and  Sargent,  C.  L.  Chemi- 
cal Methods  for  Ascertaining  the  Lime  Requirements  of  Soils. 
Rhode  Island  Agr.  Exp.  Sta.,  Bui.  62,  pp.  65-88.     1899. 

2  Truog,  E.  A.  New  Method  for  the  Determination  of  Soil 
Acidity.     Science,  N.  S.,  Vol.  40,  pp.  246-248.     1914. 


388       SOILS:    PROPERTIES  AND  MANAGEMENT 

boiling.  A  strip  of  moistened  lead  acetate  paper  is  held 
over  the  mouth  of  the  flask  for  two  minutes  while  the 
boiling  proceeds.  If  the  soil  is  acid,  the  paper  will  be 
darkened  on  the  underside;  if  the  soil  is  not  acid,  no 
darkening  will  occur. 

This  method  is  evidently  designed  to  test  the  need  of 
the  soil  for  lime  as  well  as  actual  acidity,  for  the  absorp- 
tion of  calcium  from  the  dissociated  chloride  would  leave 
free  hydrochloric  acid.  The  action  of  this  acid  on  zinc 
sulfide  would  generate  hydrogen  sulfide,  thus  blackening 
the  lead  acetate  paper. 

A  somewhat  similar  principle  is  involved  in  the  proposal 
to  use  a  solution  of  potassium  nitrate  in  the  litmus  paper 
test. 

296.  Litmus  paper  and  potassium  nitrate.  —  This  is 
performed  in  the  same  manner  as  the  former  litmus 
paper  test,  except  for  the  substitution  of  a  saturated 
solution  of  potassium  nitrate  instead  of  distilled  water 
for  moistening  the  soil. 

297.  Acid  test  for  carbonates.  —  In  this  test  a  dry 
sample  of  the  soil  is  treated  with  a  few  drops  of  dilute 
hydrochloric  acid.  Effervescence  indicates  the  presence 
of  carbonates  or  bicarbonates  in  sufficient  quantities  to 
insure  an  alkaline  soil,  although  sometimes  lime  may 
still  be  beneficial. 

Whitson  and  Weir  x  have  objected  to  this  method  on 
the  ground  that  the  displacement  of  air  in  the  pore  spaces 
of  the  soil  by  the  dilute  acid  may  be  mistaken  for  evolu- 
tion of  carbon  dioxide.  In  the  hands  of  an  experienced 
and  careful  operator  this  would  not  necessarily  invalidate 
the  method. 

1  Whitson,  A.  R.,  and  Weir,  W.  W.  Soil  Acidity  and  Lim- 
ing.    Wisconsin  Agr.  Exp.  Sta.,  Bui.  230,  pp.  7-11.     1913. 


ACID,    OR   SOUR,    SOILS  389 

298.  Plants  as  indicators  of  acidity.  —  In  addition 
to  these  chemical  tests  for  acidity  there  may  also  be 
mentioned  what  is  perhaps  the  most  reliable  indication 
of  the  need  of  lime,  namely,  the  failure  of  a  soil  to  produce 
red  clover,  and  the  presence  of  those  weeds  that  have 
previously  been  shown  to  thrive  on  sour  soil  (par.  289). 
When  a  soil  bears  this  relation  to  the  plant  growth  it  may 
safely  be  assumed  that  those  plants  included  in  the  list  of 
crops  that  are  injured  by  sour  soils  will  yield  better  if  the 
soil  is  limed  than  if  it  is  not  so  treated.  The  crops  adapted 
to  sour  soils  may  not  be  injured. 

299.  Quantitative  determinations  of  acidity.  —  A  num- 
ber of  quantitative  methods  for  determining  the  degree 
of  acidity  or  the  lime  requirements  of  soils  have  been 
devised.     Only  a  few  of  these  need  be  mentioned. 

300.  Potassium  nitrate  method.1  —  The  soil  is  shaken 
with  a  normal  solution  of  potassium  nitrate  for  three 
hours,  and  then  allowed  to  stand  overnight.  An  aliquot 
portion  of  the  supernatant  liquid  is  boiled  in  order  to 
expel  carbon  dioxide,  and  when  cool  it  is  titrated  with 
a  standard  solution  of  sodium  hydroxide. 

This  method  does  not  estimate  either  the  free  acid 
or  the  lime  requirement  of  the  soil.  What  it  does  is 
to  give  the  absorptive  power  of  the  soil  for  potassium 
when  in  equilibrium  with  a  solution  containing  the 
acid  with  which  the  potassium  was  originally  in  com- 
bination. There  is  a  substitution  of  bases  during  the 
contact  of  the  nitrate  solution  with  the  soil,  and  a 
partial  decomposition  of  these  salts  during  the  titration 
with  alkali. 

1  Official  and  Provisional  Methods  of  Analysis.  Association 
of  Official  Agricultural  Chemists.  U.  S.  D.  A.,  Bur.  Chem., 
Bui.  107  (revised),  p.  20.     1908. 


390       SOILS:    PROPEBTIES  AND  MANAGEMENT 

301.  Limewater  method.  —  A  measured  quantity  of 
a  standard  solution  of  limewater  is  brought  into  contact 
with  the  soil  and  absorption  is  accomplished  by  evapora- 
tion, after  which  water  is  added  and  the  filtrate  is  tested 
.vith  phenolphthalein.  Failure  to  produce  a  pink  color 
shows  that  the  lime  requirement  of  the  soil  has  not  been 
reached;  an  alkaline  reaction  shows  that  an  excess  of 
lime  has  been  added.  A  number  of  tests  must  be  made 
in  order  to  reach  a  point  below  which  the  indicator  shows 
no  color  and  above  which  it  does.  The  lime  requirement 
may  thus  be  indicated.  This  determination  was  devised 
by  Veitch,1  and  is  a  useful  method  since  it  indicates  to 
within  a  few  hundred  pounds  the  quantity  of  lime  re- 
quired to  satisfy  the  absorptive  power  of  a  soil. 

302.  Resume.  —  In  conclusion,  a  few  facts  regarding 
so-called  acid  soils  may  be  restated :  (1)  acidity  is  not 
always  due  to  free  acids,  but  often  to  the  lack  of  an  abun- 
dance of  bases ;  (2)  it  is  not  injurious  to  all  plants,  but  is 
likely  to  depress  the  yields  of  the  majority  of  agricultu- 
rally important  crops,  while  some  valuable  ones  are  bene- 
fited by  it;  (3)  it  may  be  overcome  sometimes  by  aera- 
tion of  the  soil,  and  always  by  the  application  of  lime  or 
wood  ashes.  The  correction  of  acidity  by  means  of  lime 
will  be  discussed  in  a  later  chapter,  as  will  also  the  rela- 
tion of  certain  bacteria  to  acidity. 


1  Veitch,  F.  P.  The  Estimation  of  Soil  Acidity  and  the  Lime 
Requirements  of  Soils.  Jour.  Am.  Chem.  Soc,  Vol.  24,  pp. 
1120-1128.     1902. 


CHAPTER   XVIII 

ALKALI  SOILS 

It  has  already  been  shown  that  soils  are  acted  upon  by 
a  great  variety  of  weathering  agents  which  gradually 
render  soluble  a  portion  of  the  most  susceptible  constitu- 
ents. This  soluble  material  becomes  a  part  of  the  soil 
solution  and  may  come  in  contact  with  the  roots  of  any 
crop  growing  on  the  soil.  In  humid  regions,  where  a 
large  quantity  of  water  percolates  through  the  soil,  this 
soluble  matter  has  little  opportunity  to  collect.  In  arid 
regions,  however,  where  loss  by  drainage  is  slight,  these 
salts  may  often  collect  in  large  amounts.  During  periods 
of  drought  they  are  carried  upward  by  the  capillary  rise 
of  the  soil  water,  while  during  periods  of  rainfall  they  may 
move  downward  again  in  proportion  to  the  leaching  action. 
At  one  time  the  lower  soil  may  contain  considerably  more 
soluble  salt  than  the  upper ;  at  another  time  the  condition 
may  be  reversed,  in  which  case  the  solution  in  contact 
with  plant  roots  may  contain  so  much  soluble  matter 
that  vegetation  is  injured  or  destroyed.  This  excess  of 
soluble  salts  usually  has  a  marked  alkaline  reaction,  but 
in  any  case  it  produces  what  is  termed  an  alkali  soil. 

303.  Composition  of  alkali  salts.  —  The  materials  dis- 
solved in  the  soil  water  consist  of  all  the  substances  found 
in  the  soil,  but  as  the  rates  of  solubility  of  these  substances 
vary  greatly  there  is  accumulated  a  much  larger  quantity 
of  some  substances  than  of  others.     Carbonates,  sulfates, 

391 


392  SOILS:    PROPERTIES  AND  MANAGEMENT 

and  chlorides  of  sodium,  potassium,  calcium,  and  mag- 
nesium occur  in  the  largest  amounts.  Sodium  may  be 
present  as  carbonate,  sulfate,  chloride,  phosphate,  and 
nitrate.  Potassium  may  be  similarly  combined.  Mag- 
nesium is  likely  to  appear  as  a  sulfate  or  a  chloride,  and 
calcium  as  a  sulfate,  a  chloride,  or  a  carbonate.  One 
salt  will  predominate  in  some  soils,  and  other  salts  in  other 
soils.  A  base  may  be  present  in  combination  with  several 
different  acids.  The  nature  of  the  prevailing  salt  greatly 
influences  the  effect  on  vegetation.     The  table  on  page 

393  gives  the  composition  of  the  soluble  salts  from  a 
number  of  alkali  soils. 

A  few  years  ago  Headden  1  called  attention  to  large 
accumulations  of  nitrates  in  certain  localities  in  Colorado. 
These  salts  dissolve  in  the  soil  water  and  are  frequently 
present  in  such  large  quantities  as  to  be  injurious  to 
vegetation. 

304.  White  and  black  alkali.  —  Sulfates  and  chlorides 
of  the  alkalies,  when  concentrated  on  the  surface  of  the 
soil,  produce  a  white  incrustation,  which  is  very  common 
in  alkali  regions  during  a  dry  period  as  a  result  of  evapora- 
tion of  moisture.  Incrustations  of  this  character  are 
called  white  alkali. 

Carbonates  of  the  alkalies,  particularly  sodium  car- 
bonate, dissolve  organic  matter  from  the  soil,  thus  giving 
a  dark  color  to  the  solution  and  to  the  incrustation.  For 
this  reason  alkali  containing  large  quantities  of  these 
salts  is  called  black  alkali.  Black  or  brown  alkali  may 
also  be  produced  by  calcium  chloride  or  by  an  excess  of 
sodium  nitrate. 

1  Headden,  W.  P.  Deterioration  in  the  Quality  of  Sugar 
Beets  due  to  Nitrates  Formed  in  the  Soil.  Colorado  Agr. 
Exp.  Sta.,  Bui.   183.     1912. 


ALKALI  SALTS 


393 


Percentage  Composition  of  Alkali  Salts  in  Soils 


IdB 

O  X 

Yakima, 
Washington,3 
12-24  Inches 

Billings, 
Montana4 

Yuma, 
Arizona  5 

E 
3 
hi 
O 

1 

el 

KCl      . 

K2S04  . 

K2CO3 

Na2S04 

NaN03 

Na2C03 

NaCl    . 

Na3HP04 

MgS04 

MgCl2  . 

CaCl2   . 

NaHCOg 

CaS04 

Ca(HC03) 

Mg(HCO, 

(NII4)2CO 

2    . 

• 

1.64 

33.07 
6.61 

12.71 
17.29 

21.48 

3.95 

25.28 
19.78 
32.58 
14.75 
2.25 

1.41 

5.61 

9.73 
13.86 

36.72 

1.87 
16.48 
15.73 

• 

1.60 
85.57 

0.55 
8.90 

0.67 
2.71 

21.41 
35.12 

7.28 

4.06 

22.06 
10.07 

4.00 

81.15 

7.71 
0.25 
0.28 
6.61 

22.10 

13.77 

6.88 
3.98 

21.02 
32.25 

1  Headden,    W.    P.     The    Fixation   of   Nitrogen.     Colorado 
Agr.  Exp.  Sta.,  Bui.  155,  p.  10.     1910. 

2  Hilgard,  E.  W.     Soils,  p.  442.     New  York,  1906. 

3  Dorsey,  C.  W.     Alkali  Soils  of  the  United  States.     U.  S. 
D.  A.,  Bur.  Soils,  Bui.  35,  p.  79.     1906. 

4  Ibid.,  p.  103.  5  Ibid.,  p.  109. 


394       SOILS:    PROPERTIES  AND  MANAGEMENT 

Black  alkali  is  much  more  destructive  to  vegetation 
than  is  white.  A  quantity  of  white  alkali  that  would 
not  seriously  interfere  with  the  growth  of  most  crops 
might  completely  prevent  the  development  of  useful 
plants  if  the  alkali  were  black. 

305.  Effect  of  alkali  on  crops.  —  The  presence  of 
relatively  large  amounts  of  salts  dissolved  in  water  and 
brought  into  contact  with  a  plant  cell  has  been  shown  to 
cause  a  shrinkage  of  the  protoplasmic  lining  of  the  cell, 
the  shrinking  increasing  with  the  concentration  of  the 
solution.  This  causes  the  plant  to  wilt,  to  cease  growth, 
and  finally  to  die.  The  nature  of  the  salt,  and  the  species 
and  even  the  individuality  of  the  plant,  determine  the 
point  of  concentration  at  which  the  plant  succumbs. 

The  directly  injurious  effect  of  the  chlorides,  sulfates, 
nitrates,  and  other  salts  of  the  alkalies  and  alkali  earths 
is  due  to  this  action  on  the  cell  contents  of  the  plants. 
The  carbonates  of  the  alkalies  have,  in  addition,  a  cor- 
roding effect  on  the  plant  tissues,  dissolving  the  parts 
of  the  plant  with  which  they  come  in  contact.  Indirectly 
alkali  salts  may  injure  plants  by  their  influence  on  the 
soil  tilth,  soil  organisms,  and  fungous  and  bacterial 
diseases. 

306.  Effect  on  different  plants.  —  The  factors  that 
determine  the  tolerance  of  plants  toward  alkali  are : 
(1)  the  physiological  constitution  of  the  plant;  (2)  the 
rooting  habit.  The  first  is  not  well  understood,  but 
resistance  varies  with  species,  and  even  with  individuals 
of  the  same  species.  So  far  as  the  rooting  habit  influences 
tolerance  of  alkali,  the  advantage  is  with  the  deep-rooted 
plants  such  as  alfalfa  and  sugar  beets,  probably  because 
a  part  of  the  root  is  in  a  less  strongly  impregnated  part 
of  the  soil. 


ALKALI  SALTS 


395 


Of  the  cereals,  barley  and  oats  are  the  most  tolerant, 
these  being  able  in  some  cases  to  produce  good  crops  on 
soil  containing  two-tenths  per  cent  of  white  alkali.  Of  the 
forage  crops,  a  number  of  valuable  grasses  are  able  to 
grow  on  soil  containing  considerably  more  than  two-tenths 
per  cent  of  alkali.  Timothy,  smooth  brome,  and  alfalfa 
are  the  cultivated  forage  plants  most  tolerant  of  alkali, 
although  they  do  not  equal  the  native  grasses  in  this 
respect.  Cotton  also  tolerates  a  considerable  amount  of 
alkali. 

Lough  ridge,1  after  experiments  and  observation  for  a 
number  of  years,  has  obtained  data  regarding  the  resist- 
ance of  various  crops  to  the  several  alkali  salts.  His 
results  are  given  in  part  below,  expressed  in  pounds  to 
an  acre  to  a  depth  of  four  feet :  — 


Crop 

Na2S04 

Na2C03 

NaCl 

Total  Alkali 

Grapes     .     .     . 

40,800 

7,550 

9,640 

45,760 

Oranges  .     .     . 

18,600 

3,840 

3,360 

21,840 

Pears 

17,800 

1,760 

1,360 

20,920 

Apples 

14,240 

640 

1,240 

16,120 

Peaches  .     .     . 

9,600 

680 

1,000 

11,280 

Rye     ...     . 

9,800 

960 

1,720 

12,480 

Barley      .     .     . 

12,020 

12,170 

5,100 

25,520 

Sugar  beets 

52,640 

4,000 

5,440 

59,840 

Sorghum       .     . 

61,840 

9,840 

9,680 

81,360 

Alfalfa     .     .     . 

102,480 

2,360 

5,760 

110,320 

Saltbush       .     . 

125,640 

18,560 

12,520 

156,720 

1  Loughridge,  R.  H.  Tolerance  of  Alkali  by  Various  Cul- 
tures. California  Agr.  Exp.  Sta.,  Bui.  133.  1901.  See  also 
Kearney,  T.  H.,  and  Harter,  L.  L,  Comparative  Tolerance  of 
Various  Plants  for  the  Salts  Common  in  Alkali  Soils.  U.  S.  D.  A., 
Bur.  Plant  Indus.,  Bui.  113.     1907. 


396     SOILS:  properties  and  management 

Although  in  general  the  results  as  to  the  resistance  to 
alkali  of  the  various  crops  are  so  conflicting,  the  Bureau 
of  Soils,1  in  its  alkali  mapping,  has  been^able  to  make  ■ 
rough  classification  as  follows  :  — 


Percentage  of  Total 

Salts 

Percentage  op 
Black  Alkali 

Crops 

0  to  0.20 
0.20  to  0.40 
0.40  to  0.60 

0.60  to  1.00 
1.00  to  3.00 

Less  than  0.05 
0.05  to  0.10 
0.10  to  0.20 

0.20  to  0.30 
0.30  and  above 

All  crops  grow 

All  but  most  sensitive 

Old   alfalfa,    sugar   beet, 

barley,  and  sorghum 
( )nl y  most  resistant  plants 
No  plants 

307.  Other  conditions  that  influence  the  action  of 
alkali.  —  The  higher  the  water  content  of  the  soil,  the 
less  is  the  injury  to  plants  from  alkali ;  but  should 
the  same  soil  become  dry,  the  previous  large  quantity  of 
water  would,  by  bringing  into  solution  a  larger  amount 
of  alkali,  render  the  solution  stronger  than  it  would 
otherwise  have  been,  and  thus  cause  greater  injury  (see 
Fig.  57). 

The  distribution  of  the  alkali  at  different  depths  may 
have  an  important  bearing  on  its  effect  on  plants. 
Young  plants  and  shallow-rooted  crops  may  be  entirely 
destroyed  by  the  concentration  of  alkali  at  the  surface, 
while  the  same  quantity  evenly  distributed  through  the 
soil,  or  carried  by  moisture  to  a  lower  depth,  would  have 
caused  no  injury.  A  loam  soil,  by  reason  of  its  greater 
water-holding  capacity  and  adsorptive  power,  will  carry 
more  alkali  without  injury  to  plants  than  will  a  sandy 


1  Dorsey,  C.  W.     Alkali  Soils  of  the  United  States. 
D.  A.,  Bur.  Soils,  Bui.  35,  pp.  23-25.     1936. 


U.  S. 


ALKALI  SALTS 


397 


soil.  Certain  of  the  alkali  salts  exert  a  deflocculating 
action  on  clay  soils  and  effect  an  indirect  injury  in  that 
way. 


Fig.  57. — Diagram  showing  the  amount  and  composition  of  alkali  salts 
at  various  depths.     Tulare,  California. 


308.  Accumulation  of  alkali.  —  The  alkali  salts,  being 
readily  soluble,  are  carried  by  the  soil  water  where  there 
is  any  lateral  movement,  as  is  often  the  case  where  land 
slopes  to  some  one  point.  Low-lying  lands  adjacent  to 
such  slopes  are  thus  likely  to  contain  considerable  alkali, 
and  the  "  alkali  spots "  of  semiarid  regions  and  the 
large  accumulations  of  alkali  in  many  of  the  valley  lands 
of  arid  regions  are  traceable  to  this  cause. 

309.  Irrigation  and  alkali.  —  In  irrigated  regions,  the 
injurious  effect  of  alkali  is  in  many  cases  discovered  only 


398       SOILS:    PROPERTIES  AND  MANAGEMENT 

after  irrigation  has  been  practiced  for  a  few  years.  This 
is  due  to  what  is  known  asa  "  rise  of  alkali,"  and  comes 
about  through  the  accumulation,  near  the  surface  of  the 
soil,  of  salts  that  were  formerly  distributed  throughout 
a  depth  of  perhaps  many  feet.  Before  the  land  was 
irrigated,  the  rainfall  penetrated  only  a  slight  depth  into 
the  soil,  and  when  evaporation  took  place,  salts  were 
drawn  to  the  surface  from  only  a  small  volume  of  soil. 
When,  however,  irrigation  water  is  turned  on  the  land, 
the  soil  becomes  wet  to  a  depth  of  perhaps  fifteen  or 
twenty  feet.  During  the  portion  of  the  year  in  which 
the  soil  is  allowed  to  dry,  large  quantities  of  salts  are 
carried  toward  the  surface  by  the  upward-moving  capil- 
lary water.  Although  these  salts  are  in  part  carried 
down  again  by  the  next  irrigation,  the  upward  movement 
constantly  exceeds  the  downward  one.  This  is  because 
the  descending  water  passes  largely  through  the  non- 
capillary  interstitial  spaces,  while  the  ascending  water 
passes  entirely  through  the  capillary  spaces.  The  smaller 
spaces,  therefore,  contain  a  considerable  quantity  of 
soluble  salts  after  the  downward  movement  ceases  and 
the  upward  movement  begins.  In  other  words,  the 
volume  of  water  carrying  the  salts  downward  in  the 
capillary  spaces  is  less  than  that  carrying  them  upward 
through  these  spaces.  Surface  tension  causes  the  salts 
to  accumulate  largely  in  the  capillary  spaces,  and  it  is 
therefore  the  direction  of  the  principal  movement  through 
these  spaces  that  determines  the  point  of  accumulation 
of  the  alkali. 

There  are  large  areas  of  land  in  Egypt,  in  India,  and 
even  in  France  and  Italy,  as  well  as  in  this  country,  that 
have  suffered  in  this  way,  and  not  infrequently  they  have 
reverted  to  a  desert  state. 


ALKALI  SALTS  399 

310.  The  handling  of  alkali  lands.1  —  Ordinarily  there 
are  two  general  ways  in  which  alkali  lands  may  be  handled 
in  order  to  avoid  the  injurious  effect  of  soluble  salts. 
The  first  of  these  is  eradication,  the  second  may  be 
designated  as  control.  In  the  former  case,  an  at- 
tempt is  made  to  actually  eliminate  by  various  means 
some  of  the  alkali.  In  the  latter,  methods  of  soil 
management  are  employed  which  will  keep  the  salts 
well  distributed  throughout  the  soil.  In  many  cases 
soils  would  grow  excellent  crops  if  the  alkali  could 
only  be  kept  well  distributed  through  the  soil  layers 
so  that  no  toxic  action  could  occur,  at  least  within 
the  root  zone.  In  general,  steps  should  always  be  taken 
toward  the  control  of  alkali,  whether  eradication  is  at- 
tempted or  not.  Under  irrigation,  careful  control  is 
always  wise. 

311.  Eradication  of  alkali.  —  Of  methods  designed  to 
at  least  partially  free  the  soil  of  alkali,  the  commonest 
are :  (1)  leaching  with  underdrainage,  (2)  correction  with 
gypsum,  (3)  scraping,  and  (4)  flushing. 

312.  Leaching  with  underdrainage.  —  Of  the  various 
methods  for  removing  an  excess  of  soluble  salts,  the  use 
of  tile  drains  is  the  most  thorough  and  satisfactory. 
When  this  method  is  used  in  an  irrigated  region,  heavy 
and  repeated  applications  of  water  must  be  made,  to 
leach  out  the  alkali  from  the  soil  and  drain  it  off  through 
the  tile.     When  used  for  the  amelioration  of  alkali  spots 

1  Dorsey,  C.  W.  Reclamation  of  Alkali  Soils.  U.  S.  D.  A., 
Bur.  Soils,  Bui.  34.  1906.  Also,  Hilgard,  E.  W.  Utilization 
and  Reclamation  of  Alkali  Lands,  Soils.  New  York,  1911. 
Also,  Brown,  C.  F.,  and  Hart,  R.  A.  Reclamation  of  Seeped 
and  Alkali  Lands.  Utah  Agr.  Exp.  Sta.,  Bui.  111.  1910. 
Also,  Dorsey,  C.  W.  Reclamation  of  Alkali  Soils  at  Billings, 
Montana.     U.  S.  D.  A.,  Bur.  Soils,  Bui.  44.     1907. 


400       SOILS:    PROPERTIES  AND  MANAGEMENT 

in  a  semiarid  region,  the  natural  rainfall  will  in  time 
effect  the  removal. 

In  laying  tiles  it  is  necessary  to  have  them  at  such  a 
depth  that  soluble  salt  in  the  soil  beneath  them  will  not 
readily  rise  to  the  surface.  This  will  depend  on  those 
properties  of  the  soil  governing  the  capillary  move- 
ment of  water.  Three  or  four  feet  in  depth  is  usually 
sufficient,  but  the  capillary  movement  should  first  be 
estimated. 

After  the  drains  have  been  placed,  the  land  is  flooded 
with  water  to  a  depth  of  several  inches.  The  water  is 
allowed  to  soak  into  the  soil  and  to  pass  off  through  the 
drains,  leaching  out  part  of  the  alkali  in  the  process. 
Before  the  soil  has  time  to  become  very  dry  the  flooding 
is  repeated,  and  the  operation  is  kept  up  until  the  land 
is  brought  into  a  satisfactory  condition. 

Crops  that  will  stand  flooding  may  be  grown  during 
this  treatment,  and  they  will  serve  to  keep  the  soil 
from  puddling,  as  it  is  likely  to  do  if  allowed  to  become 
dry  on  the  surface.  If  crops  are  not  grown,  the  soil 
should  be  harrowed  between  floodings.  The  operation 
should  not  be  carried  to  a  point  where  the  soluble 
salts  are  reduced  below  the  needs  of  the  crop,  or  so 
low  that  they  lose  entirely  their  effect  on  the  retention 
of  moisture. 

313.  Correction  with  gypsum.  —  The  use  of  gypsum 
on  black  alkali  land  has  sometimes  been  practiced  for 
the  purpose  of  converting  the  alkali  carbonates  into 
sulfates,  thus  ameliorating  the  injurious  properties  of 
the  alkali  without  decreasing  the  amount.  The  quantity 
of  gypsum  required  may  be  calculated  from  the  amount 
and  composition  of  the  alkali.  The  soil  must  be  kept 
moist,   in  order  to  bring  about  the  reaction,   and  the 


ALKALI  SALTS  401 

gypsum  should  be  harrowed  into  the  surface,  not  plowed 
under.     The  reaction  is  as  follows  :  — 

Na2C03  +  CaS04  =  CaC03  +  Na2S04 

When  soil  containing  black  alkali  is  to  be  tile-drained, 
it  is  recommended  that  the  land  shall  first  be  treated 
with  gypsum,  as  the  substitution  of  alkali  sulfates  for 
carbonates  causes  the  soil  to  assume  a  much  less  compact 
condition  and  thus  facilitates  drainage.  It  also  prevents 
the  loss  of  organic  matter  dissolved  by  the  carbonate  of 
soda  and  the  soluble  phosphates,  both  of  which  are  pre- 
cipitated by  the  change. 

314.  Scraping.  —  Removal  of  the  alkali  incrustation 
that  has  accumulated  at  the  surface  is  sometimes  re- 
sorted to.  Very  often  the  rise  of  alkali  is  encouraged 
by  applications  of  irrigation  water,  which  is  allowed 
to  evaporate  unretarded.  The  salts  are  thus  carried 
upward  by  the  capillary  movement  of  the  soil  water. 
This  method  of  alkali  eradication  is  never  very  effi- 
cient, and  is  often  dangerous,  as  it  encourages  the 
presence  of  very  large  amounts  of  alkali  salts  in  the  sur- 
face soil. 

315.  Flushing.  —  Often  alkali  accumulations  may  be 
washed  from  the  soil  surface  by  turning  on  a  rapidly 
moving  stream  of  water.  The  texture  of  the  soil,  as  well 
as  the  slope  of  the  land,  must  be  just  right  for  such  a 
procedure.  Generally  so  much  water  enters  the  soil 
that  the  land  remains  heavily  impregnated  with  alkali 
salts.  Both  this  method  and  the  previous  one,  even 
if  successful,  are  only  temporary.  Moreover,  lands 
carrying  so  much  alkali  as  to  admit  of  either  one  of  these 
procedures  may  be  so  heavily  charged  as  never  to  yield 
to  any  form  of  either  eradication  or  control. 

2d 


402       SOILS:    PROPERTIES  AND  MANAGEMENT 

316.  Control  of  alkali.  —Where  excessive  amounts  of 
soluble  salts  do  not  exist  in  a  soil,  the  control  of  the  alkali 
with  a  view  of  keeping  it  well  distributed  in  the  soil 
column  is  the  best  practice.  The  retardation  of  evapora- 
tion is,  of  course,  the  main  object  in  this  procedure.  The 
intensive  use  of  the  soil  mulch  is  therefore  to  be  advocated, 
especially  in  all  irrigation  operations  where  alkali  concen- 
trations are  likely  to  occur.  Such  a  method  of  soil 
management  not  only  saves  moisture,  but  also  prevents 
the  excessive  translocation  of  soluble  salts  into  the  root 
zone.  This  method  of  control  is  the  most  economical, 
the  cheapest,  and  the  one  to  be  advocated  on  all  occasions, 
no  matter  what  may  have  been  the  previous  means  of 
dealing  with  the  alkali  situation. 

317.  Cropping  with  tolerant  plants.  —  Certain  soils 
that  are  strongly  impregnated  with  alkali  may  be  grad- 
ually improved  by  cropping  with  sugar  beets  and  other 
crops  that  are  tolerant  of  alkali  and  that  remove  large 
quantities  of  salts.  This  is  more  likely  to  be  efficacious 
where  irrigation  is  not  practiced.  Certain  crops,  more- 
over, while  somewhat  seriously  injured  while  young,  are 
very  resistant  once  their  root  systems  are  developed. 
A  good  example  is  alfalfa,  the  young  plants  being  very 
tender  while  the  more  mature  ones  are  extremely  resistant. 
Temporary  eradication  of  alkali  may  allow  such  a  crop  to 
be  established.  It  will  then  maintain  itself  in  spite  of  the 
concentrations  that  may  later  occur. 

318.  Alkali  spots.  —  In  semiarid  regions  small  areas  of 
alkali  are  often  found,  varying  from  a  few  square  yards 
to  several  acres  in  size.  The  quantities  of  alkali  in  these 
are  usually  not  sufficient  to  prevent  the  growth  of  plants 
in  years  of  good  rainfall,  but  in  periods  of  drought  the 
concentration  of  the  salts  and  the  compact  condition  that 


ALKALI  SALTS  403 

they  tend  to  produce  combine  to  injure  the  crop.  The 
methods  already  mentioned  for  treating  alkali  land  are 
of  service  on  these  small  areas,  and,  in  addition,  the 
plowing-under  of  fresh  farm  manure  has  been  found  to 
improve  their  productiveness.  This,  with  surface  drain- 
age, deep  tillage,  and  good  cultivation  in  order  to  prevent 
the  soil  from  drying  out,  will  usually  remedy  the  difficulty. 
In  many  cases  these  spots  become  highly  productive  under 
proper  treatment. 


CHAPTER   XIX 

ABSORPTION   OF    NUTRITIVE   SALTS    BY 
AGRICULTURAL  PLANTS 

All  the  salts  taken  up  by  the  roots  of  agricultural 
plants  are  in  solution  when  absorbed.  The  movement 
into  the  root  thus  depends  on  the  presence  of  moisture, 
which  is  the  medium  of  transfer.  The  root-hairs  are  the 
great  absorbing  organs  of  the  plant,  and  through  the 
cells  of  their  delicate  tissues  the  solutions  of  the  various 
salts  are  passed. 

319.  How  plants  absorb  nutrients.  —  The  nature  and 
quantity  of  material  absorbed  by  a  plant  is  determined 
by  the  law  of  diffusion.  From  the  cells  of  the  root-hairs 
the  dissolved  salts  are  transferred  to  other  parts  of  the 
plant,  where  they  undergo  the  metabolic  processes  that 
determine  which  constituents  shall  be  retained  in  the 
tissues  of  the  plant.  The  unused  ions  that  remain  in 
the  plant  juices  prevent  by  their  presence  the  further 
absorption  of  those  particular  substances  from  the  soil 
water.  It  thus  happens  that  the  composition  of  the 
ash  of  a  plant  may  be  very  different  from  that  of  the 
substances  presented  to  it  in  solution.  For  example, 
aluminium,  although  always  present  in  the  soil  in  a  very 
slightly  soluble  form,  is  present  in  mere  traces  in  the  ash 
of  most  plants.  On  the  other  hand,  iodine,  although 
present  in  sea  water  only  in  the  most  minute  quantities, 
is  present  in  large  quantities  in  the  ash  of  certain  marine 
algse. 

404 


ABSORPTION   OF  NUTRITIVE  SALTS  405 

A  plant  will,  in  general,  take  up  more  of  a  nutritive 
substance  if  it  is  presented  in  large  amount,  as  compared 
with  the  other  soluble  substances  in  the  nutrient  solution, 
than  if  it  is  presented  in  small  amount.  Thus,  the  per- 
centage of  nitrogen  in  maize,  oats,  and  wheat  may  be 
increased  by  increasing  the  ratio  of  nitrogen  to  other 
nutritive  substances  in  the  nutrient  media.  This  is 
also  true  of  potassium  and  phosphorus,  respectively. 
This  fact  is  accounted  for  by  the  maintenance  of  the 
d.ffusive  equilibrium  at  a  higher  level  for  a  particular 
ion  which  is  relatively  abundant  in  the  nutrient  solution, 
thus  preventing  the  return  of  the  excess  from  the  plant. 

320.  Relation  between  root-hairs  and  soil  particles.  — 
In  a  rich,  moist  soil  the  number  of  root-hairs  is  very 
great,  while  in  a  poor  or  a  very  dry  soil  or  in  a  saturated 
soil  there  are  comparatively  few  root-hairs.  The  con- 
nection between  the  root-hairs  and  the  soil  particles  is 
extremely  intimate.  When  in  contact  with  a  particle 
of  soil,  a  root-hair  in  many  cases  almost  incloses  it,  and 
by  means  of  its  mucilaginous  wall  forms  a  contact  so  close 
as  practically  to  make  the  solution  between  the  particle 
and  the  cell  wall  distinct  from  that  between  the  soil 
particles  themselves. ' 

There  has  been  considerable  difference  of  opinion  as  to 
how  a  plant  can  obtain  its  mineral  nutrients  from  a  sub- 
stance so  difficultly  soluble  as  the  soil.  This  has  arisen 
because  of  the  conflicting  nature  and  the  inadequate 
character  of  the  data  available. 

321.  Liebig  and  Sachs  on  solvent  action  of  plant 
roots.  —  Liebig  l  called  attention  to  the  fact  that  a  plant 
may  obtain  one  hundred  times  as  much  phosphorus  and 

1  Liebig,  J.     Die  Chemie  in  Ihrer  Anwendung  auf  Agrikultur. 

1862. 


406       SOILS:    PROPERTIES  AND  MANAGEMENT 

nitrogen  and  fifty  times  as  much  potassium  as  can  be 
extracted  from  the  same  volume  of  soil  with  pure  water 
or  with  water  containing  carbon  dioxide.  It  has,  of 
course,  been  recognized  that  the  soil  water  is  aided  in  its 
solvent  action  by  a  variety  of  substances  that  may  be 
normally  present  in  solution,  beginning  with  the  gases 
taken  up  by  rain  in  its  descent  through  the  atmosphere, 
and  further  added  to  by  the  carbon  dioxide  and  the  or- 
ganic and  mineral  substances  obtained  from  the  soil.  It 
has  been  held  that  the  plant  roots  aid  solution  of  mineral 
matter  by  excretion  of  acids,  which  act  effectively  as 
solvents.  The  well-known  root  tracings  on  limestone 
and  marble  have  been  taken  as  proof  of  the  excretion  of 
such  acids.  Sachs,1  and  later  other  investigators,  grew 
plants  of  various  kinds  in  soil  and  other  media  in  which 
was  placed  a  slab  of  polished  marble  or  dolomite  or  cal- 
cium phosphate,  covered  with  a  layer  of  washed  sand. 
After  the  plants  had  made  sufficient  growth  the  slabs  were 
removed,  and  on  the  surfaces  were  found  corroded  trac- 
ings, corresponding  to  the  lines  of  contact  between  the 
rootlets  and  the  minerals. 

322.  Czapek's  experiments.  —  In  order  to  test  this 
theory,  Czapek 2  repeated  the  experiments  of  Sachs, 
using  plates  of  gypsum  mixed  with  the  ground  mineral 
that  he  wished  to  test,  and  this  mixture  he  spread  over  a 
glass  plate.  Using  these  plates  in  the  same  manner  as 
previously  described,  Czapek  found  that,  while  plates  of 
calcium  carbonate  and  of  calcium  phosphate  were  cor- 
roded by  the  plant  roots,  plates  of  aluminium  phosphate 

1  Sachs,   J.     Aufiosung  des    Marmors   durch   Mais-Wurzeln 
Bot.  Zeitung,  18  Jahrgang,  Seite  117-119.     1860. 

2  Czapek,  J.  Zur  Lehre  von  den  Wurzelausscheidung.  Jahrb. 
f.  Wiss.  Bot.,  Band  29,  Seite  321-390.     1896. 


ABSORPTION  OF  NUTRITIVE  SALTS  407 

were  not.  He  concludes  that  if  the  tracings  are  due  to 
acids  excreted  by  the  plant  roots,  the  acids  so  excreted 
must  be  those  that  have  no  solvent  action  on  aluminium 
phosphate.  This  would  limit  the  excreted  acids  to  car- 
bonic, acetic,  propionic,  and  butyric.  Czapek  also  re- 
plies to  the  argument  that  the  acids  producing  the  tracings 
must  be  non-volatile  ones  because  of  the  definite  lines 
made  in  the  mineral,  by  stating  that  the  excretion  of 
carbon  dioxide  alone  would  be  sufficient  to  account  for 
the  observations  since  it  dissolves  in  water  to  form  car- 
bonic acid,  and  that  carbonic  acid  is  always  present  in 
the  cell  walls  of  the  root  epidermis.  By  means  of  micro- 
chemical  analyses  of  the  exudations  of  root-hairs  grown 
in  a  water-saturated  atmosphere,  Czapek  found  potassium, 
magnesium,  calcium,  phosphorus,  and  chlorine  in  the 
exudate.  He  concludes  that  the  solvent  action  of  plant 
roots  is  due  to  acid  salts  of  mineral  acids,  particularly 
acid  potassium  phosphate.  He  has  not  proved,  however, 
that  the  exudations  were  not  from  dead  root-hairs  nor 
from  the  dead  cells  of  the  rootcap.  In  either  case  they 
would  have  some  solvent  action,  but  whether  sufficient 
to  make  them  of  importance  is  doubtful. 

323.  Secretion  of  an  oxidizing  enzyme  by  plant  roots.  — 
Molisch  1  found  that  root-hairs  secrete  a  substance  having 
properties  corresponding  to  those  of  an  oxidizing  enzyme. 
His  work  has  been  repeated  by  others  who  have  failed  to 
obtain  similar  results,  but  lately  Schreiner  and  Reed  2  have 

1  Molisch,  H.  Ueber  Wurzelausscheidungen  und  deren 
Einwirkung  auf  Organische  Substanzen.  Sitzungsber.  Akad. 
Wiss.  Wien-Math.  Nat.,  Band  96,  Seite  84-109.  1888.  Ab- 
stract in  Chem.  Centrlb.,  Band  18,  Seite  1513,  1888,  and  in 
Centrlb.  f.  Agr.  Chem.,  Band  17,  Seite  428,  1888. 

2  Schreiner,  Oswald,  and  Reed,  H.  S.  Studies  on  the  Oxidiz- 
ing Powers  of  Roots.     Bot.  Gazette,  Vol.  47,  p.  355.     1909. 


408       SOILS:    PROPERTIES  AND  MANAGEMENT 

demonstrated  an  oxidizing  action  of  roots  that  is  appar- 
ently due  to  a  peroxidase.  Oxidation  alone,  however, 
would  hardly  suffice  to  account  for  the  solvent  action 
accompanying  the  development  of  plant  roots,  although 
it  is  doubtless  an  important  function  and  useful  in  other 
ways. 

324.  Importance  of  carbon  dioxide  as  a  solvent.  — 
Stoklasa  and  Ernst x  have  contributed  much  to  this  sub- 
ject during  the  last  decade.  Stoklasa's  earlier  experiments, 
conducted  by  maintaining  the  plant  roots  in  a  saturated 
atmosphere,  gave  only  carbon  dioxide  in  the  exudate. 
In  this  he  is  in  agreement  with  most  of  the  recent  inves- 
tigators of  this  subject.  Stoklasa  emphasizes  the  im- 
portance of  carbon  dioxide  as  a  solvent  by  showing  the 
quantity  produced  by  plants  and  by  microorganisms. 
He  estimates  that  in  one  acre  of  soil  to  a  depth  of  sixteen 
inches  there  are  sixty-eight  pounds  of  carbon  dioxide 
produced  by  bacterial  respiration  in  two  hundred  days, 
and  fifty-four  pounds  of  carbon  dioxide  excreted  by  plant 
roots  in  one  hundred  days;  these  periods  he  considers 
as  representing  the  year's  activity  of  bacteria  and  higher 
plants. 

In  later  experiments,  Stoklasa  and  Ernst 2  found  that 
when  plants  do  not  have  a  sufficient  supply  of  oxygen  in 
the  air  surrounding  their  roots,  they  secrete  acetic  and 
formic  acids  from  the  root-hairs.  These  investigators 
believe  that  these  acids  are  toxic  rather  than  beneficial, 


1  Stoklasa,  J.,  and  Ernst,  A.  Ueber  den  Ursprung  die  Menge 
und  die  Bedeutnng  des  Kohlendioxvds  im  Boden.  Centrlb.  f. 
Bakt.,  II,  Band  14,  Seite  723-736.    *1905. 

2  Stoklasa,  J.,  and  Ernst,  A.  Beitrage  zur  Losung  der  Frage 
der  Chemischen  Natur  des  Wurzelsekretes.  Jahrb.  f .  Wiss. 
Bot.,  Band  46,  Seite  55-102.     1908-1909. 


ABSORPTION   OF  NUTRITIVE  SALTS  409 

and  that  they  are  responsible,  in  large  measure,  for  the 
injurious  effect  on  plants  of  a  very  compact  condition  of 
soil.  In  the  same  communication  these  authors  report 
an  experiment  in  which  it  was  found  that  the  kinds  of 
plants  that  excrete  the  largest  quantity  of  carbon  dioxide 
from  their  roots  are  the  ones  that  absorb  the  greatest 
quantities  of  phosphorus  from  gneiss  and  from  basalt. 
This,  however,  does  not  necessarily  connote  any  conse- 
quential relation  between  these  physiological  functions. 

Barakov  1  drew  air  through  planted  soils  contained  in 
large  tanks.  He  found  that  the  maximum  production 
of  carbon  dioxide  occurred  at  the  time  when  the  plants 
were  blossoming,  whether  the  plants  blossomed  early 
or  late  in  the  season.  This  he  considered  to  indicate 
that  the  plant  assists  most  vigorously  in  the  solution  of 
nutrient  materials  at  the  time  when  it  is  most  active  in 
absorbing  them. 

325.  Insufficiency  of  carbon  dioxide.  —  Pfeiffer  and 
Blanck2  passed  carbon  dioxide  through  soil  contained  in 
vessels  in  which  plants  were  growing.  The  soil  in  some  ves- 
sels contained  a  difficultly  soluble  tricalcic  phosphate,  that 
in  other  vessels  the  more  easily  soluble  dicalcic  phosphate, 
and  that  in  still  other  vessels  was  unfertilized.  Another 
set  of  vessels  having  the  same  fertilizer  treatment  received 
no  carbon  dioxide.  The  soil  receiving  carbon  dioxide 
produced  larger  yields  of  dry  matter  and  phosphorus  in 

1  Barakov,  F.  The  Carbon  Dioxide  Content  of  Soils  at 
Different  Periods  of  Plant  Growth.  Jour.  Exp.  Agr.  (Russian), 
Vol.  11,  pp.  321-342.  1910.  The  authors  are  indebted  to 
Dr.  J.  Davidson  for  a  translation  of  this  paper. 

2  Pfeiffer,  Th.,  and  Blanck,  E.  Die  .  Saureausscheidung 
der  Wurzeln  und  die  Loslichkeit  der  Bodennahrstoffe  in  Koh- 
lensaurehaltigem  Wasser.  Landw.  Vers.  Stat.,  Band  77,  Seite 
217-268.     1912. 


410       SOILS:    PROPERTIES  AND  MANAGEMENT 

the  crop  on  the  soil  to  which  dicalcic  phosphate  had  been 
applied  than  did  the  soil  not  receiving  carbon  dioxide; 
but  the  soil  to  which  no  phosphate  was  added  yielded 
equally  well  whether  it  received  carbon  dioxide  or  not. 
The  plants  used  were  oats,  peas,  and  lupines.  These 
investigators  conclude  that  carbon  dioxide  is  not  a  suffi- 
cient solvent  to  account  for  the  mineral  nutrients  obtained 
from  soils  by  plants. 

326.  The  present  status  of  the  question.  —  The  avail- 
able evidence  on  excretion  of  acids  other  than  carbonic 
by  the  roots  of  plants  does  not  admit  of  any  very  satis- 
factory conclusion  as  to  their  relative  importance  in  the 
acquisition  of  plant-food  materials.  There  can  be  no 
doubt,  however,  that  carbon  dioxide  resulting  from  root 
exudation  and  from  decomposition  of  organic  matter  in 
the  soil  plays  a  very  prominent  part  in  this  operation. 
The  very  large  quantity  of  carbon  dioxide  in  the  soil, 
amounting  in  some  cases  to  from  .5  to  nearly  10  per  cent 
of  the  soil  air,  or  several  hundred  times  that  of  the  at- 
mospheric air,  must  aid  greatly  in  dissolving  the  soil 
particles. 

Whatever  may  be  the  concentration  of  the  soil  water, 
it  seems  probable  that  the  liquid  which  is  found  where 
the  root-hair  comes  in  contact  with  the  soil  particle,  and 
which  is  separated,  in  part  at  least,  from  the  remainder 
of  the  soil  water,  must  have  a  density  much  greater  than 
that  found  elsewhere  in  the  soil.  That  portion  of  the 
soil  water  immediately  in  contact  with  the  soil  grain  is  a 
much  stronger  solution  than  the  water  farther  from  the 
soil  surfaces,  because  of  the  adsorptive  action  of  the 
particles. 

Many  plants  grown  in  solutions  of  nutritive  salts  have 
few  or  no  root-hairs,  but  absorb  through  the  epidermal 


ABSORPTION   OF  NUTRITIVE  SALTS  411 

tissue  of  the  roots.  If  the  plant  depended  wholly  on  the 
prepared  solution  in  the  soil  water,  a  similar  structure 
would  doubtless  suffice.  The  special  modification  by 
which  the  root-hairs  come  in  intimate  contact  with  the 
soil  particle  and  almost  surround  it,  indicates  a  direct 
relation  between  the  soil  particles  and  the  plant,'  and 
not  merely  between  the  soil  solution  and  the  plant. 

New  root-hairs  are  constantly  being  formed,  and  the 
old  ones  become  inactive  and  disappear.  The  contact 
of  a  root-hair  with  a  soil  particle  is  not  long-continued. 
Whether  the  period  of  contact  is  determined  by  the 
ability  of  the  root  to  absorb  nutriment  from  the  particle 
is  not  known.  Certain  it  is  that  only  a  small  portion  of 
the  particle  is  removed. 

327.  Possible  root  action  on  colloidal  complexes.  — 
It  has  already  been  stated  that  there  is  some  evidence 
to  lead  to  the  belief  that  the  surfaces  of  soil  particles  are 
covered  to  a  large  extent  with  colloidal  complexes,  com- 
posed of  both  organic  and  inorganic  matter  having  vigor- 
ous absorptive  properties  and  holding  the  bases  and  phos- 
phorus in  an  absorbed  condition.  Roots  of  growing 
plants  have  been  found  to  cause  coagulation  of  at  least 
some  colloids,  possibly  by  leaving  an  acid  residue  in  the 
nutrient  solution  by  reason  of  the  selective  absorption  of 
bases  and  rejection  of  the  acids  of  the  dissolved  salts. 
It  is  conceivable  that  the  root-hair,  by  removing  bases 
from  the  solution  existing  between  the  cell  wall  and  the 
colloidal  covering  of  the  soil  particle,  may  cause  coagula- 
tion of  the  colloidal  matter  and  thus  liberate  the  plant- 
food  materials  held  by  absorption.  The  liberated  ma- 
terial, being  of  a  readily  soluble  nature,  would  be  taken 
up  by  the  solution  between  the  rootlet  and  the  soil  particle, 
from  which  the  root-hair  could  readily  absorb  it.     Such 


412       SOILS:    PROPERTIES  AND  MANAGEMENT 

an  hypothesis  would  account  for  the  ability  of  plants 
to  obtain  a  quantity  of  nutrient  materials  far  in  excess  of 
what  can  be  accounted  for  by  the  solvent  action  of  pure 
water,  and  even  beyond  what  many  investigators  are 
willing  to  attribute  to  the  solvent  action  of  water  charged 
with  carbon  dioxide. 

328.  Why  crops  vary  in  their  absorptive  powers.  — 
As  has  already  been  pointed  out  (pars.  331-336),  crops  of 
different  kinds  vary  greatly  in  their  ability  to  draw 
nourishment  from  the  soil.  The  difference  between  the 
nitrogen,  phosphorus,  and  potassium  taken  up  by  a  corn 
crop  of  average  size  and  a  wheat  crop  of  average  size  is 
very  striking.  In  the  table  on  page  419  it  is  seen  that 
two  tons  of  red  clover  contain  three  times  as  much  potash, 
nearly  ten  times  as  much  lime,  and  somewhat  more  phos- 
phoric acid  than  does  a  crop  of  thirty  bushels  of  wheat 
including  the  straw. 

The  difference  in  absorbing  power  may  be  due  to  either 
one  or  both  of  two  causes :  (1)  a  larger  absorbing  system ; 
(2)  a  more  active  absorbing  system.  The  former  is  deter- 
mined by  the  extent  of  the  root-hair  surfaces;  the  latter 
by  the  intensity  of  the  absorbing  action. 

329.  Extent  of  absorbing  system.  —  Plants  with  large 
root  systems  may  be  expected  to  absorb  the  larger  amounts 
of  nutrients  from  the  soil.  Such  is  usually  the  case, 
although  the  extent  of  the  root  system  is  not  necessarily 
proportional  to  the  total  area  of  the  absorbing  surfaces 
of  the  root-hairs. 

330.  Absorptive  Activity.  —  The  absorptive  activity  of 
a  plant  under  any  given  condition  of  soil  and  climate  de- 
pends on  :  (l)  the  rapidity  and  completeness  with  which 
the  pilant  elaborates  the  substances  taken  from  the  soil 
into  plant  substance,  or  otherwise  removes  them   from 


ABSORPTION  OF  NUTRITIVE  SALTS  413 

solution ;  (2)  the  extent  to  which  the  exudations  from  the 
root-hairs  —  whether  these  be  carbon  dioxide,  salts  of 
mineral  acids,  or  organic  acids  —  act  on  the  soil  particles. 
.  The  first  of  these  is  a  function  of  the  vital  energy  of  the 
plant  and  its  ability  to  utilize  sunshine  and  carbon  dioxide 
to  produce  organic  matter.  It  may  be  compared  to. the 
property  which  enables  one  animal  to  do  more  work  than 
another  animal  of  the  same  weight  on  a  similar  ration. 

The  removal  from  the  ascending  water  current  in  the 
plant  of  substances  derived  from  the  soil  is  accomplished 
in  the  leaves.  By  the  dissociation  of  these  substances, 
ions  are  constantly  furnished  for  metabolism  into  materials 
that  may  be  built  into  the  tissues  of  the  plant.  The  re- 
maining ions  are  kept  in  the  solution.  There  is  a  con- 
stant tendency  to  bring  the  composition  and  density  of 
the  solution  into  equilibrium,  by  diffusion  and  diosmosis, 
with  the  solution  between  the  soil  particle  and  the  root- 
hair.  The  rapidity  with  which  the  metabolic  process 
removes  a  substance  from  the  solution  in  the  plant,  there- 
fore, determines  the  rate  at  which  it  is  removed  from  a 
solution  of  given  composition  and  density  in  the  soil. 
Plants  making  a  rapid  growth  remove  more  nutrients  in 
a  given  time  than  those  making  a  slower  growth,  when 
the  nutrient  solution  is  of  a  given  composition  and  density. 

Another  factor  that  affects  the  rate  of  absorption  of 
salts  from  the  soil  is  the  solvent  influence  of  exudates  from 
the  root-hairs.  This  subject  has  already  been  treated  (pars. 
321-326),  and  it  only  remains  to  be  said  that  this  action 
apparently  varies  with  different  kinds  of  plants,  and 
probably  accounts  in  no  small  measure  for  the  difference 
in  the  ability  of  different  plants  to  withdraw  salts  from 
the  soil.  40^ 

These  several  factors,  which,  when  combined,  deter- 


414      SOILS:    PROPERTIES  AND   MANAGEMENT 

mine  the  so-called  "  feeding  power  "  of  the  plant,  are 
recognized  by  the  popular  terms  "  weak  feeder "  and 
"  strong  feeder,"  —  applied,  on  the  one  hand,  to  such 
crops  as  wheat  or  onions,  which  require  very  careful  soil 
preparation  and  manuring,  and,  on  the  other  hand,  to 
maize,  oats,  or  cabbage,  which  demand  relatively  less 
care.  In  the  manuring  and  rotating  of  crops,  this  differ- 
ence in  absorptive  power  must  be  considered,  in  order  not 
only  to  secure  the  maximum  effect  on  the  crop  manured, 
but  also  to  get  the  greatest  residual  effect  of  the  manure 
on  succeeding  crops. 

331.  The  absorptive  power  of  cereals.  —  Cereals  have 
the  power  of  utilizing  the  potassium  and  phosphorus  of 
the  soil  to  a  considerable  degree,  but  they  generally  re- 
quire fertilization  with  nitrogen  salts.  Most  of  the  cereals, 
such  as  wheat,  rye,  oats,  and  barley,  take  up  the  principal 
part  of  their  nitrogen  early  in  the  season,  before  the  nitri- 
fication processes  have  been  sufficiently  operative  to  fur- 
nish a  large  supply  of  nitrogen;  hence  nitrogen  is  the 
fertilizer  constituent  that  usually  gives  the  best  results, 
and  should  be  added  in  a  soluble  form.  Wheat,  in  partic- 
ular, needs  a  large  amount  of  soluble  nitrogen  early  in 
its  spring  growth.  Since  it  is  a  "  delicate  feeder,"  it  does 
best  after  a  cultivated  crop  or  a  fallow,  by  which  the 
nitrogen  has  been  converted  into  a  soluble  form.  Oats 
can  make  better  use  of  the  soil  fertility  and  do  not  require 
so  much  manuring.  Maize  is  a  very  coarse  "  feeder," 
and,  while  it  removes  a  large  quantity  of  plant-food  from 
the  soil,  it  does  not  require  that  this  shall  be  added  in  a 
soluble  form.  Farm  manure  and  other  slowly  acting 
manures  may  well  be  applied  for  the  maize  crop.  The 
long  growing  period  required  by  the  maize  plant  gives  it 
opportunity  to  utilize  the  nitrogen  as  it  becomes  avail- 


ABSORPTION  OF  NUTRITIVE  SALTS  415 

able  during  the  summer,  when  ammonification  and  nitri- 
fication are  active.  Phosphorus  is  the  substance  usually 
most  needed  by  maize. 

332.  The  feeding  of  grass  crops.  —  Grasses,  when  in 
meadow  or  in  pasture,  are  greatly  benefited  by  manures. 
They  are  less  vigorous  "  feeders  "  than  the  cereals,  have 
shorter  roots,  and,  when  left  down  for  more  than  one 
year,  the  lack  of  aeration  in  the  soil  causes  decomposition 
to  decrease.  There  is  usually  a  more  active  fixation  of 
nitrogen  in  grass  lands  than  in  cultivated  lands,  but  this 
becomes  available  very  slowly. 

Different  soils  and  different  climatic  conditions  neces- 
sitate different  methods  of  manuring  for  grass.  Farm 
manures  may  well  be  applied  to  meadows  in  all  situations, 
while  the  use  of  nitrogen  is  generally  profitable. 

333.  Leguminous  crops.  —  Most  of  the  leguminous 
crops  are  deep-rooted  and  are  vigorous  "  feeders."  Their 
ability  to  take  nitrogen  from  the  air  makes  the  use  of  that 
fertilizer  constituent  unnecessary  except  in  a  few  in- 
stances, such  as  young  alfalfa  on  poor  soil,  where  a  small 
application  of  nitrate  of  soda  is  usually  beneficial.  Lime 
and  potassium  are  the  substances  most  beneficial  to  leg- 
umes on  the  majority  of  soils. 

334.  Root  crops.  —  Many  root  crops  will  utilize  very 
large  quantities  of  plant-food  if  it  is  in  a  form  in  which 
they  can  use  it.  Phosphates  and  nitrogen  are  the  sub- 
stances generally  required,  the  latter  especially  by  beets 
and  carrots. 

335.  Vegetables.  —  In  growing  vegetables,  the  object 
is  to  produce  a  rapid  growth  of  leaves  and  stalks  rather 
than  seeds,  and  often  this  growth  is  made  very  early  in 
the  season.  As  a  consequence,  a  soluble  form  of  nitrogen 
is  very  desirable.     Farm  manure  should  also  have  a  promi- 


416      SOILS:    PROPERTIES  AND  MANAGEMENT 

nent  part  in  the  treatment,  as  it  keeps  the  soil  in  a  mechan- 
ical condition  favorable  to  retention  of  moisture,  which 
vegetables  require  in  large  amounts,  and  it  also  supplies 
needed  fertility.  The  very  intensive  method  of  culture 
employed  in  the  production  of  vegetables  necessitates  the 
use  of  much  greater  quantities  of  manures  than  are  used 
for  field  crops,  and  the  great  value  of  the  product  justifies 
the  practice. 

336.  Fruits.  —  In  manuring  fruits,  with  the  exception 
of  some  of  the  small,  rapidly-growing  ones,  it  is  the  aim 
to  maintain  a  continuous  supply  of  nutrients  available 
to  the  plant,  but  not  sufficient  for  stimulation  except 
during  the  early  life  of  the  tree,  when  rapid  growth  of 
wood  is  desired.  An  acre  of  apple  trees  in  bearing  re- 
moves as  much  plant-food  material  from  the  soil  in  a 
season  as  does  an  acre  of  wheat.  Farm  manure  and  a 
complete  fertilizer  may  be  used,  of  which  the  constituents 
should  be  in  a  fairly  available  form,  as  a  constant  supply 
is  necessary.  A  young  growing  orchard  requires  con- 
siderably more  nitrogen  than  does  an  old  orchard.  Some 
nitrate  of  soda  in  early  spring  is  desirable. 

337.  Mineral  substances  absorbed  by  plants.  —  The 
plant,  in  its  process  of  growth,  withdraws  from  the  soil 
certain  mineral  substances  that  are  presented  to  its  roots 
in  a  dissolved  condition.  As  the  salts  in  solution  are 
rather  numerous,  and  since  the  diffusion  by  which  the 
absorption  is  accomplished  does  not  admit  of  the  entire 
exclusion  of  any  ion  capable  of  diosmosis,  there  are  to  be 
found  in  the  plant  most  of  the  mineral  constituents  of  the 
soil.  Some  of  these  are  concerned  in  the  vital  processes 
of  the  plant  and  are  essential  to  its  growTth ;  others  seem 
to  have  no  specific  function,  but  are  generally  present. 

The  substances  commonly  met  with  in  the  ash  of  plants 


ABSORPTION   OF  NUTRITIVE  SALTS  417 

are  potassium,  sodium,  calcium,  magnesium,  iron,  man- 
ganese, aluminium,  phosphorus,  sulfur,  silicon,  and  chlo- 
rine. In  addition  to  these,  nitrogen  is  absorbed  from  the 
soil  in  the  form  of  soluble  salts.  Of  these  the  substances 
known  to  be  absolutely  essential  to  the  normal  growth  of 
plants  to  maturity,  are  potassium,  calcium,  magnesium, 
iron,  phosphorus,  sulfur,  and  nitrogen,  while  the  others 
are  probably  beneficial  to  the  plant  in  some  way  not  yet 
discovered. 

Of  the  substances  acting  as  plant  nutrients,  each  must 
be  present  in  an  amount  sufficient  to  make  possible  the 
maximum  growth  consistent  with  other  conditions,  or 
the  yield  of  the  crop  will  be  curtailed  by  its  deficiency. 
To  some  extent  certain  essential  substances  may  be  re- 
placed by  others,  as,  for  instance,  potassium  by  sodium; 
but  such  substitution  is  probably  possible  only  in  some 
physiological  role  other  than  that  of  an  elemental  con- 
stituent of  an  organic  compound.  The  substances  that  are 
likely  to  be  so  deficient  in  an  available  form  in  any  soil 
as  to  curtail  the  yield  of  crops,  are  potassium,  phosphorus, 
nitrogen,  and  possibly  sulfur;  while  the  addition  of  cer- 
tain forms  of  calcium  is  likely  to  be  beneficial  because  of 
its  relation  to  other  constituents  and  properties  of  the  soil. 
It  is  for  the  purpose  of  supplying  these  substances,  and 
to  some  extent  to  improve  the  mechanical  condition  of 
the  soil,  that  mineral  manures  are  used. 

338.  Relation  of  plant  growth  to  concentration  of  nu- 
trient solution.  —  It  has  already  been  stated  that  the 
addition  of  soluble  salts  to  a  soil  has  been  found  by  some 
experimenters  to  apparently  increase  the  concentration 
of  the  soil  solution  (par.  250).  It  has  also  been  found 
that  plant  growth,  as  measured  by  weight  of  plants,  in- 
creases with  the  concentration  of  the  nutrient  solution  in 
2e 


418     SOILS:   properties  and  management 

which  the  plants  are  grown.1  This  is  the  way  in  which 
it  is  generally  believed  that  soluble  fertilizer  salts  benefit 
plant  growth.  Insoluble  plant-food  materials  have  a 
similar,  but  less  active,  result  because  they  do  not  increase 
the  concentration  of  the  soil  solution  to  as  great  an  extent. 

339.  Quantities  of  plant-food  material  removed  by 
crops.  —  The  utilization  of  mineral  substances  by  crops 
is  a  source  of  loss  of  fertility  to  agricultural  soils.  In  a 
state  of  nature,  the  loss  in  this  way  is  comparatively  small, 
as  the  native  vegetation  falls  on  the  ground,  and  in  the 
process  of  decomposition  the  ash  is  almost  entirely  returned 
to  the  soil.  Under  natural  conditions,  soil  usually  in- 
creases in  fertility ;  for,  while  there  is  some  loss  through 
drainage  and  other  sources,  this  is  more  than  counter- 
balanced by  the  action  of  the  natural  agencies  of  disin- 
tegration and  decomposition,  and  the  fixation  of  atmos- 
pheric nitrogen  affords  a  constant,  though  small,  supply 
of  that  important  soil  ingredient. 

When  land  is  put  under  cultivation,  a  very  different 
condition  is  presented.  Crops  are  removed  from  the 
land,  and  only  partially  returned  to  it  in  manure  or  straw. 
This  withdraws  annually  a  certain  small  proportion  of 
the  total  quantity  of  mineral  substances,  but,  what  is  of 
more  immediate  importance,  it  withdraws  all  of  this  in  a 
readily  available  form. 

The  following  table,  computed  by  Warington, 2  shows 

iHafl,  A.  D.,  Brenchley,  W.  E.,  and  Underwood,  L.  M. 
The  Soil  Solution  and  the  Mineral  Constituents  of  the  Soil. 
Philosoph.  Trans.  Royal  Soc.  London,  Series  B,  Vol.  204,  pp. 
179-200.  1913.  Also,  Lyon,  T.  L.,  and  Bizzell,  J.  A.  The 
Plant  as  an  Indicator  of  the  Relative  Density  of  Soil  Solutions. 
Proc.  Am.  Soc.  Agron.,  Vol.  4,  pp.  35-49.     1912. 

2  Warington,  R.  Chemistry  of  the  Farm,  pp.  64-65.  Lon- 
don.   1894. 


ABSORPTION   OF  NUTRITIVE  SALTS 


419 


the  quantities  of  nitrogen,  potassium,  phosphorus,  and 
lime  removed  from  an  acre  of  soil  by  some  of  the  common 
crops.     The  entire  harvested  crop  is  included  :  — 


Total 

NlTRO- 

Potash 

Lime 

Phos- 
phoric 

Crop 

Yield 

Ash 

N 

K20 

CaO 

Acid 
P2O5 

(Pounds) 

(Pounds) 

(Pounds) 

(Pounds) 

(Pounds) 

Wheat    .     .     .     . 

30  bushels 

172 

48 

28.8 

9.2 

21.1 

Barley    .     . 

40  bushels 

157 

48 

35.7 

9.2 

20.7 

Oats       .     . 

45  bushels 

191 

55 

46.1 

11.6 

19.4 

Maize     .     . 

30  bushels 

121 

43 

36.3 

— 

18.0 

Meadow  hay 

l£  tons 

203 

49 

50.9 

32.1 

12.3 

Red  clover 

2  tons 

258 

102 

83.4 

90.1 

24.9 

Potatoes 

6  tons 

127 

47 

76.5 

3.4 

21.5 

Turnips 

17  tons 

364 

192 

148.8 

74.0 

33.1 

340.  Quantities  of  plant-food  materials  contained  in 
soils.  —  Comparing  the  figures  given  above  with  those 
showing  the  percentages  of  the  fertilizing  constituents  in 
certain  soils,  it  is  evident  that  there  is  a  supply  in  most 
arable  soils  that  will  afford  nutriment  for  average  crops 
for  a  very  long  period  of  time.     (See  pars.  46,  48,  52,  53.) 

341.  Possible  exhaustion  of  mineral  nutrients.  —  On 
the  other  hand,  when  it  is  considered  that  the  soil  must 
be  depended  upon  to  furnish  food  for  humanity  and 
domestic  animals  as  long  as  they  shall  continue  to  in- 
habit the  earth,  at  least  so  far  as  is  now  known,  the  very 
apparent  possibility  of  exhausting,  even  in  a  period  of 
several  hundred  years,  the  supply  of  plant  nutrients  be- 
comes a  matter  of  grave  concern.  The  visible  sources  of 
supply,  to  replace  or  supplement  those  in  the  soils  now 
cultivated,  are,  for  the  mineral  substances,  the  subsoil 
and  the  natural  deposits  of  phosphates,  potash  salts,  and 


420       SOILS:    PROPERTIES  AND  MANAGEMENT 

limestone ;  and  for  nitrogen,  deposits  of  nitrates,  the  by- 
product of  coal  distillation,  and  the  nitrogen  of  the 
atmosphere.  The  last  of  these  is  inexhaustible,  and  the 
exhaustion  of  the  nitrogen  supply,  which  a  few  years  ago 
was  thought  to  be  a  matter  of  less  than  half  a  century, 
has  now  ceased  to  cause  any  apprehension.  The  conser- 
vation or  extension  of  the  supply  of  mineral  nutrients  is 
now  of  supreme  importance.  The  utilization  of  city  refuse 
and  the  discovery  of  new  mineral  deposits  are  develop- 
ments well  within  the  range  of  possibility,  but  neither  of 
these  promises  to  afford  more  than  partial  relief.  The 
utilization  of  the  subsoil  through  the  gradual  removal  by 
natural  agencies  of  the  topsoil  will,  without  doubt,  tend 
to  constantly  renew  the  supply.  The  removal  of  topsoil 
by  wind  and  erosion  is,  even  on  level  land,  a  very  con- 
siderable factor.  The  large  amount  of  sediment  carried 
in  streams  immediately  after  a  rain,  especially  in  summer, 
gives  some  idea  of  the  extent  of  this  shifting.  This  affects 
chiefly  the  surface  soil,  and  thereby  brings  the  subsoil 
into  the  range  of  root  action. 

There  is  little  doubt  that  a  moderate  supply  of  plant- 
food  materials  will  always  be  available  in  most  soils,  but 
for  progressive  agriculture  manures  must  be  used. 


CHAPTER  XX 
ORGANISMS    IN    THE   SOIL 

A  vast  number  of  organisms,  animal  and  vegetable,  live 
in  the  soil.  By  far  the  greater  part  of  these  belong  to 
plant  life,  and  these  comprise  the  forms  of  greatest  influ- 
ence in  producing  the  changes  in  structure  and  composition 
that  contribute  to  soil  productiveness.  Most  of  the 
organisms  are  so  minute  as  to  be  seen  only  by  the  aid  of 
the  microscope,  while  a  much  smaller  proportion  range 
from  these  to  the  size  of  the  larger  rodents.  They  may 
thus  be  classed  as  microorganisms  and  macroorganisms. 
The  latter  class  will  be  considered  first. 

MACROORGANISMS   OF  THE   SOIL 

Of  the  macroorganisms  in  the  soil  the  animal  forms 
belong  chiefly  to  (1)  rodents,  (2)  worms,  and  (3)  insects; 
and  the  plant  forms  to  (1)  the  large  fungi  and  (2)  plant 
roots. 

342.  Rodents.  —  The  burrowing  habits  of  rodents  — 
of  which  the  ground  squirrel,  the  mole,  the  gopher,  and  the 
prairie  dog  are  familiar  examples  —  result  in  the  pulveri- 
zation and  transfer  of  very  considerable  quantities  of  soil. 
While  the  activities  of  these  animals  are  often  not  favor- 
able to  agriculture,  the  effect  on  the  character  of  the  soil 
is  rather  beneficial  and  is  analogous  to  that  of  good  tillage. 
Their  burrows  also  serve  to  aerate  and  drain  the  soil,  and 

421 


422       SOILS:    PROPERTIES  AND  MANAGEMENT 

in  permanent  pastures  and  meadows  are  of  much  value 
in  this  way. 

343.  Worms.  —  The  common  earthworm  is  the  most 
conspicuous  example  of  the  benefit  that  may  accrue  from 
this  form  of  life.  Darwin,  as  the  result  of  careful  measure- 
ments, states  that  the  quantity  of  soil  passed  through 
these  creatures  may,  in  a  favorable  soil  in  a  humid  climate, 
amount  to  ten  tons  of  dry  earth  per  acre  annually.  The 
earthworm  obtains  its  nourishment  from  the  organic 
matter  of  the  soil,  but  takes  into  its  alimentary  canal  the 
inorganic  matter  as  well,  expelling  the  latter  in  the  form 
of  casts  after  it  has  passed  entirely  through  the  body. 
The  ejected  material  is  to  some  extent  disintegrated,  and 
is  in  a  flocculated  condition.  The  holes  left  in  the  soil 
serve  to  increase  aeration  and  drainage,  and  the  move- 
ments of  the  worms  bring  about  a  notable  transportation 
of  lower  soil  to  the  surface,  which  aids  still  more  in  effect- 
ing aeration.  Darwin's  studies  led  him  to  state  that  from 
one-tenth  to  two-tenths  of  an  inch  of  soil  is  yearly  brought 
to  the  surface  of  land  in  which  earthworms  exist  in  normal 
numbers. 

Instances  are  on  record  of  land  flooded  for  a  consider- 
able period  so  that  the  worms  were  destroyed,  and  the 
productiveness  of  the  soil  was  seriously  impaired  until  it 
was  restocked  with  earthworms. 

Wollny  conducted  experiments  with  soil,  the  soil  in  one 
case  containing  earthworms  and  in  another  case  not  con- 
taining them.  Although  there  was  much  variation  in  his 
results,  they  were  in  every  case  in  favor  of  the  soil  con- 
taining the  worms,  and  in  a  number  of  the  tests  the  yield 
on  rich  soil  was  several  times  as  great  as  where  no  worms 
were  present. 

Earthworms  naturally  seek  a  heavy,  compact  soil,  and 


ORGANISMS  IN   THE  SOIL  423 


it  is  in  soil  of  this  character  that  they  are  most  needed 
because  of  the  stirring  and  aeration  they  accomplish. 
Sandy  soil  and  the  soils  of  arid  regions,  in  which  are 
found  few  or  no  earthworms,  are  not  usually  in  need  of 
their  activities. 

344.  Insects.  —  There  is  a  less  definite,  and  probably 
less  effective,  action  of  a  similar  kind  produced  by  insects. 
Ants,  beetles,  and  the  myriads  of  other  burrowing  insects 
and  their  larvae  effect  a  considerable  movement  of  soil 
particles,  with  a  consequent  aeration  of  the  soil.  At  the 
same  time  they  incorporate  into  the  soil  a  considerable 
quantity  of  organic  matter. 

345.  Large  fungi.  —  The  larger  fungi  are  chiefly  con- 
cerned in  bringing  about  the  first  stages  in  the  decom- 
position of  woody  matter,  which  is  disintegrated  through 
the  growth  in  its  tissues  of  the  root  mycelia  of  the  fungi. 
These  break  down  the  structure,  and  thus  greatly  facili- 
tate the  work  of  the  decay  bacteria.  Action  of  this  kind 
is  largely  confined  to  the  forest  and  is  not  of  great  im- 
portance in  cultivated  soil. 

Another  function  of  the  large  fungi  is  exercised  in  the 
intimate,  and  possibly  symbiotic,  relation  of  the  fungal 
hyphse  to  the  roots  of  many  forest  trees,  in  soil  where 
nitrification  proceeds  very  slowly,  if  at  all,  for  nitrates  are 
apparently  not  abundant  in  forest  soils.  This  envelop- 
ing system  of  hyphse,  which  may  consist  of  masses  in  a 
definite  zone  of  the  cortex  with  occasional  filaments  pass- 
ing outward  into  the  soil,  or  which  may  surround  the  root 
with  a  dense  mass  of  interwoven  hyphse,  is  called  mycor- 
rhiza. 

The  cereal,  cruciferous,  leguminous,  and  solanaceous 
plants  are  not  associated  with  mycorrhiza.  Mycotrophic 
plants  are  usually  those  that  live  in  a  humous  soil  filled 


424      SOILS:    PROPERTIES  AND  MANAGEMENT 

with  the  mycelia  of  fungi.  It  is  thought  that  the  mycor- 
rhiza  aid  the  higher  plants  to  obtain  nutriment  that  they 
must  strive  for  in  competition  with  the  fungi. 

Mycotrophic  plants  are  also  able  to  grow  with  a  \ cry 
small  transpiration  of  moisture,  as  is  well  known  to  be 
the  case  with  many  conifers;  and  this  restricted  tran- 
spiration would  doubtless  result  in  lack  of  nutriment  were 
it  not  for  the  assistance  of  the  mycorrhiza. 

346.  Plant  roots.  —  The  roots  of  plants  assist  in  pro- 
moting productiveness  of  the  soil  both  by  contributing 
organic  matter  and  by  leaving,  on  their  decay,  openings 
which  render  the  soil  more  permeable  to  water  and  which 
also  facilitate  drainage  and  aeration.  The  dense  mass  of 
rootlets,  with  their  minute  hairs  that  are  left  in  the  soil 
after  every  harvest,  furnish  a  well-distributed  supply  of 
organic  manure,  which  is  not  confined  to  the  furrow  slice, 
as  is  artificially  incorporated  manure.  The  drainage  and 
aeration  of  the  lower  soil,  due  to  the  openings  left  by  the 
decomposed  roots,  are  of  the  greatest  importance  in  heavy 
soil,  and  the  beneficial  effects  of  clover  and  other  deep- 
rooted  plants  are  due  in  no  small  measure  to  this  function. 


Fig.  58. — Nema- 
todes entering 
a  plant  root. 


MICROORGANISMS   OF  THE   SOIL 

Of  the  microorganisms  commonly  exist- 
ing in  soils,  the  greater  part  belong  to 
plant  rather  than  to  animal  life.  Of  the 
latter,  the  only  organisms  of  well-known 
economical  importance  are  the  nematodes 
(Fig.  58),  whose  injurious  effect  on  plant 
growth  is  accomplished  through  the  for- 
mation of  galls  on  the  roots,  in  which  the 
young  are  hatched  and  live  to  sexual 
maturity. 


ORGANISMS  IN  THE  SOIL  425 

347.  Plant  microorganisms.  —  The  microscopic  plants 
of  the  soil  may  be  classed  as  slime  molds,  bacteria,  fungi, 
and  algae. 

348.  Plant  microorganisms  injurious  to  higher  plants. 
—  Injurious  plant  microorganisms  are  confined  mostly 
to  fungi  and  bacteria.  They  may  be  entirely  parasitic  in 
their  habits,  or  only  partially  so.  They  injure  plants  by 
attacking  the  roots.  Those  that  attack  other  parts  of 
plants  may  live  in  the  soil  during  their  spore  stage,  but 
they  are  not  strictly  microorganisms  of  the  soil.  Some 
of  the  more  common  diseases  produced  by  soil  organisms 
are :  wilt  of  cotton,  cowpeas,  watermelon,  flax,  tobacco, 
tomatoes,  and  other  plants ;  damping-off  of  a  large  num- 
ber of  plants ;   root-rot ;   galls. 

These  fungi  or  bacteria  may  live  for  long  periods,  prob- 
ably indefinitely,  in  the  soil,  if  the  conditions  necessary 
for  their  growth  are  maintained.  Some  of  them  will  die 
within  a  few  years  if  their  host  plants  are  not  grown  on 
the  soil,  but  others  are  able  to  maintain  existence  on 
almost  any  organic  substance.  Once  a  soil  is  infected, 
it  is  likely  to  remain  so  for  a  long  time,  or  indeed  indefi- 
nitely. Infection  is  easily  carried.  Soil  from  infected 
fields  may  be  carried  on  implements,  plants,  or  rubbish 
of  any  kind,  in  soil  used  for  inoculation  of  leguminous 
crops,  or  even  in  stable  manure  containing  infected  plants 
or  in  the  feces  resulting  from  the  feeding  of  infected  plants. 
Flooding  of  land  by  which  soil  is  washed  from  one  field 
to  another  may  be  a  means  of  infection. 

Prevention  is  the  best  defense  from  diseases  produced 
by  these  soil  organisms.  Once  disease  has  procured  a 
foothold,  it  is  practically  impossible  to  eradicate  all  its 
organisms.  Rotation  of  crops  is  effective  for  some  dis- 
eases, but  entire  absence  of  the  host  crop  is  oftener  neces- 


426     soils:  properties  and  management 

sary.  The  use  of  lime  is  beneficial  in  the  case  of  certain 
diseases.  Chemicals  of  various  kinds  have  been  tried 
with  little  success.  Steam  sterilization  is  a  practical 
method  of  treating  greenhouse  soils  for  a  number  of  dis- 
eases. The  breeding  of  plants  immune  to  the  disease  af- 
fecting its  particular  species  has  been  successfully  carried 
out  in  the  case  of  the  cowpea  and  cotton  plants,  and  can 
doubtless  be  accomplished  with  others. 

In  regions  in  which  farming  is  confined  largely  to  one 
crop  or  to  a  limited  number  of  cereals,  it  is  the  common 
experience  that  yields  decrease  greatly  in  the  course  of  a 
score  of  years  after  the  virgin  soil  is  broken.  The  cause 
for  this  is  attributed  by  Bolley  l  in  large  measure  to  a 
diseased  condition  of  the  plants  due  to  the  growth  of 
various  fungi  that  inhabit  the  soil  and  attack  the  crops 
grown  on  it.  He  reports  that  he  has  experimented  with 
pure  cultures  taken  from  wheat  grains,  straw,  and  roots, 
and  has  demonstrated  that  certain  strains  or  species  of 
Fusarium,  Helminthosporium,  Alternaria,  Macrosporium, 
Colletotrichum,  and  Cephalothecium  are  directly  capable 
of  attacking  and  destroying  growing  plants  of  wheat,  oats, 
barley,  brome  grass,  and  quack  grass,  and  that  within 
limits  the  disease  may  be  transferred  from  one  type  of 
crop  to  another. 

349.  Plant  microorganisms  not  injurious  to  higher 
plants.  —  The  vegetable  microorganisms  of  the  soil  all 
take  an  active  part  in  removing  dead  plants  and  animals 
from  the  surface  of  the  soil,  and  in  bringing  about  the  other 
operations  that  are  necessary  for  the  production  of  plants. 
The  first  step  in  the  preparation  for  plant  growth  is  to 
remove  the  remains  of  plants  and  animals  that  would 

1  Bolley,  H.  L.  Wheat.  North  Dakota  Agr.  Exp.  Sta., 
Bui.  107.     1913. 


ORGANISMS  IN   THE  SOIL  427 

otherwise  accumulate  to  the  exclusion  of  other  plants. 
These  are  decomposed  through  the  action  of  organisms 
of  various  kinds,  the  intermediate  and  final  products  of 
decomposition  assisting  plant  production  by  contributing 
nitrogen  and  certain  mineral  compounds  that  are  a 
directly  available  source  of  plant  nutriment,  and  also  by 
the  effect  of  certain  of  the  decomposition  products  on  the 
mineral  substances  of  the  soil,  by  which  they  are  rendered 
soluble  and  hence  available  to  the  plant. 

Through  these  operations  the  supply  of  carbon  and 
nitrogen  required  for  the  production  of  organic  matter  is 
kept  in  circulation.  The  complex  organic  compounds 
in  the  bodies  of  dead  plants  or  animals,  in  which  condi- 
tion plants  cannot  use  them,  are,  under  the  action  of 
microorganisms,  converted  by  a  number  of  stages  into 
the  very  simple  compounds  used  by  plants.  In  the  course 
of  this  process  a  part  of  the  nitrogen  is  sometimes  lost 
into  the  air  by  conversion  into  free  nitrogen,  but  fortu- 
nately this  may  be  recovered  and  even  more  nitrogen 
taken  from  the  air  by  certain  other  organisms  of  the  soil. 

The  slime  molds,  bacteria,  fungi,  and  algae  all  play  a 
part  in  these  processes,  but  none  of  them  so  actively 
during  every  stage  of  the  processes  as  do  the  bacteria. 
Molds  and  fungi  are  particularly  active  in  the  early  stages 
of  decomposition  of  both  nitrogenous  and  non-nitrogenous 
organic  matter.  Molds  are  also  capable  of  ammonifying 
proteins,  and  even  re-forming  the  complex  protein  bodies 
from  the  nitrogen  of  ammonium  salts.  Certain  of  the 
molds  and  of  the  algse  are  apparently  able  to  fix  atmos- 
pheric nitrogen,  and  contribute  a  supply  of  carbohydrates 
required  for  the  use  of  the  nitrogen-fixing  bacteria.  Among 
these  are  Aspergillus  niger  and  Fenicillium  glaucum. 

It  also  seems  probable  that  the  fungi  associated  with 


428       SOILS:    PROPERTIES  AND  MANAGEMENT 


the  roots  of  many  forest  trees  and  known  as  mycorrhizal 
fungi  have  the  ability  to  fix  atmospheric  nitrogen,  and  that 
in  some  .way  the  trees  obtain  a  part,  at  least,  of  the  nitro- 
gen so  fixed.  The  growth  of  forests  on  poor,  sandy  soil 
containing  practically  no  nitrogen  has  been  urged  as  an 
example  of  this  process. 

350.  Bacteria.  —  Of  the  several  forms  of  microorgan- 
isms found  in  the  soil,  bacteria  are  the  most  important. 
In  fact,  the  abundant  and  continued  growth  of  plants  on 
the  soil  is  absolutely  dependent  on  the  presence  of  bacteria, 
for  through  their  action  chemical  changes  are  brought 
about  which  result  in  making  soluble  both  organic  and 
inorganic  material  necessary  for  the  life  of  higher  plants, 
and  which,  in  part  at  least,  would  not  otherwise  occur. 

Bacteria  are  thus  trans- 
formers, not  producers,  of 
fertility  in  the  soil,  although, 
as  will  be  seen  later,  certain 
kinds  of  bacteria  take  nitro- 
gen from  the  air  and  leave  it 
in  the  soil.  With  this  excep- 
tion, however,  they  add  no 
plant-food  to  the  soil.  It  is 
their  action  in  rendering 
available  to  the  plant  ma- 
terial already  present  in  the 
soil  that  constitutes  their 
greatest  present  value  in  crop 
production.  It  is  to  their 
activity  in  conveying  nitro- 
gen from  the  air  to  the  soil 
that  we  are  indebted  for  most  of  our  supply  of  nitrogen 
in  virgin  soils  (see  Fig.  59). 


Fig.  59. — Some  types  of  soil  mi- 
croorganisms highly  magnified, 
(a) ,  nitrate  formers  ;  (6) ,  ni- 
trite formers;  (c),  B.  graveo- 
lens  ;  (d) ,  B.  fusiformis  ;  (e) ,  B. 
subtilis  ;  (/),  Clostridium  pas- 
teurianum. 


ORGANISMS  IN  THE  SOIL  429 

It  is  not  usually  the  entire  absence  of  bacteria  from  the 
soil  that  is  to  be  avoided  in  practice,  for  all  arable  soils  con- 
tain bacteria,  although  sometimes  not  all  of  the  desirable 
forms ;  but,  as  great  bacterial  activity  is  required  for  the 
large  production  of  crops,  the  practical  problem  is  to  main- 
tain a  condition  of  soil  most  favorable  to  such  activity. 

351.  Distribution  of  bacteria.  —  Bacteria  are  found 
almost  universally  in  soils,  although  they  are  much  more 
numerous  in  some  soils  than  in  others.  A  number  of  in- 
vestigators have  stated  that  in  soils  from  different  locali- 
ties and  of  different  types  that  they  have  examined,  the 
numbers  of  bacteria  were  proportional  to  the  productive- 
ness of  the  soils.  The  number  of  bacteria  present  has  in 
some  cases  been  shown  to  be  proportional  to  the  amount 
of  humus' contained  in  the  soil.  It  is  natural  to  expect 
that  within  certain  limits  both  these  findings  will  hold.  The 
conditions  obtaining  in  a  productive  soil  are  those  favorable 
to  the  development  of  certain  forms  of  bacteria,  and  these 
kinds  constitute  a  very  large  proportion  of  those  generally 
found  in  soils.  However,  there  is  evidence  that  compara- 
tively unproductive  soils  may  contain  a  large  number  of 
bacteria  that  are  presumably  not  favorable  to  plant  growth. 

Samples  of  soil  taken  from  certain  productive  and  rela- 
tively unproductive  parts  of  a  field  on  the  Cornell  Uni- 
versity farm  contained  a  larger  number  of  bacteria  in  the 
poor  soil,  although  the  two  soils  were  equally  well  drained 
and  the  good  soil  had  slightly  more  organic  matter. 
They  had  also  received  practically  the  same  treatment 
during  the  preceding  few  years :  — 

Character  of  Number  of  bacteria 

soil  per  gram  of  dry  soil 

Good 1,200,000 

Poor  :   i 1,600,000 


430       SOILS:    PROPERTIES  AND  MANAGEMENT 

After  wheat  had  been  growing  for  two  months  on  these 
soils  in  the  greenhouse,  the  soils  being  maintained  at  the 
same  moisture  content,  the  samples  showed  the  following 
count : — 

Character  of  Number  of  bacteria 

soil  to  a  gram  of  dry  soil 

Good 760,000 

Poor 1,120,000 

Another  reason  why  this  relation  between  the  number 
of  bacteria  and  soil  productiveness  does  not  hold  is  that 
the  bacteria  having  the  same  functions  in  relation  to  plant- 
food  do  not  always  have  the  same  physiological  efficiency. 
In  other  words,  they  do  not  have  the  same  virulence,  a 
small  number  in  some  cases  being  able  to  bring  about  the 
same  changes  that  in  other  cases  require  a  much  greater 
number. 

Bacteria  are  found  chiefly  in  the  upper  layers  of  soil, 
although  not  in  large  numbers  at  the  immediate  surface 
of  the  ground.  In  humid  regions  the  layer  between  the 
first  inch  and  the  sixth  or  the  seventh  inch  contains,  in 
most  soils,  the  great  bulk  of  bacteria  present.  In  arid  or 
semiarid  regions,  bacteria  are  found  at  greater  depths 
and  the  densest  population  is  located  at  lower  levels  than 
in  humid  regions.  This  is  largely  because  of  the  deeper 
penetration  of  the  air  and  the  conditions  that  accom- 
pany it. 

352.  Numbers  of  bacteria.  — The  number  of  bacteria  in 
a  soil  will  naturally  vary  with  the  conditions  that  favor  or 
discourage  their  growth.  In  very  sandy  soils,  forest  soils, 
desert  soils,  water-logged  soils,  and  soils  low  in  humus, 
the  bacteria  are  either  absent  or  comparatively  few  in 
numbers.     In  soils  very  rich  in  organic  matter,  especially 


ORGANISMS  IN    THE   SOIL  431 

where  animal  manure  has  been  applied  or  where  a  carcass 
has  been  buried,  the  number  becomes  very  large,  as 
many  as  100,000,000  to  a  gram  of  soil  having  been  found ; 
while  in  soil  of  ordinary  fertility  and  tilth  the  numbers 
range  from  1 ,000,000  to  5,000,000  to  a  gram.  The  extreme 
rapidity  with  which  reproduction  occurs  makes  it  possible 
for  the  number  to  increase  enormously  when  conditions 
are  favorable  for  their  growth. 

The  table  on  page  432  shows  the  number  of  bacteria 
to  a  gram  of  soil  found  in  different  parts  of  the  United 
States  during  some  portion  of  the  growing  season. 

The  figures  showing  the  number  of  bacteria  in  each 
gram  of  soil  that  are  presented  in  this  table  cannot  be 
used  for  a  comparison  of  the  relative  numbers  of  bacteria 
in  soils  of  different  regions  of  this  country,  because  dif- 
ferent methods  were  used  by  the  experimenters  in  making 
the  estimations.  They  are,  however,  an  indication  of 
what  may  be  considered  the  ordinary  range  in  arable 
soils.    • 

353.  Numbers  as  influenced  by  season.  —  It  might  be 
supposed  that,  like  most  plants,  bacteria  would  develop 
most  rapidly  in  summer  months  and  that  they  would  be 
found  in  largest  numbers  at  that  season,  at  least  in  regions 
of  low  temperatures  during  the  winter  months.  That 
this  is  not  always  the  case  has  been  shown  by  Conn,1 
who  found  as  the  result  of  periodical  enumeration  of  bac- 
teria throughout  a  term  of  two  years  that  the  highest 
counts  were  obtained  during  the  winter  months,  when 
the  soil  was  frozen.  This  does  not  mean  that  all  classes 
of  bacteria  are  present  in  largest  numbers  at  that  season, 
but,  as  explained  by  Conn,  it  seems  likely  that  certain 

1  Conn,  H.  J.  Bacteria  in  Frozen  Soils  II.  Centrlb.  f. 
Bakt.,  II,  Band  32,  Seite  70-97.     1912. 


432       SOILS:    PROPERTIES  AND   MANAGEMENT 


Number   of   Bacteria    to    a   Gram   of    Soil    during   Some 
Period  of  the  Growing  Season 


State 

Soil 

Depth 

Crop 

Investi- 
gator 

Number 

op 
Bacteria 

Delaware  . 

Stiff  clay 

3 
inches 

Orchard  in  high 
state  of  culti- 
vation.       In 
cover  crops 

Chester  » 

2,200,000 

Delaware  . 

Adjoining  soil  above 

3 

Meadow    for 

Chester  1 

450,000 

and  of  same  char- 

inches 

twelve   years 

acter 

Delaware  . 

Of    same     type     as 

3 

Vegetables  and 

Chester » 

1,800,000 

above 

inches 

heavily     ma- 
nured 

Delaware  . 

Scarlet    clover 

Chester  * 

3,360,000 

plowed  under 

and        alter- 

nated     with 

maize  for  ten 

years 

Kansas 

Loam,  rich  in  humus 

30 

Alfalfa,        five 

Mayo  and 

33,931,808 

inches 

years 

Kinsley  * 

Kansas 

Loam,  richer  in  hu- 

30 

Alfalfa 

Mayo  and 

53,596,060 

mus  than  soil  above 

inches 

Kinsley 2 

Kansas 

Thin     soil,     gumbo 

30 

Mixed   grasses 

Mayo  and 

78,534 

subsoil 

inches 

Kinsley  2 

Kansas 

Loam,  low  in  humus 

30 
inches 

Oats  and  wheat 

Mayo  and 
Kinsley  2 

8,543,006 

Iowa      .     . 

Marshall    loam,    no 
lime  applied 

top 
soil 

Oats 

Brown  3 

1,930,000 

Iowa 

Marshall  loam,  1,000 
pounds     lime    per 
acre 

top 
soil 

Oats 

Brown  * 

2,342,000 

Iowa      .     . 

Marshall  loam,  2,000 
pounds      lime     per 
acre 

top 

soil 

Oats 

Brown  3 

2,787,000 

Iowa      .     . 

Marshall  loam,  6,000 
pounds     lime     per 
acre 

top 
soil 

Oats 

Brown  3 

3,766,000 

1  Chester,    F.    D.     The    Bacteriological    Analysis    of    Soils. 
Delaware  Agr.  Exp.  Sta.,  Bui.  65.     1904. 

2  Mayo,   U.   S.,   and  Kinsley,  A.   F.     Bacteria  of  the   Soil. 
Kansas  Agr.  Exp.  Sta.,  Bui.  117.     1903. 

3  Brown,   P.   E.     Some  Bacterial  Effects  of  Liming.     Iowa 
Agr.  Exp.  Sta.,  Research  Bui.  2.  .  1911. 


ORGANISMS  IN   THE  SOIL 


433 


forms  predominate  in  summer  and  others  in  winter  (see 
Fig.  60). 


Fig.  60. — Periodical  enumeration  of  bacteria  in  soil  of  two  plats  during 
two  years,  expressed  in  millions  to  a  gram  of  dry  soil. 


Brown  and  Smith  l  obtained  results  that  in  the  main 
confirmed  Conn's  work,  and  they  advanced  the  theory 
that  the  concentration  of  the  soil  solution  immediately 
surrounding  the  soil  particles,  together  with  the  high 
surface  tension  exerted  by  the  soil  particles,  prevents  the 
freezing  of  the  surface  film  and  that  this  water  forms  a 
suitable  medium  for  the  development  of  bacteria. 

354.  Conditions  affecting  growth.  —  Many  conditions 
of  the  soil  affect  the  growth  of  bacteria.  Among  the  most 
important  of  these  are  the  supply  of  oxygen  and  moisture, 
the  temperature,  the  presence  of  organic  matter,  and  the 
acidity  or  the  basicity  of  the  soil. 

355.  Oxygen.  —  All  soil  bacteria  require  for  their 
growth  a  certain  amount  of  oxygen.  Some  bacteria,  how- 
ever, can  continue  their  activities  with  much  less  oxygen 
than  can  others.  Those  requiring  an  abundant  supply 
of  oxygen  have  been  called  aerobic  bacteria,  while  those 
preferring  little  or  no  air  are  designated  as  anaerobic 

1  Brown,  P.  E.,  and  Smith,  R.  E.  Bacterial  Activities  in 
Frozen  Soil.     Iowa  Agr.  Exp.  Sta.,  Research  Bui.  4.     ?912, 

2f 


434       SOILS:    PROPERTIES  AND   MANAGEMENT 

bacteria.  This  is  an  important  distinction,  because  those 
bacteria  that  are  of  the  greatest  benefit  to  the  soil  are,  in 
the  main,  aerobes,  and  those  that  are  injurious  in  their 
action  are  chiefly  anaerobes.  However,  it  seems  likely 
that  an  aerobic  bacterium  may  gradually  accommodate 
itself  within  certain  limits  to  an  environment  containing 
less  oxygen,  and  an  anaerobic  bacterium  may  accommo- 
date itself  to  the  presence  of  a  larger  amount  of  oxygen. 
Thus  a  bacterium  may  be  most  active  in  the  presence  of 
an  abundant  supply  of  oxygen,  but,  when  subjected  to 
conditions  in  which  the  supply  is  small,  growth  continues 
but  with  lessened  vigor.  The  term  facultative  bacteria 
has  been  used  to  designate  those  bacteria  that  are  able 
to  adapt  themselves  to  considerable  variation  in  oxygen 
supply.  The  structure,  tilth,  and  drainage  of  the  soil 
consequently  determine  largely  whether  aerobic  or  an- 
aerobic bacteria  shall  be  more  active. 

356.  Moisture.  —  Bacteria  require  some  moisture  for 
their  growth.  A  notable  decrease  in  the  moisture  con- 
tent of  the  soil  may  temporarily  decrease  the  number  of 
bacteria  by  limiting  their  development  to  the  films  of 
moisture  surrounding  the  particles.  With  a  decrease  in 
the  moisture  content  of  a  soil,  there  occurs  an  increase 
in  the  oxygen  in  the  interstitial  spaces.  Those  bacteria 
that  thrive  in  the  presence  of  oxygen  are  thereby  favored, 
and  the  character  of  the  bacterial  flora  is  correspondingly 
changed.  When  the  soil  remains  saturated,  or  nearly 
so,  for  any  considerable  period,  the  anaerobic  forms 
assert  themselves,  and  the  usually  beneficial  activities 
of  the  aerobic  bacteria  are  temporarily  suspended. 
The  most  favorable  moisture  condition  for  the  activity 
of  the  most  desirable  bacteria  is  that  found  in  a  well- 
drained  soil. 


ORGANISMS  IN   THE  SOIL  435 

357.  Temperature.  —  Soil  bacteria,  like  other  plants, 
continue  life  and  growth  under  a  considerable  range  of 
temperature.  Freezing,  while  rendering  bacteria  dor- 
mant, does  not  kill  them,  and  growth  begins  slightly 
above  that  point.  It  has  been  shown  that  nitrification 
goes  on  at  temperatures  as  low  as  from  37°  to  39°  F.  It 
is  not,  however,  until  the  temperature  is  considerably 
higher  that  their  functions  are  pronounced.  From  70° 
to  110°  F.  their  activity  is  greatest,  and  it  diminishes 
perceptibly  below  or  above  those  points.  The  thermal 
death  point  of  most  forms  of  bacteria  is  found  at  some 
point  between  110°  and  160°  F.,  but  the  spore  forms  even 
resist  boiling.  Only  in  some  desert  soils  does  the  natural 
temperature  reach  a  point  sufficiently  high  to  actually 
destroy  bacteria,  and  there  only  near  the  surface.  In  fact, 
it  is  seldom  that  soil  temperatures  become  sufficiently  high 
to  curtail  bacterial  activity. 

358.  Organic  matter.  —  The  presence  of  a  certain 
amount  of  organic  matter  is  essential  to  the  growth  of 
most,  but  not  all,  forms  of  soil  bacteria.  The  organic 
matter  of  the  soil,  consisting  as  it  does  of  the  remains  of 
a  large  variety  of  substances,  furnishes  a  suitable  food 
supply  for  a  very  great  number  of  forms  of  organisms. 
The  action  of  one  set  of  bacteria  on  the  cellular  matter  of 
plants  embodied  in  the  soil  produces  compounds  suited  to 
other  forms,  and  so  from  one  stage  of  decomposition  to 
another  this  constantly  changing  material  affords  sus- 
tenance to  a  bacterial  flora  the  extent  and  variety  of  which 
it  is  difficult  to  conceive.  Not  only  do  bacteria  affect  the 
organic  matter  of  the  soil,  but,  in  the  case  of  certain 
forms,  their  activities  produce  changes  in  the  inorganic 
matter  that  cause  it  to  become  more  soluble  and  more 
easily  available  to  the  plant. 


436      SOILS:    PROPERTIES  AND  MANAGEMENT 

A  soil  low  in  organic  matter  usually  has  a  lower  bac- 
terial content  than  one  containing  a  larger  amount,  and, 
under  favorable  conditions,  the  beneficial  action,  to  a  cer- 
tain point  at  least,  increases  with  the  content  of  organic 
substance;  but,  as  the  products  of  bacterial  life  are 
generally  injurious  to  the  organisms  producing  them, 
such  factors  as  the  rate  of  aeration  and  the  basicity  of 
the  soil  must  determine  the  effectiveness  of  the  organic 
matter. 

359.  Soil  acidity.  —  A  soil  having  an  acid  reaction 
makes  a  poor  medium  for  the  growth  of  certain  bacteria. 
A  neutral  or  a  slightly  alkaline  soil  furnishes  the  most 
favorable  condition  for  the  development  of  the  forms  of 
bacteria  most  beneficial  to  arable  land.  The  activities  of 
many  soil  bacteria  result  in  the  formation  of  acids  which 
are  injurious  to  the  bacteria  themselves,  and,  unless  there 
is  present  some  basic  substance  with  which  these  can 
combine,  bacterial  development  is  inhibited  by  their  own 
products.  This  is  one  of  the  reasons  why  lime  is  so  often  of 
great  benefit  when  applied  to  soils,  and  especially  to  those 
on  which  alfalfa  and  red  clover  are  growing.  For  the 
same  reason,  the  presence  of  lime  hastens  decay  of  or- 
ganic matter  in  certain  soils,  and  the  conversion  of  nitrog- 
enous material  with  a  minimum  loss  into  compounds 
available  to  the  plants.  As  showing  the  value  of  lime 
in  the  process  of  nitrate  formation,  it  has  been  pointed 
out  that  in  the  presence  of  an  adequate  supply  of  lime 
the  availability  of  ammonium  salts  is  almost  as  high 
as  that  of  nitrate  salts,  but  where  the  supply  of  lime 
is  insufficient  the  value  of  ammonium  salts  is  relatively 
rather  low. 

360.  Functions  of  soil  bacteria.  —  Bacteria  have  a  part 
in  many  of  the  processes  of  the  soil  which  greatly  affects 


ORGANISMS  IN   THE  SOIL  437 

its  productiveness.  It  has  become  customary  to  refer  to 
the  changes  produced  by  certain  forms  of  bacteria  as  their 
function  in  contributing  to  soil  productiveness. 

361.  Decomposition  of  mineral  matter.  —  Certain  bac- 
teria decompose  some  of  the  mineral  matter  of  the 
soil  and  render  it  more  easily  available  to  the  plant. 
While  the  nature  of  the  processes  and  their  extent 
are  not  known,  there  is  sufficient  evidence  to  justify 
the  above  statement.  It  is  well  known  that  several 
forms  of  bacteria  are  instrumental  in  decomposing  rock, 
and  that  sulfur  and  iron  compounds  are  acted  upon  by 
other  forms. 

To  what  extent  the  very  difficultly  soluble  forms  of 
phosphorus,  as  tricalcium  phosphate  for  example,  are 
rendered  soluble  and  available  to  agricultural  plants  by 
microorganisms,  is  a  matter  of  great  importance.  The 
extent  to  which  the  subject  has  been  investigated  is 
rather  limited,  but,  in  the  main,  there  is  indicated  a  con- 
siderable action  of  both  bacteria  and  fungi  on  tricalcium 
phosphate. 

362.  Influence  of  certain  bacteria  and  molds  on  the 
solubility  of  phosphates.  —  Some  very  significant  experi- 
ments were  performed  by  Stoklasa,  Duchacek,  and  Pitra,1 
who  found  that  bone  meal,  when  brought  into  contact 
with  pure  cultures  of  certain  bacteria,  was  apparently 
rendered  soluble,  the  extent  to  which  the  solubility  pro- 
gressed varying  with  the  different  forms  of  bacteria 
brought  into  contact  with  it.  The  percentage  of  the  total 
phosphorus  in  the  meal  that  was  rendered  soluble  was  as 
follows :  — 

1  Stoklasa,  J.,  Duchacek,  F.,  and  Pitra,  J.  Uber  den  Einfluss 
der  Bakterien  auf  die  Knochenzersetzung.  Centrlb.  f.  Bakt., 
II,  Band  6,  Seite  526-535,  554-558.     1900. 


438       SOILS:    PROPERTIES  AND  MANAGEMENT 


Not  inoculated 
B.  megatherium 
B.  fluorescens    .     . 
B.  proteus  vulgaris 
B.  butyricus  Hueppe 
B.  mycoides       .     . 
B.  mesentericus 


Per  cent 
3.83 
21.56 
9.19 
14.7!) 
15.55 
23.03 
20.60 


Lohnis l  quotes  Grazia  e  Cerza  to  have  found  that 
Aspergillus  nigrr,  PeniciUiutn  glaucvm,  and  P.  brevicaule, 
isolated  from  garden  soil,  when  placed  in  nutrient  solu- 
tion with  tricalcium  phosphate,  assimilated  one-fifth  to 
one-third  of  the  phosphorus  in  sixty  days. 

There  is  some  difference  of  opinion  whether  the  solvent 
action  arising  from  bacterial  growth  is  due  entirely  to  the 
acids  that  are  produced  by  the  bacteria  exerting  such 
action,  or  whether  there  is  also  some  other  influence  exer- 
cised by  bacteria.  Stoklasa  accounts  for  the  solvent 
action  of  the  bacteria  in  his  experiments  by  the  bacterial 
secretion  of  proteolytic  and  diastatic  enzymes  acting  on 
the  bone  meal.  In  opposition  to  this  idea,  Krober2 
maintains  that  the  solvent  action  depends  on  the  kind  of 
fermentation  that  the  organic  matter  undergoes,  acid  fer- 
mentation rendering  the  phosphates  more  soluble,  while 
ammoniacal  fermentation  results  in  no  solvent  action  on 
tricalcium  phosphate  and,  in  the  presence  of  sufficient 
basic  material,  may  render  the  monocalcium  and  dical- 


1  Lohnis,  F.  Handbuch  d.  Landw.  Bakteriologie,  Seite  700. 
Berlin.    1910. 

2  Krober,  E.  Tiber  das  Loslichwerden  der  Phosphorsaure 
aus  Wasserunloslichen  Verbindungen  unter  der  Einwirkung  von 
Bakterien  und  Hefen.  Jour.  f.  Landw.,  Band  57,  Seite  5-80. 
1909-1910. 


ORGANISMS  IN  THE  SOIL  439 

cium  phosphates  insoluble.  He  would  limit  the  solvent 
action  of  bacteria  to  the  effect  of  the  acids  they  produce. 

Sackett,  Patten,  and  Brown  *  have  in  a  measure  repeated 
Stoklasa's  experiments  and  obtained  somewhat  similar 
results,  which  lead  them  to  conclude  that  there  is  a 
solvent  agent  other  than  the  acids  produced  by  the 
bacteria. 

It  would  appear  from  these  experiments  that  bacteria, 
and  possibly  fungi,  commonly  found  in  soils  act  on  tri- 
calcium  phosphate  in  such  a  manner  as  to  render  a  part 
of  it  soluble.  Nevertheless,  experiments  that  have  been 
conducted  for  the  purpose  of  ascertaining  whether  tri- 
calcium  phosphate  in  soils  is  rendered  more  readily  avail- 
able to  plants  when  large  quantities  of  decomposing  organic 
matter  are  present  than  when  this  is  not  the  case,  have 
not,  in  the  main,  indicated  that  the  decomposing  organic 
matter  increases  availability  of  the  phosphorus  (par.  439). 
An  explanation  of  this  may  possibly  be  found  in  the 
occurrence  of  a  reverse  biological  process  which  results 
in  the  transformation  of  soluble  phosphates  into  insoluble 
ones,  the  occurrence  of  such  a  process  having  been  found 
by  Stoklasa  2  and  others. 

The  carbon  dioxide  produced  by  bacteria  is  a  solvent 
for  many  of  the  silicates  of  the  soil,  and  may  free  calcium 
and  potassium  from  hornblende  and  feldspar. 

Various  groups  of  sulfur  bacteria,  through  the  produc- 
tion of  H2S  and  H2S04,  act  on  iron  in  the  soil  and  con- 
vert  it   into   sulfide   and    sulfate.     Carbon   dioxide   also 

1  Sackett,  W.  G.,  Patten,  A.  J.,  and  Brown,  C.  W.  The 
Solvent  Action  of  Soil  Bacteria  upon  the  Insoluble  Phosphates 
of  Raw  Bone  Meal  and  Natural  Rock  Phosphate.  Michigan 
Agr.  Exp.  Sta.,  Special  Bui.  43.     1908. 

2  Stoklasa,  J.  Biochemischer  Kreislauf  des  Phosphat-Ions 
imBoden.     Centrlb.  f.  Bakt.,  II,  Band  29,  Seite  385-519.     1913. 


440       SOILS:    PROPERTIES  AND  MANAGEMENT 

plays  a  part  in  the  solution  of  iron.  The  lower  fungi  and 
the  algae  precipitate  iron  from  solution  as  iron  oxide-. 

363.  Decomposition  of  non-nitrogenous  organic  matter. 
—  The  organic  matter  commonly  decomposed  in  soils 
contains  a  large  proportion  of  compounds  containing  no 
nitrogen.  Many  non-nitrogenous  substances  decompose 
rather  rapidly,  and  the  organic  nitrogen  disappears  less 
rapidly  than  the  carbon,  hydrogen,  and  oxygen  of  organic 
bodies. 

Humus  always  contains  a  higher  percentage  of  nitrogen 
than  do  the  plants  from  which  it  is  formed. 

The  non-nitrogenous  substances  consist  of  cellulose  and 
allied  compounds  forming  the  cell  walls  of  plants,  and  the 
carbohydrates,  organic  acids,  fats,  and  the  like,  contained 
in  them.  The  dissolution  of  cellulose  is  brought  about 
by  the  action  of  the  enzyme  cytase  secreted  by  a  number 
of  fungi,  and  is  also  probably  accomplished  by  the  Bacillus 
amylobacter,  but  whether  through  the  secretion  of  an 
enzyme  is  not  known.  Other  bacteria  have  been  reported 
to  secrete  a  cytase  that  acts  on  certain  constituents  of  the 
cell  wall.  It  is  probable  that  numerous  organisms  capa- 
ble of  fermenting  cellulose  and  allied  substances  exist  in 
the  soil,  accomplishing  this  decomposition  through  the 
production  of  cytase. 

The  effect  of  cytase  on  cellulose  and  other  fiber  is  to 
hydrolyze  it  with  the  formation  of  sugar,  as  glucose, 
mannose,  zylose,  arabinose,  and  the  like. 

Starch  is  converted  into  glucose  by  a  ferment  (diastase) 
either  present  in  the  plant  itself  or  possibly  secreted  by 
fungi  or  bacteria.  All  the  sugars  are  finally  converted 
into  organic  acids  which  may  combine  with  mineral  bases. 
Distinct  organisms  have  been  isolated  that  can  utilize 
for  their  development   formates,   acetates,   propionates, 


ORGANISMS  IN   THE  SOIL  441 

butyrates,  and  the  like,  the  final  product  being  carbon 
dioxide  and  water.  Thus,  step  by  step,  the  non-nitroge- 
nous matter  incorporated  with  the  soil  is  carried  by 
one  and  another  form  of  organism  from  the  most  com- 
plex to  the  simplest  combinations.  , 

The  final  product  of  the  decomposition  of  carbonaceous 
matter  being  carbon  dioxide,  there  is  a  return  to  the  air 
of  the  compound  from  which  the  carbon  of  the  decompos- 
ing substance  was  originally  derived.  In  the  plant,  un- 
less it  is  saprophytic,,  the  carbon  of  the  tissues  comes 
largely  from  the  carbon  dioxide  of  the  air,  from  which 
more  complex  carbon-bearing  compounds  are  produced  and 
utilized  in  its  functions  or  in  its  tissues.  A  portion  of  the 
carbon  is  returned  to  the  air  by  the  plant  in  the  form  of 
carbon  dioxide ;  the  remainder  is  retained  by  the  plant, 
and  may  be  returned  by  the  process  of  decay  or  may  be 
consumed  by  an  animal,  and,  as  the  result  of  its  physio- 
logical processes,  either  exhaled  as  carbon  dioxide  or 
deposited  in  the  tissues  to  be  later  decomposed  and  con- 
verted into  carbon  dioxide.  The  soil  is  thus  the  scene  of 
at  least  a  part  of  the  varied  transformations  through 
which  carbon  is  continually  passing  as  it  is  utilized  by 
higher  plants,  animals,  bacteria,  and  fungi. 

The  non-nitrogenous  organic  substances  in  their  various 
stages  furnish  food  for  a  large  number  of  bacteria,  among 
which  are  those  concerned  in  the  decomposition  of  mineral 
matter  and  in  the  processes  of  nitrification  and  nitrogen 
fixation.  There  are,  therefore,  two  ways  in  which  these 
substances  are  of  great  importance  in  soil  fertility :  (1)  as 
a  source  of  carbon  dioxide  and  of  organic  acids ;  (2)  as 
a  food  supply  for  useful  soil  bacteria. 

364.  Decomposition  of  nitrogenous  organic  matter.— 
The  decomposition  of  nitrogenous  organic  matter  is  ac- 


442       SOILS:    PROPERTIES  AND  MANAGEMENT 

complished  by  a  series  of  changes  from  one  compound  to 
another,  as  was  seen  to  be  the  case  with  the  non-nitroge- 
nous materials.  The  final  products  are  carbon  dioxide, 
water,  usually  some  hydrocarbon  gases  resulting  from 
the  carbon  and  hydrogen  of  the  organic  matter,  and  also 
some  hydrogen  sulfide  or  other  gas  containing  sulfur 
or  a  final  oxidation  of  the  sulfur  of  the  proteids  into  sul- 
fates ;  while  the  nitrogen  is  ultimately  converted  into 
nitrates,  or  into  free  nitrogen,  although  a  portion  of  the 
original  nitrogen  sometimes  escapes  into  the  air  in  the 
intermediate  stage,  ammonia. 

The  processes  will  be  discussed  under  the  following 
heads,  which  represent  certain  more  or  less  definite  stages 
in  the  decomposition  :  1 ,  decay  and  putrefaction ;  2,  am- 
monification ;  3,  nitrification ;  4,  denitrification ;  5,  fixa- 
tion of  atmospheric  nitrogen.  These  various  processes 
form  what  has  been  termed  the  nitrogen  cycle. 


CHAPTER  XXI 
THE   NITROGEN   CYCLE 

Of  the  various  elements  composing  the  nutrients  used 
by  plants,  nitrogen  has  the  highest  commercial  value. 
It  is,  moreover,  absorbed  in  large  quantities  by  agricul- 
tural plants  and  the  supply  is  constantly  liable  to  loss  in 
drainage  water  and  in  the  gaseous  form.  Its  importance 
to  agriculture  has  led  to  much  study  of  its  occurrence, 
combinations,  reactions,  and  movements  in  the  soil. 

When  it  is  recalled  that  the  nitrogen  gas  of  the  atmos- 
phere is  the  one  primitive  source  of  the  world's  supply  of 
nitrogen,  it  becomes  apparent  that  the  agencies  that  have 
been  instrumental  in  its  transfer  from  one  condition  to 
another  have  been  extremely  active.  The  movement  of 
nitrogen  from  air  to  soil,  from  soil  to  plant,  from  plant 
back  to  soil  or  to  animal,  and  from  animal  back  to  soil, 
with  a  return  to  air  at  various  stages,  involves  many 
forces,  many  factors,  many  organisms,  and  many  re- 
actions. 

365.  Decay  and  putrefaction.  —  Decomposition  of  the 
nitrogenous  organic  matter  of  the  soil,  consisting  largely 
of  the  proteins,  begins  with  either  one  of  two  processes 
—  decay  or  putrefaction.  Decay  is  produced  by  aerobic 
'bacteria,  and  naturally  occurs  when  the  conditions  are 
most  favorable  for  their  development.  When  the  condi- 
tions are  otherwise,  the  growth  of  these  bacteria  is  checked, 
and  then  further  decomposition  would  be  extremely  slow 

443 


444       SOILS:    PROPERTIES  AND   MANAGEMENT 

were  it  not  for  the  other  process  —  putrefaction.  Putre- 
faction is  produced  by  anaerobic  bacteria.  In  the  same 
body,  and  consequently  in  the  same  soil,  decay  and  putre- 
faction may  be  in  progress  simultaneously,  decay  taking 
place  on  the  outside  and  on  the  surfaces  of  other  parts 
exposed  to  the  air,  while  putrefaction  occurs  on  the  in- 
terior, where  the  supply  of  oxygen  is  limited.  By  means 
of  the  two  processes,  decomposition  is  greatly  facilitated. 
Decay  (see  Fig.  61)  produces  a  very  rapid  and  complete 
decomposition  of  the  substance  in  which  it  operates,  most 
of  the  carbon  and  hydrogen  being  quickly  converted  into 
carbon  dioxide  and  water,  and  the  nitrogen  into  ammonia 
and  probably  some  free  nitrogen.  The  latter  is  possibly 
due  to  the  oxidation  of  ammonia,  thus 

4  NH3  +  3  02  =  6  H20  +  2  N2 

The  sulfur  of  the  proteins  finally  appears  in  the  form  of 
sulfates. 

What  the  intermediate  products  are  has  not  been  deter- 
mined, but  in  the  decay  of  meat,  in  which  there  was  an 
abundant  supply  of  oxygen,  succinic,  palmytic,  oleic,  and 
phenyl-propionic  acids  have  been  found. 

Putrefaction  results  in  a  large  number  of  complex  inter- 
mediate compounds  and  proceeds  much  more  slowly. 
Many  of  the  substances  thus  produced  are  highly  poison- 
ous, and  most  of  them  have  a  very  offensive  odor.  They 
may  be  further  broken  down  by  decay  when  the  condi- 
tions are  suitable,  or  by  a  continuation  of  the  process  of 
putrefaction.  In  either  case,  the  poisonous  properties 
and  the  odor  are  removed. 

In  the  process  of  decomposition  of  organic  matter  two 
classes  of  substances  are  produced :  (1)  those  that  have 
been  excreted  or  secreted  by  the  bacterium,  and  therefore 


THE  NITROGEN    CYCLE  445 

have  passed  through  the  metabolic  processes  of  the  organ- 
ism; (2)  those  that  have  been  formed  because  of  the 
removal  of  certain  atoms  by  bacteria  or  enzymes  from 
compounds,  thus  necessitating  a  readjustment  of  the 
remaining  atoms  and  the  consequent  formation  of  a  new 
compound. 

Putrefaction  is  carried  on  by  a  large  number  of  forms 
of  bacteria,  the  resulting  product  depending  on  the  sub- 
stance in  process  of  decomposition  and  on  the  bacteria 
involved.  Some  of  the  characteristic,  although  not  con- 
stant, products  formed  in  the  putrefaction  of  albumin 
and  proteins  are  albumoses,  peptones,  and  amino  acids, 
followed  by  the  formation  of  cadaverine,  putrescine,  ska- 
tol,  and  indol.  Where  an  abundant  supply  of  oxygen  is 
present,  or  where  a  sufficient  supply  of  carbohydrates 
exist,  these  substances  are  not  formed.  There  are  many 
other  products  of  putrefaction,  including  a  number  of 
gases,  as  carbon  dioxide,  hydrogen  sulfide,  marsh  gas, 
phosphine,  hydrogen,  nitrogen,  and  the  like. 

It  will  be  noticed  that  these  changes,  like  those  occur- 
ring in  the  non-nitrogenous  organic  matter,  involve  a 
breaking-down  of  the  more  complex  compounds  and  the 
formation  of  simpler  ones ;  and  that  a  very  large  number 
of  bacteria  are  concerned  in  the  various  steps,  while  even 
the  same  substances  may  be  decomposed  and  the  same 
resulting  compounds  formed  by  a  number  of  different 
species  of  bacteria. 

Present-day  knowledge  of  the  subject  does  not  make 
it  possible  to  present  a  list  of  the  bacteria  concerned  in 
each  step,  or  to  name  all  the  intermediate  products 
formed;  but  for  the  student  of  the  soil  the  principal 
consideration  is  a  knowledge  of  the  circumstances  under 
which  the  nitrogen  is  made  available  to  plants,  and  the 


446       SOILS:    PROPERTIES  AND  MANAGEMENT 

conditions  that  are  likely  to  result  in  its  loss  from  the 

soil. 

366.  Ammonification.  —  Decay  and  putrefaction  may 
be  considered  as  the  beginning  of  the  process  of  ammoni- 
fication. Ammonification  (see  Fig.  61),  as  its  name 
implies,  is  that  stage  of  the  process  during  which  am- 
monia is  formed  from  the  intermediate  products. 

Like  the  other  processes  of  decomposition,  there  are 
many  species  of  bacteria  capable  of  forming  ammonia 
from  nitrogenous  organic  substances.  Different  forms 
display  different  abilities  in  converting  nitrogen  of  the 
same  organic  material  into  ammonia,  some  acting  more 
rapidly  or  more  thoroughly  than  others.  In  tests  by 
certain  investigators  in  which  the  same  bacteria  are  used 
on  different  substances,  the  order  of  their  efficiency  is 
changed  with  the  change  of  substance.  It, seems  likely, 
therefore,  that  certain  forms  are  most  efficient  when 
acting  on  certain  organic  compounds;  that,  in  other 
words,  each  species  is  best  adapted  to  the  decom- 
position of  certain  substances,  while  capable  of  attack- 
ing others  although  less  effectively.  This  characteristic 
preference  of  a  class  of  bacteria  for  the  decomposition  of 
certain  substances  is  made  evident  by  the  experiments 
of  Sackett,1  who  found  that  in  some  soils  dried  blood  was 
ammonified  more  rapidly  than  was  cottonseed  meal,  while 
in  other  soils  the  reverse  was  true. 

367.  Bacteria  and  substances  concerned  in  ammoni- 
fication. —  Among  the  bacteria  producing  ammonifica- 
tion are  B.  mycoides,  B.  subtilis,  B.  mesentericus  vul- 
gatus,  B.  janthinus,  and  B.  proteus  vulgaris.  Of  these, 
B.   mycoides  has  been  very  carefully   studied,   and   the 

1  Sackett,  W.  G.  The  Ammonifying  Efficiency  of  Certain 
Colorado  Soils.     Colorado  Agr.  Exp.  Sta.,  Bui.  184.     1912. 


THE  NITROGEN   CYCLE  447 

findings  of  Marchal *  may  be  taken  as  representative 
of  the  process  of  ammonification.  He  found  that  when 
this  bacterium  was  seeded  on  a  neutral  solution  of  albumin, 
ammonia  and  carbon  dioxide  were  produced,  together 
with  small  amounts  of  peptone,  leucine,  tyrosine,  and 
formic,  butyric,  and  propionic  acids.  He  concludes  that 
in  the  process,  atmospheric  oxygen  is  used,  and  that 
the  carbon  of  the  albumin  is  converted  into  carbon  dioxide, 
the  sulfur  into  sulfuric  acid,  and  the  hydrogen  partly 
into  water,  and  partly  into  ammonia  by  combining 
with  the  nitrogen  of  the  organic  substance.  He  suggests 
that  a  complete  decomposition  of  the  albumin  occurs 
according  to  the  following  reaction  :  — 

C72H112  N18S022  +  77  02 

=  29  H20  +  72  C02  +  S03  +  18  NH3 

The  greatest  activity  occurred  at  a  temperature  of  86°. 
F.,  and  as  low  as  68°  F.  action  was  rather  strong.  Access 
of  an  increased  amount  of  air,  produced  by  increasing  the 
surface  of  the  liquid,  increased  the  rate  of  ammonification. 
A  slightly  acid  reaction  in  the  liquid  produced  the  maxi- 
mum activity,  but  in  a  neutral  or  even  slightly  acid  me- 
dium the  process  was  continued,  although  much  less 
actively. 

Marchal  found  that  B.  mycoides  was  also  capable  of 
ammonifying  casein,  fibrin,  legumin,  glutin,  myosin, 
serin,  peptones,  creatine,  leucine,  tyrosine,  and  asparagine, 
but  not  urea. 

368.  Nitrification.  —  Some  agricultural  plants  can  util- 
ize ammonium  salts  as  a  source  of   nitrogen.     This  has 

1  Marchal,  E.  Sur  la  Production  de  rAmmoniaque  dans 
le  Sol  par  les  Microbes.  Bulletins  de  l'Acad.  Royale  de  Belg., 
3  series,  F.  25,  pp.  727-776.     1893. 


448       SOILS:    PROPERTIES   AND  MANAGEMENT 

been  determined  for  maize,  rice,  peas,  barley,  and  po- 
tatoes. Other  plants,  such  as  beets,  show  a  decided 
preference  for  nitrogen  in  the  form  of  nitrates.  Whether 
any  of  the  common  crops  can  thrive  as  well  on  ammo- 
nium salts  as  on  nitrates  has  not  been  finally  demon- 
strated. In  most  arable  soils  the  transformation  of  nitro- 
gen does  not  stop  with  its  conversion  into  ammonia,  but 
goes  on  by  an  oxidation  process  to  the  formation  of  first 
nitrous,  and  then  nitric,  acids  (see  Fig.  61).  This  may  be 
considered  to  proceed  according  to  the  following  equa- 
tions :  — 

2  NH3  +  3  02  =  2  HN02  +  2  H20 

2  HN02  -f  02  =  2  HN03 

The  acid  in  either  case  combines  with  one  of  the  bases 
of  the  soil,  usually  calcium,  so  that  calcium  nitrate 
results. 

Each  of  these  steps  is  brought  about  by  a  distinct 
bacterium,  but  the  bacteria  are  closely  related.  Collec- 
tively they  are  called  nitrobacteria.  Nitrosomonas  and 
Nitrosococcus  are  the  bacteria  concerned  in  the  conver- 
sion of  ammonia  into  nitrous  acid  or  nitrites.  The  former 
are  supposed  to  be  characteristic  of  European,  and  the 
latter  of  American,  soils.  They  are  sometimes  referred 
to  as  nitrous  ferments. 

Nitrobacter  are  those  bacteria  that  convert  nitrites 
into  nitrates.  They  are  also  designated  nitric  ferments. 
There  seem  to  be  some  differences  in  bacteria  from  dif- 
ferent soils,  but  the  differences  are  slight  and  the  condi- 
tions favoring  the  actions  of  the  bacteria  are  similar.  It 
is  also  true  that  the  conditions  favoring  the  action  of 
Nitrosomonas  and  Nitrobacter  are  similar,  and  they 
are  generally  found   in  the  same  soils,   although   some 


THE  NITROGEN   CYCLE 


449 


experiments  show  that  in  the  same  soil  nitrites  may 
sometimes  accumulate,  indicating  conditions  more  favor- 
able to  the  development  of  the  Nitrosomonas  bacteria. 


/////vgen  ofa> 


*  To  a/?ima/ 


. ' .  Green  '  Farm 
\  :  '.  Manure  ;  . 

*:'  ■'  oecAY  "\ 

___3gj  humus    •    . 


complex//-  ccmpour.  w 

•V?/trates  * "Mfr/fes *-— <>*  : 


Fig.  61.— Diagrammatic  representation  of  the  movements  of  nitrogen 
between  soil,  plant,  animal,  and  atmosphere.  This  has  been  termed 
the  nitrogen  cycle. 

The  formation  of  nitrates  usually  follows  closely  on  the 
production  of  nitrites,  so  that  there  is  rarely  more  than 
a  trace  of  the  latter  to  be  found  in  soils.  A  soil  favorable 
to  the  process  of  nitrification  is  usually  well  adapted  to 
all  the  processes  of  nitrogen  transformation. 

Marked  differences  have  been  found  in  the  nitrifying 
power  of  bacteria  from  different  soils.  Highly  productive 
soils  have  generally  been  found  to  contain  bacteria  having 
greater  nitrifying  efficiency  than  those  from  less  produc- 
tive soils,  but  this  may  not  always  be  the  case,  as  other 
factors  may  limit  the  productiveness. 

369.  Effect  of  organic  matter  on  nitrification.  —  A 
peculiarity  in  the  artificial  culture  of  nitrifying  bacteria 
2g 


450       SOILS:    PROPERTIES  AND  MANAGEMENT 

is  that  they  cannot  be  grown  in  artificial  media  containing 
organic  matter.  This  property  for  a  long  time  prevented 
the  isolation  and  identification  of  these  organisms,  as  it 
was  hardly  conceivable  that  organisms  living  in  the  dark, 
where  energy  cannot  be  obtained  from  sunlight,  could 
exist  without  using  the  energy  stored  by  organic  matter. 
It  has  been  suggested,  in  explanation  of  this,  that  the 
energy  produced  by  the  oxidation  involved  in  the  process 
of  nitrification  makes  possible  the  growth  of  the  organisms 
under  these  apparently  impossible  conditions.  Some 
experimenters  report  having  grown  nitrobacteria  in  or- 
ganic media,  but  it  is  generally  believed  at  present  that 
this  is  not  possible  and  that  there  has  been  some  error  in 
the  work  of  these  experimenters. 

The  presence  of  peptone  in  the  proportion  of  500 
parts  per  million  completely  prevents  the  development 
of  nitrobacteria,  and  one-half  that  quantity  checks  it; 
while  150  parts  of  ammonia  to  the  million  has  a  similar 
effect.  In  a  normal  soil  the  quantity  of  soluble  ammo- 
nium salts  is  well  below  this  amount,  as  must  also  be  that 
of  soluble  organic  matter.  In  confirmation  of  the  inhibit- 
ing effect  of  organic  matter  on  the  nitrobacteria,  cases 
have  been  reported  of  soils  very  rich  in  organic  matter 
in  which  no  bacteria  of  this  type  exist. 

It  has  also  been  stated  that  very  heavy  manuring 
with  organic  manures  results  in  decreased  nitrification 
in  the  soil.  While  this  may  be  true  where  farm  manure 
is  used  in  the  quantities  sometimes  applied  in  gardening 
operations,  it  is  not  likely  to  be  the  case  in  soils  on  which 
ordinary  field  crops  are  grown.  The  principle  is  well 
illustrated  by  the  dry-earth  closet.  Manure  mixed  with 
earth  in  relatively  small  proportions  and  kept  aerated 
by  occasional  mixing  undergoes  a  very  thorough  decom- 


THE  NITROGEN    CYCLE 


151 


position  of  the  manure  but  without  any  corresponding 
increase  in  nitrates.  On  the  other  hand,  under  field  con- 
ditions, manure  used  in  relatively  small  amounts  does 
not  undergo  this  serious  loss. 

The  application  of  twenty  tons  of  farm  manure  to  the 
acre  to  sod  on  a  clay  loam  soil  for  three  consecutive  years, 
at  Cornell  University,  resulted  in  a  larger  production 
of  nitrates  on  the  manured  soil  than  on  a  contiguous  plat 
of  similar  soil  left  unmanured.  This  was  true  during  the 
third  year  of  the  applications,  when  the  land  was  in  sod, 
and  also  during  the  fourth  year,  when  no  manure  was 
applied  to  either  plat  and  when  both  plats  were  planted 
to  corn,  as  may  be  seen  from  the  following  table :  — 

Nitrates    Produced    on    Heavily    Manured    and    on    Un- 
manured Soil 


Land  in  timothy 

April  23       .  . 

May  3    .     .  . 

May  14       .  . 

May  30       .  . 

June  1    .     .  . 

June  13  .     .  . 

June  20       .  . 

July  24        .'  . 

August  14  . 
Land  in  maize 

May  19       .  . 

June  22       .  . 

July  6     .     .  . 

July  28        .  . 

August  10 


NO3  in  Parts  to  a  Million, 
Dry  Soil 

Unmanured 
soil 

Twenty  tons 
manure  to  the 
acre  for  three 

years 

8.2 

21.0 

4.1 

4.6 

3.3 

4.5 

2.0 

4.0 

2.4 

2.0 

0.8 

1.1 

1.3 

3.0 

2.2 

2.8 

1.8 

3.0 

17.5 

20.1 

42.8 

79.3 

50.0 

105.0 

195.0 

304.0 

151.0 

184.0 

452       SOILS:    PROPERTIES  AND  MANAGEMENT 

370.  Effect  of  soil  aeration  on  nitrification.  —  Probably 
the  most  potent  factor  governing  nitrification  in  the  soil 
is  the  supply  of  air.  In  clay,  and  even  in  loam  soils,  the 
tendency  to  compactness  is  such  as  to  prevent  the  pres- 
ence of  sufficient  air  to  enable  nitrification  to  proceed 
as  rapidly  as  desirable  unless  the  soil  is  well  tilled.  Col- 
umns of  soil  eight  inches  in  diameter  and  eight  inches  in 
depth  were  removed  from  a  field  of  clay  loam  on  the  Cor- 
nell University  farm,  and  carried  to  the  greenhouse  with- 
out disturbing  the  structure  of  the  soil  as  it  existed  in 
the  field.  At  the  same  time,  vessels  of  similar  size  were 
filled  with  soil  dug  from  a  spot  near  by.  These  may  be 
termed  unaerated  and  aerated  soils.  Both  were  kept 
at  the  same  temperature  and  moisture  content  in  the 
greenhouse,  but  no  plants  were  grown  on  them.  The 
production  of  nitrates  was  as  follows :  — 


Time  of  Analysis 

Nitrates  in  Dry  Soil,  Parts 
to  the  Million 

Unaerated  soil 

Aerated  soil 

When  taken  from  field 

After  standing  one  month     .... 
After  standing  two  months  .     .     .     . 

3.2 
4.2 
9.0 

3.2 
17.6 
45.6 

371.  Effect  of  sod  on  nitrification.  —  Nitrification 
proceeds  slowly  on  sod  land,  especially  if  the  soil  is  heavy. 
On  the  same  type  of  soil  as  that  used  in  the  experiment 
last  described,  the  average  quantities  of  nitrates  for  each 
month  of  the  growing  season  in  the  surface  eight  inches 
of  sod  land,  as  compared  with  maize  land  under  the  same 
manuring,  were  as  follows :  — 


THE  NITROGEN   CYCLE 


453 


Month 

Nitrates  in  Dry  Soil,  Parts 
to  the  Million 

Sod  land 

Maize  land 

April 

May 

8.9 
3.0 
2.4 
4.0 
5.4 

17.1 

June 

40.3 

July 

194.0 

August 

186.7 

The  amount  of  nitrogen  removed  by  the  maize  crop 
was  greater  than  that  removed  by  the  timothy;  conse- 
quently the  greater  amount  in  the  former  soil  cannot  be 
due  to  the  effect  of  the  crop. 

So  far  as  the  conservation  of  nitrogen  is  concerned, 
sod  is  an  ideal  crop,  for  nitrates  are  formed  very  little 
faster  than  they  are  used,  and  are  not  carried  off  in  large 
quantities  by  the  drainage  water. 

In  the  corn  land  as  much  as  175  pounds  of  nitrate 
nitrogen  was  present  in  the  first  twelve  inches  of  one 
acre,  or  fully  three  times  as  much  as  was  used  by  the 
crop. 

372.  Depths  at  which  nitrification  takes  place.  —  War- 
ington  x  concluded  from  his  experiments  that  nitrification 
takes  place  only  in  the  surface  six  feet  of  soil.  Hall 2 
has  pointed  to  the  fact  that  no  more  nitrates  were  leached 
from  the  60-inch  lysimeter  at  Rothamsted  than  from  the 
one  40  inches  deep ;   which  is  very  good  evidence  that  in 


1Warington,  R.  On  the  Distribution  of  the  Nitrifying 
Organism  in  the  Soil.  Trans.  Chem.  Soc,  Vol.  51,  p.  118. 
1887. 

2  Hall,  A.  D.  The  Book  of  the  Rothamsted  Experiments, 
p.  230.     New  York,   1905. 


454       SOILS:    PBOPEUTIES  AND  MANAGEMENT 

that  particular  soil  nitrification  does  not  take  place  below 
40  inches  from  the  surface.  In  more  porous  soils,  how- 
ever, nitrification  probably  extends  deeper,  especially 
in  the  rich  and  porous  subsoils  of  arid  and  semiarid  regions. 

In  all  probability,  nitrification  is  largely  confined  to 
the  furrow  slice,  where  the  opening-up  of  the  soil  by  til- 
lage has  provided  the  necessary  air,  and  where  the  tem- 
perature rises  to  a  point  more  favorable  to  the  action 
of  nitrifying  bacteria.  The  results  from  the  aerated  and 
unaerated  soils  as  shown  above  represent  the  differences 
that  doubtless  exist  between  the  furrow  slice  and  the  sub- 
soil so  far  as  nitrification  is  concerned. 

373.  Loss  of  nitrates  from  the  soil.  —  Nitrogen  hav- 
ing been  converted  into  the  form  of  nitric  acid,  it  im- 
mediately combines  with  available  bases  in  the  soil, 
forming  salts,  all  of  which  are  very  easily  soluble  and 
which  are  carried  in  solution  by  the  soil  water.  In  a 
region  of  much  rainfall,  the  removal  of  nitrates  in  the 
drainage  water  is  very  rapid.  Hall l  states  that  nitrates 
formed  during  the  summer  or  the  autumn  of  one  year  are 
practically  all  removed  from  the  soil  of  the  Rothamsted 
fields  before  the  crops  of  the  following  year  have  advanced 
sufficiently  to  utilize  them.  It  was  formerly  customary 
to  fertilize  with  ammonium  salts  in  autumn,  but  the 
drainage  water  showed  on  analysis  such  a  large  quantity 
of  nitrates  during  the  months  intervening  between  the 
time  of  fertilizing  and  the  opening  of  the  growing  season 
that  the  practice  was  discontinued. 

In  regions  of  less  rainfall  or  of  greater  surface  evapora- 
tion, the  loss  in  this  way  is  less,  reaching  a  minimum  in  an 
arid    region    when    irrigation    is    not    practiced.     Under 

1  Hall,  A.  D.     The  Soil,  p.  176.     New  York,  1903. 


THE  NITROGEN   CYCLE  455 

such  conditions,  there  is  a  return  of  nitrates  to  the  upper 
soil  as  capillary  water  moves  upward  to  replace  evapo- 
rated water.  In  fact,  wherever  evaporation  takes  place 
to  any  considerable  extent  there  is  some  movement  of 
this  kind.  The  need  for  catch  crops  to  take  up  and  pre- 
serve nitrogen  is  therefore  greater  in  a  humid  region  than 
in  an  arid  or  a  semiarid  one.  A  system  of  cropping  that 
allows  the  land  to  stand  idle  for  some  time,  or  a  crop  that 
requires  intertillage,  as  does  maize,  fails  to  utilize  all  the 
nitrates  produced,  and  promotes  the  loss  of  nitrogen  in 
drainage  water. 

374.  Nitrate  reduction.  —  The  nitrogen-transforming 
bacteria  thus  far  studied  have  been  those  that  cause 
the  oxidation  of  nitrogen  as  the  result  of  their  activi- 
ties. A  number  of  forms  of  bacteria  that  accomplish  a 
reverse  action  may  now  be  considered.  The  several 
processes  involved  are  commonly  designated  by  the 
general  term  denitrification,  and  comprise  the  follow- 
ing:  1,  reduction  of  nitrates  to  nitrites  and  ammonia; 
2,  reduction  of  nitrates  to  nitrites,  and  of  these  to  ele- 
mentary nitrogen. 

The  number  of  organisms  that  possess  the  ability  to 
accomplish  one  or  more  of  these  processes  is  very  large  — 
in  fact,  greater  than  the  number  involved  in  the  oxida- 
tion processes ;  but,  in  spite  of  their  numbers,  permanent 
loss  of  nitrogen  in  ordinary  arable  soils  is  unimportant 
in  amount,  although  in  heaps  of  barnyard  manure  it 
may  be  a  very  serious  cause  of  loss.  *~ 

Some  of  the  specific  bacteria  reported  as  bringing  about 
nitrate  reduction  are :  B.  ramosus  and  B.  pestifer,  which 
reduce  nitrates;  B.  mycoides,  B.  subtilis,  B.  mesentericus 
vulgatus,  and  many  other  ammonification  bacteria  which 
are  capable  of  converting  nitrates  into  ammonia. 


456       SOILS:    PROPERTIES  AND  MANAGEMENT 

B.  denitrificans  alpha  and  B.  denitrificans  beta  reduce 
nitrates  with  the  evolution  of  gaseous  nitrogen. 

375.  Nitrate-assimilating  organisms.  —  In  addition  to 
the  nitrate-reducing  bacteria  already  mentioned,  there  are 
other  bacteria  which  also  utilize  nitrates ;  but,  like  higher 
plants,  these  convert  the  nitrogen  into  organic  nitrog- 
enous substances.  However,  as  they  operate  in  the 
dark  and  cannot  obtain  energy  from  sunlight,  they  must 
have  organic  acids  or  carbohydrates  as  a  source  of  energy. 
While  these  bacteria  cannot  be  considered  as  nitrate 
reducers,  they  help  to  deplete  the  supply  of  nitrates  when 
conditions  are  favorable  for  their  development.  What 
these  conditions  are  is  not  well  understood,  nor  can  any 
estimate  be  made  as  to  the  extent  of  their  operations. 

376.  Denitrification.  —  The  term  denitrification  may 
be  used  to  include  both  the  process  of  nitrate  reduction 
and  that  of  nitrate  assimilation  (see  Fig.  61). 

Most  of  the  denitrifying  bacteria  perform  their  func- 
tions only  under  a  limited  amount  of  oxygen,  while  others 
can  operate  in  the  presence  of  a  more  liberal  supply ; 
but,  in  general,  thorough  aeration  of  the  soil  practically 
prevents  denitrification.  Straw  apparently  carries  an 
abundant  supply  of  denitrifying  organisms,  and  also 
furnishes  a  supply  of  carbohydrates  which  favor  their 
action;  so  that  stable  manure  is  very  likely  to  undergo 
denitrification,  and  straw  or  coarse  stable  manure  are 
conducive  to  the  growth  of  denitrifying  bacteria  in  the  soil. 

Under  ordinary  farm  conditions,  denitrification  is 
of  no  significance  in  the  soil  where  proper  drainage  and 
good  tillage  are  practiced.     Warington  1  showed  that  if 

1  Warington,  R.  Investigations  at  Rothamsted  Experi- 
mental Station.  U.  S.  D.  A.,  Office  of  Exp.  Sta.,  Bui.  8,  p.  64. 
1892.  r  - 


THE  NITROGEN   CYCLE  457 

an  arable  soil  is  kept  saturated  with  water  to  the  exclu- 
sion of  air,  nitrates  added  to  the  soil  are  decomposed, 
with  the  evolution  of  nitrogen  gas.  As  lack  of  drainage 
is  usually  most  pronounced  in  early  spring,  when  the  soil 
is  likely  to  be  depleted  of  nitrates,  it  is  not  likely  that 
much  loss  arises  in  this  way  unless  a  nitrate  fertilizer  has 
been  added.  Among  the  many  difficulties  arising  from 
poor  drainage,  denitrification  of  an  expensive  fertilizer 
may  be  a  very  considerable  item. 

The  addition  of  a  nitrate  fertilizer  to  a  well-drained  soil 
receiving  stable  manure  is  not  likely  to  result  in  a  loss  of  ni- 
trates unless  the  dressings  of  manure  have  been  extremely 
heavy.  Hall *  states  that  at  Rothamsted,  where  large  quan- 
tities of  nitrate  of  soda  are  used  every  year  in  connection 
with  annual  dressings  of  farm  manure,  the  nitrate  produces 
nearly  as  large  an  increase  when  added  to  the  manured 
as  when  added  to  the  unmanured  plat.  In  other  words, 
there  appears  to  be  no  loss  of  nitrate  by  denitrification. 

It  is  possible  to  reach  a  point  in  manuring  at  which 
denitrification  may  take  place.  Market-gardeners  some- 
times reach  this  point,  when  fifty  tons  or  more  of  farm 
manure,  in  addition  to  a  nitrate  fertilizer,  are  added  to 
the  soil.  Plowing  under  heavy  crops  of  green  manure 
may  produce  the  same  result.  In  either  case,  the  best 
way  to  overcome  the  difficulty  is  to  allow  the  organic 
matter  to  partly  decompose  before  adding  the  fertilizer. 
The  removal  of  the  easily  decomposable  carbohydrates 
needed  by  the  denitrifying  organisms  decreases  or  pre- 
cludes their  activity. 

377.  Nitrogen  fixation  through  symbiosis  with  higher 
plants.  —  It  has  long  been  recognized  by  farmers  that 

1  Hall,  A.  D.  The  Book  of  the  Rothamsted  Experiments, 
pp.   114-115.     New  York,   1905. 


458       SOILS:    PROPERTIES  AND  MANAGEMENT 

certain  crops,  as  clover,  alfalfa,  peas,  beans,  and  some 
others,  improve  the  soil,  making  it  possible  to  grow  larger 
crops  of  cereals  after  these  crops  have  been  on  the  land. 
Within  the  past  century  the  benefit  has  been  traced  to 
an  increase  in  the  nitrogen  content  of  the  soil,  and  the 
specific  plants  so  affecting  the  soil  were  found  to  be,  with 
a  few  exceptions,  those  belonging  to  the  family  of  legumes. 
It  has  furthermore  been  demonstrated  that  under  certain 
conditions  these  plants  utilize  the  uncombined  nitrogen  of 
the  atmosphere  (see  Fig.  61),  and  that  they  contain, 
both  in  the  aerial  portions  and  in  the  roots,  a  very  high 
percentage  of  nitrogen.  In  consequence,  the  decomposi- 
tion of  even  the  roots  of  the  plants  in  the  soil  leaves  a 
large  amount  of  nitrogenous  matter. 

378.  Relation  of  bacteria  to  nodules  on  roots.  —  It  has 
also  been  shown  that  the  utilization  of  atmospheric  nitro- 
gen is  accomplished  through  the  aid  of  certain  bacteria 
that  live  in  nodules  (tubercles)  on  the  roots  of  the  plants. 
These  bacteria  take  free  nitrogen  from  the  air  in  the  soil, 
and  the  host  plant  secures  it  in  some  form  from  the  bac- 
teria or  their  products.  The  presence  of  a  certain  species 
of  bacteria  is  necessary  for  the  formation  of  tubercles. 
Leguminous  plants  grown  in  cultures  or  in  soil  not  con- 
taining the  necessary  bacteria  do  not  form  nodules  and  do 
not  utilize  atmospheric  nitrogen,  the  result  being  that 
the  crop  produced  is  less  in  amount  and  the  percentage 
of  nitrogen  in  the  crop  is  less  than  if  nodules  were  formed. 

The  nodules  are  not  normally  a  part  of  leguminous 
plants,  but  are  evidently  caused  by  some  irritation  of  the 
root  surface,  much  as  a  gall  is  caused  to  develop  on  a  leaf 
or  a  branch  of  a  tree  by  an  insect.  In  a  culture  contain- 
ing the  proper  bacteria,  the  prick  of  a  needle  on  the  root 
surface  will  cause  a  nodule  to  form  in  the  course  of  a  few 


THE  NITROGEN  CYCLE  459 

days.  The  entrance  of  the  organism  is  effected  through 
a  root-hair  which  it  penetrates,  and  it  may  be  seen  as  a 
filament  extending  the  entire  length  of  the  hair  and  into 
the  cells  of  the  cortex  of  the  root,  where  the  growth  of 
the  tubercle  starts. 

Even  where  the  causative  bacteria  occur  in  cultures 
or  in  the  soil,  a  leguminous  plant  may  not  secure  any 
atmospheric  nitrogen,  or  perhaps  only  a  small  quantity, 
if  there  is  an  abundant  supply  of  readily  available  com- 
bined nitrogen  on  which  the  plant  may  draw.  The  bac- 
teria have  the  ability  to  utilize  combined  nitrogen  as 
well  as  uncombined  nitrogen,  and  prefer  to  have  it  in 
the  former  condition.  On  soils  rich  in  nitrogen,  legumes 
may  therefore  add  little  or  no  nitrogen  to  the  soil ;  while 
in  properly  inoculated  soils  deficient  in  nitrogen  an  impor- 
tant gain  of  nitrogen  results. 

While  B.  radicicola  is  considered  the  organism  common 
to  all  leguminous  plants,  it  is  now  known  that  the  organ- 
isms from  one  species  of  legume  are  not  equally  well  adapted 
to  the  production  of  tubercles  on  each  of  the  other  species 
of  legumes.  They  show  greater  activity  on  some  species 
than  on  others,  but  do  not  develop  so  successfully  on  all 
species  as  on  the  one  from  which  the  organisms  were 
taken.  It  was  rather  generally  believed  at  one  time  that 
the  longer  any  species  of  legume  is  in  contact  with  the 
organisms  from  another  species,  the  more  active  this 
species  becomes  and  the  greater  is  the  utilization  of 
atmospheric  nitrogen.  Considerable  doubt  has  been  cast 
on  this  view  in  recent  years,  and  it  is  now  generally  con- 
ceded that  the  bacteria  of  certain  legumes  are  not  capable 
of  inoculating  certain  other  species  of  legumes. 

379.  Transfer  of  nitrogen  to  the  plant.  —  It  has  been 
shown  by  several  investigators  that  bacteria  from  the 


460       SOILS:    PROPERTIES  AND  MANAGEMENT 

nodules  of  legumes  are  able  to  fix  atmospheric  nitrogen 
even  when  not  associated  with  leguminous  plants.  There 
would  seem  to  be  no  doubt,  therefore,  that  the  fixation  of 
nitrogen  in  the  tubercles  of  legumes  is  accomplished  di- 
rectly by  this  organism,  not  by  the  plant  itself  nor  through 
any  combination  of  the  plant  and  the  organism  —  though 
both  of  these  hypotheses  have  been  advanced.  The  part 
played  by  the  plant  is  doubtless  to  furnish  the  carbohydrates 
which  are  required  in  large  quantities  by  all  nitrogen-fixing 
organisms  and  which  the  legumes  are  able  to  supply  in 
large  amounts.  The  utilization  of  large  quantities  of 
carbohydrates  by  the  nitrogen-fixing  bacteria  in  the  tuber- 
cles may  also  account  for  the  small  proportion  of  non- 
nitrogenous  organic  matter  in  the  plants. 

How  the  plant  absorbs  this  nitrogen  after  it  has  been 
secured  by  the  bacteria  is  less  well  understood.  Early 
in  the  growth  of  the  tubercle,  a  mucilaginous  substance 
is  produced,  which  permeates  the  tissues  of  the  plant  in 
the  form  of  long,  slender  threads  containing  the  bacteria. 
These  threads  develop  by  branching  or  budding,  and  form 
what  have  been  called  Y  and  T  forms,  known  as  bac- 
terids, which  are  peculiar  to  these  bacteria.  The  threads 
finally  disappear,  and  the  bacteria  diffuse  themselves  more 
or  less  through  the  tissues  of  the  root.  What  part  the 
bacteroids  play  in  the  transfer  of  nitrogen  is  not  known. 
It  has  been  suggested  that  in  this  form  the  nitrogen  is 
absorbed  by  the  tissues  of  the  plant.  It  seems  quite  likely 
that  the  nitrogen  compounds  produced  within  the  bacteria 
cells  are  diffused  through  the  cell  wall  and  absorbed  by 
the  plant. 

380.  Soil  inoculation  for  legumes.  —  Immediately  fol- 
lowing the  discovery  of  the  nitrogen-fixing  bacteria,  the 
possibility  was  conceived  of  securing  a  better  growth  of 


THE  NITBOGEN   CYCLE  461 

leguminous  crops  on  soils  not  having  previously  grown 
such  crops  successfully.  Extensive  experiments  showed 
the  practicability  of  inoculating  land  for  a  certain  legumi- 
nous crop  by  spreading  on  its  surface  soil  from  a  field 
on  which  the  same  crop  is  successfully  growing.  It  is 
manifestly  much  better  to  apply  the  organisms  from  a 
certain  species  of  legumes  from  a  field  having  grown  the 
same  species,  than  to  attempt  to  use  organisms  from  an- 
other species  of  legumes.  The  fact  that  soil  inoculation 
by  means  of  soil  from  other  fields  may  possibly  transmit 
weed  seeds  and  fungous  diseases,  and  also  necessitates 
the  transportation  of  a  great  bulk  and  weight  of  material, 
has  led  to  numerous  efforts  to  inoculate  soil  by  means 
of  pure  cultures.  The  pure  culture  may  also  make  it 
possible  to  bring  to  the  soil  bacteria  of  greater  physio- 
logical efficiency  than  those  already  there. 

The  first  attempt  at  inoculation  by  pure  cultures 
was  made  in  Germany,  the  cultures  being  sold  under  the 
name  of  "  nitragin."  Careful  experiments  made  with 
this  material  previous  to  the  year  1900  did  not  show 
it  to  be  very  efficient ;  but  in  recent  years  improvements 
in  the  method  of  manipulating  the  cultures  have  resulted 
in  much  greater  success.  In  "  nitragin "  the  medium 
used  for  growing  the  organisms  is  gelatin,  and  before  use 
this  was  formerly  dissolved  in  water ;  but  now  a  solution 
of  greater  density  is  used  in  order  to  prevent  a  change  of 
osmotic  pressure,  which  may  cause  plasmolysis  and  result 
in  the  destruction  of  the  bacteria. 

Within  recent  years  a  number  of  cultures  for  soil 
inoculation  have  been  offered  to  the  public.  The  first 
of  these  utilized  absorbent  cotton  to  transmit  the  bac- 
teria in  a  dry  state  from  the  pure  culture  in  the  laboratory 
to  the  user  of  the  culture,  who  was  to  prepare  therefrom 


462       SOILS:    PROPERTIES  AND  MANAGEMENT 

another  culture  to  be  used  for  inoculating  the  soil.  ( ire- 
ful investigation  of  this  method  showed  that  its  weakness 
lay  in  drying  the  cultures  on  the  absorbent  cotton,  which 
frequently  resulted  in  the  death  of  the  organisms.  More 
recently,  liquid  cultures  have  been  placed  on  the  market 
in  this  country,  and  these  have,  in  the  main,  proved  to 
be  more  successful,  notably  those  sent  out  by  the  United 
States  Department  of  Agriculture.  Another  very  suc- 
cessful culture  medium,  now  being  distributed  by  the 
Department  of  Plant  Physiology  at  Cornell  University,  is 
steamed  soil.  The  process  of  steaming  under  a  pressure 
of  two  or  three  atmospheres  increases  greatly  the  solu- 
bility of  both  organic  and  inorganic  matter,  and  produces 
a  medium  highly  favorable  to  the  development  of  the 
organisms  isolated  from  the  nodules  of  legumes. 

Liquid  cultures  for  legume  inoculation  have  now  been 
prepared  and  distributed  by  the  United  States  Depart- 
ment of  Agriculture  for  seven  years,  and  during  this  time 
a  record  has  been  kept  of  the  results  so  far  as  it  has  been 
possible  to  do  this.  These  are  summarized  by  Keller- 
man  *  as  follows :  average  percentage  of  success,  76 ; 
average  percentage  of  failure,  24.  If,  however,  the  doubt- 
ful reports  are  included  with  the  failures,  the  percentage 
of  success  is  reduced  to  38.  Kellerman  states  as  his 
opinion  that  inoculation  with  pure  liquid  cultures  is  as 
certain  a  means  of  infection  as  is  inoculation  with  soil 
from  fields  on  which  legumes  have  been  successfully  grown 
for  extended  periods,  if  the  soil  to  be  infected  is  one  well 
adapted  to  the  leguminous  crop;  but  on  soils  not  well 
suited  to  legumes,  the  use  of  soil  from  old  fields  is  a  much 
more  satisfactory  medium  with  which  to  attempt  inocula- 

1  Kellerman,  K.  F.  The  Present  Status  of  Soil  Inoculation. 
Centrlb.  f.  Bakt.,  II,  Band  34,  Seite  42-50.     1912. 


THE  NITROGEN   CYCLE  463 

tion.  It  is  only  a  question  of  time  until  a  successful 
method  of  inoculating  soil  from  artificial  cultures  will  be 
found.  In  the  meantime,  inoculation  by  means  of  in- 
fested soil  is  the  most  practical  method. 

381.  Nitrogen  fixation  without  symbiosis  with  higher 
plants.  —  If  a  soil  is  allowed  to  stand  idle,  either  without 
vegetation  or  in  grass,  it  will,  under  favorable  moisture 
conditions  in  the  northern  states,  accumulate  in  one  or  two 
years  an  appreciable  amount  of  nitrogen  not  present  at  the 
beginning  of  the  period.  At  the  Rothamsted  Experiment 
Station,  one  of  the  fields  in  volunteer  plants,  consisting 
mainly  of  grass  without  legumes,  gained  in  the  course  of 
twenty  years  about  twenty-five  pounds  of  nitrogen  per 
acre  annually.1  According  to  Hall,  the  nitrogen  brought 
down  by  rain  would  account  for  about  five  pounds  to  the 
acre  per  annum,  and  dust,  bird  droppings,  and  the  like,  for 
a  little  more. 

382.  Nitrogen-fixing  organisms.  —  Direct  experiment 
has  shown  that  certain  bacteria  have  the  ability  to  utilize 
atmospheric  nitrogen  and  to  leave  it  in  the  soil  in  a  com- 
bined form  (see  Fig.  61).  An  anaerobic  bacillus- — Clos- 
tridium pasteurianum  —  was  first  found  to  produce  this 
result.  Later,  a  commercial  culture  called  "  alinit  "  was 
placed  on  the  market  in  Germany,  claimed  to  contain 
Bacterium  ellenbachensis ,  with  which  the  soil  was  to  be 
inoculated,  and  it  was  claimed  that  a  large  fixation  of 
atmospheric  nitrogen  would  result.  A  number  of  tests  of 
this  material  failed  to  show  that  it  caused  any  marked 
fixation  of  atmospheric  nitrogen. 

A  number  of  other  nitrogen-fixing  organisms  have 
since  been  discovered.     There  are:     (1)  several  members 

1  Hall,  A.  D.  On  the  Accumulation  of  Fertility  by  Land 
Allowed  to  Run  Wild.     Jour.  Agr.  Sci.,  Vol.  1,  p.  241.     1905. 


464       SOILS:    PROPERTIES  AND  MANAGEMENT 

of  the  group  designated  Azotobacter,  which  are  aerobic 
bacteria,  and  which  some  investigators  hold  to  be  capable  of 
fixing  atmospheric  nitrogen  when  grown  in  pure  cultures, 
while  others  believe  them  to  be  able  to  do  so,  at  least  in 
large  amounts,  only  in  the  presence  of  certain  other 
organisms;  (2)  members  of  the  Granulobacter  group, 
which  are  large  spore-bearing  bacilli  of  anaerobic  habits; 
(3)  Bacillus  radiobacter,  which  appear  to  be  closely  related 
to  or  identical  with  the  B.  radicicola  of  legume  tubercles. 
The  last-named  has  been  shown  to  be  able  to  fix  atmospheric 
nitrogen  even  when  not  growing  in  symbiosis  with  leg- 
umes. 

There  are  doubtless  many  other  nitrogen-fixing  or- 
ganisms still  to  be  discovered. 

A  peculiarity  of  these  nitrogen-fixing  organisms  is 
their  use  of  carbohydrates,  which  they  decompose  in 
the  process  of  nitrogen  fixation.  They  secure  more 
atmospheric  nitrogen  when  in  a  nitrogen-free  medium. 
The  presence  of  soluble  lime  or  magnesium  salts,  especially 
carbonates,  is  necessary  for  the  best  performance  of  the 
nitrogen-fixing  function,  as  is  also  the  presence  of  a  some- 
what easily  soluble  form  of  phosphorus.  The  organisms 
are  exceedingly  sensitive  to  an  acid  condition  of  the  soil. 

383.  Mixed  cultures  of  nitrogen-fixing  organisms.  — 
Mixed  cultures  of  the  various  organisms  mentioned  'fix 
larger  amounts  of  nitrogen  than  do  the  pure  cultures 
of  any  one  of  them,  while  some  forms  are  incapable  of 
fixing  nitrogen  in  pure  cultures.  Certain  algae,  particularly 
the  blue-green  algae,  aid  greatly  in  promoting  growth  and 
nitrogen  fixation  by  these  organisms.  This  they  probably 
do  by  producing  carbohydrates,  which  are  used  by  the 
bacteria  as  a  source  of  energy  for  nitrogen  fixation,  the 
bacteria  furnishing  the  algae  with  nitrogenous  compounds. 


THE  NITROGEN   CYCLE  465 

To  what  extent  the  relation  is  symbiotic  is  not  known  at 
present,  but  it  seems  probable  that  a  relation  may  exist 
similar  to  that  between  leguminous  plants  and  the  nitrogen- 
gathering  bacteria  in  their  nodules. 

384.  Nitrogen  fixation  and  denitrification  antagonistic. 
—  Nitrogen  fixation  and  denitrification  are  reverse  pro- 
cesses. The  former  is,  for  most  bacteria,  favored  by 
an  abundant  supply  of  air  and  a  moderately  high  tempera- 
ture. Thus,  at  75°  F.  fixation  was  rapid,  at  59°  F.  it  was 
decreased,  and  at  44°  F.  there  was  no  fixation.  Denitri- 
fication is  favored  by  a  somewhat  limited  supply  of 
oxygen. 

There  is  no  reason  to  believe  that  the  practical  impor- 
tance of  nitrogen  fixation  without  legumes  is  equal,  under 
the  most  favorable  conditions,  to  that  with  legumes. 
A  further  knowledge  of  the  organisms  effecting  fixation 
and  of  their  habits  will  doubtless  make  possible  a  greater 
utilization  of  their  powers  to  supplement  the  use  of  leg- 
umes as  a  source  of  combined  nitrogen  in  the  soil. 

TREATMENT    OF    SOILS    WITH    VOLATILE    ANTISEPTICS    AND 
WITH  HEAT 

Attention  was  first  drawn  to  the  effects  of  carbon  bisul- 
fide on  the  soil  in  a  paper  by  Girard  l  and  one  by  Oberlin2 
which  appeared  in  1894.  Girard  noticed  that  soil  treated 
with  carbon  disulfide  for  the  purpose  of  combating  a  para- 
sitic disease  of  sugar-beet  was  more  productive  than  it 

1  Girard,  A.  Recherches  sur  l'Augmentation  des  Recoltes 
par  1' Injection  dans  le  Sol  du  Sulfure  de  Carbone  a  Doses  Mas- 
sives.     Bui.  Soc.  Nationale  d'Agric.,  Tome  54,  p.  356.     1894. 

2  Oberlin.  Bodenmiidigkeit  und  Schwefelkohlenstoff.  Mainz, 
1894. 

2h 


466       SOILS:    PROPERTIES  AND  MANAGEMENT 

was  before  such  treatment.  The  beneficial  effect  of  the 
treatment  extended  to  the  second  year. 

Oberlin  found  a  somewhat  similar  condition  where 
the  soil  of  vineyards  treated  with  carbon  bisulfide  to  kill 
phylloxera  showed  greatly  increased  productiveness  after 
the  treatment.  The  effect  of  carbon  bisulfide  on  the 
vineyard  soil  was  to  make  it  possible  to  raise  grapes  con- 
tinually on  the  same  land,  whereas  it  had  previously  been 
necessary  to  rest  the  land  by  growing  a  succession  of 
other  crops  at  intervals  of  several  years.  It  was  noticed, 
however,  that  immediately  after  treatment  the  plants  did 
not  grow  so  well  as  under  normal  conditions.  Systematic 
investigations  of  the  subject  then  began,  and  as  early  as 
1895  Pagnoul 1  reported  that  when  carbon  bisulfide  is 
applied  to  soils  nitrification  is  temporarily  depressed. 

Investigation  of  the  effect  of  heat  on  soil  had  begun 
somewhat  earlier,  when  Frank2  showed  in  1888  that  it 
increases  the  quantities  of  soluble  matter,  both  organic 
and  inorganic,  as  well  as  causing  the  soil  to  be  more  pro- 
ductive. 

The  subject  has  been  investigated  by  a  large  number  of 
persons,  and  in  addition  to  carbon  bisulfide  a  considerable 
number  of  other  volatile  antiseptics,  including  ether, 
chloroform,  and  toluene,  have  been  found  to  influence  the 
productiveness  of  soils.  The  effect  of  heat,  particularly 
in  steam,  at  various  temperatures  from  slightly  above 
normal  to  more  than  200°  C,  has  also  been  studied,  while 

1  Pagnoul,  M.  Nouvelles  Recherches  sur  les  Transforma- 
tions que  Subit  l'Azote  .dans  le  Sol.  Annales  Agronomique, 
Tome  21,  pp.  497-501.     1895. 

2  Frank,  B.  Ueber  den  Einfluss  welchen  das  Sterilisiren 
des  Erdbodens  auf  die  Pflanzen  Entwickelung  ausubt.  Ber. 
d.  Deut.  Bot.  Gesell.  (Generalversammlungs  Heft)  Band  4, 
Seite  87-97.     1888, 


THE  NITROGEN   CYCLE  467 

it  has  been  found  that  the  mere  drying  of  soils  effects 
important  changes  in  their  solubility  and  in  the  bacterial 
processes  that  occur  in  them.  As  the  result  of  the  in- 
vestigations, certain  well-established  facts  have  been 
worked  out  in  connection  with  certain  treatments  when 
applied  to  most  soils. 

385.  Effects  of  carbon  bisulfide  and  heat  on  properties 
of  soils.  —  Volatile  antiseptics  usually  increase  the  pro- 
ductiveness of  soils,  although  there  may  be  at  first  a  slight 
temporary  retardation  of  plant  growth.  It  is  of  course 
customary  to  permit  the  antiseptic  to  volatilize  from 
the  soil  before  seed  is  planted.  For  this  purpose  the  soil 
is  spread  out  in  a  thin  layer,  in  which  condition  it  is  al- 
lowed to  remain  until  the  odor  of  the  antiseptic  has  dis- 
appeared. The  soil  is  then  placed  in  vessels  and  moistened 
and  the  seeds  are  planted  in  it. 

Other  characteristic  effects  of  treatment  with  volatile 
antiseptics  reported  by  different  investigators  are :  (1) 
an  initial  decrease  in  the  numbers  of  bacteria,  followed  by 
a  long-continued  increase ;  (2)  a  disturbance  of  the  equi- 
librium of  the  flora,  by  which  certain  bacteria  multiply 
more  rapidly  than  others;  (3)  a  slight  initial  increase 
in  ammonia  content,  followed  by  a  considerable  increase 
in  the  rate  of  production  of  ammonia;  (4)  depression  of 
the  process  by  which  ammonia  is  converted  into  nitric 
acid,  and  a  very  slow  recovery  in  the  activity  of  the  bac- 
teria concerned,  as  a  result  of  which  ammonia  accumulates 
in  the  soil ;  (5)  an  increase  in  the  rate  at  which  oxidation 
takes  place  in  soils ;   (6)  destruction  of  protozoa. 

386.  Hypotheses  to  account  for  effects  of  carbon 
bisulfide  and  of  heat.  —  A  number  of  hypotheses  have 
been  formulated  by  which  to  account  for  the  increased 
plant  growth  and  for  changes  induced  in  soils  by  treat- 


468       SOILS:    PROPERTIES  AND   MANAGEMENT 

ment  with  heat  and  volatile  antiseptics.  A  number  of 
these  theories  will  be  mentioned,  but  it  should  be  remem- 
bered that  much  important  work  on  the  subject  has  been 
done  by  investigators  who  have  not  advanced  any  hy- 
potheses. 

387.  Koch's  theory.  —  Koch x  was  the  first  to  offer 
any  explanation.  In  1899  he  stated  it  as  his  opinion  that 
carbon  bisulfide  has  a  directly  stimulating  action  on  the 
plants  themselves.  lie  later2  found  ether  to  have  a 
similar  action,  and  continued  his  experiments  with  carbon 
bisulfide.  He  found  that  soil  sterilized  with  heat  pro- 
duced better  crops  when  treated  with  carbon  bisulfide 
than  when  not  so  treated,  and  concludes  that  the  effect 
of  the  antiseptic,  therefore,  cannot  be  due  to  the  effect 
of  the  antiseptic  on  bacteria.  He  also  experimented  with 
field  soils,  and  showed  that  the  size  of  the  crop  on  treated 
soils  is  not  proportional  to  the  quantity  of  nitrogen 
contained. 

The  theory  of  Koch  has  been  supported  by  Fred,3  who 
fertilized  soils  with  an  abundant  supply  of  sodium  nitrate 
and  found  that  in  every  case  in  which  carbon  bisulfide 
was  added  the  growth  and  yield  of  crop  were  much  su- 
perior to  those  in  the  corresponding  pots  not  treated  with 
that  substance.  He  concludes  that  as  there  was  no  lack 
of  plant-food   and   other   conditions  favorable  to  plant 

1  Koch,  A.  Untersuchungen  liber  die  Ursachen  der  Riiben- 
mudigkeit  mit  Besonderes  Beriicksichtigung  der  Schwefel- 
kohlenstoffbehandlung.  Arb.  Deut.  Landw.  Gesell.,  Heft  40, 
Seite  7-38.     1899. 

2  Koch,  A.  Ueber  die  Wirkung  von  Aether  Schwefelkoh- 
lenstoff  auf  Hohere  und  Niedere  Pflanzen.  Centrlb.  f.  Bakt., 
II,  Band  31,  Seite  175-185.     1911-1912. 

3  Fred,  E.  B.  Effect  of  Fresh  and  Well-rotted  Manure 
on  Plant  Growth.  Virginia  Poly.  Inst.  Agr.  Exp.  Sta.,  Ann. 
Rept.  1909-1910,  pp.  142-159. 


THE  NITROGEN  CYCLE  469 

growth,  the  effect  of  the  antiseptic  must  have  been  directly 
on  the  plants. 

388.  Hiltner  and  Stormer's  theory.  —  According  to 
Hiltner  and  Stormer,1  the  effect  of  treatment  with  carbon 
bisulfide  is  to  cause  a  disturbance  in  the  equilibrium  of 
the  different  forms  of  soil  bacteria.  These  investigators 
compared  the  numbers  in  three  groups  of  bacteria  that 
developed  on  gelatin  plates  inoculated  from  soil  infu- 
sions. The  groups  were  Streptothrix,  liquefiers,  and 
non-liquefiers.  The  normal  relation  of  these  in  the  soil 
with  which  they  worked  was  20  per  cent  Streptothrix, 
5  per  cent  liquefiers,  and  70  per  cent  non-liquefiers.  After 
treatment  with  carbon  bisulfide  the  relative  proportions 
were  5  per  cent,  10  per  cent,  and  85  per  cent,  respectively. 
From  70  to  75  per  cent  of  the  whole  number  of  bacteria 
were  destroyed  by  the  treatment,  but  the  numbers  rapidly 
increased  after  treatment,  rising  in  a  few  weeks  to  50,- 
000,000  to  a  gram  in  a  soil  that  contained  10,000,000  to  a 
gram  before  treatment.  This  increase  is  due  largely  to 
the  development  of  the  non-liquefiers,  the  Streptothrix 
remaining  at  about  the  same  actual  number. 

The  fact  that  the  equilibrium  of  the  bacterial  flora 
was  so  greatly  disturbed  by  the  treatment  with  carbon 
bisulfide  led  Hiltner  and  Stormer  to  believe  that  the  greater 
productiveness  of  the  soil  after  treatment  is  due  to  the 
greater  effectiveness  of  the  surviving  and  rapidly  develop- 
ing  forms   in   rendering   available   the   supply   of   plant 

1  Hiltner,  L.,  and  Stormer,  K.  Studien  iiber  die  Bakteri- 
enflora  des  Ackerbodens,  mit  besonderer  Berucksichtigung 
ihres  Verhaltens  nach  einer  Behandlung  mit  Schwefelkohlenstoff 
und  nach  Brache.  Arb.  Biol.  Abt.  f.  Land-  u.  Forstwirtschaft 
am  Kaiserl.  Ges.  Amt.,  Band  III,  Heft  5.  Berlin,  1903.  Ab- 
stract in  Centrlb.  f.  Agrikultur  Chemie,  33  Jahrg.,  Seite  361- 
374.     1904. 


470       SOILS:    PROPERTIES  AND   MANAGEMENT 

nutrients  in  the  soil,  and  to  a  decrease  in  the  number  of 
denitrifying  bacteria,  which  obviates  loss  of  available 
nitrogen  through  their  action. 

Heinze,1  working  with  soils  treated  with  carbon  bisul- 
fide, and  Pfeiffer,  Frank,  Friedlander,  and  Ehrenberg,2 
working  with  steamed  soils,  found  that  there  was  a  large 
fixation  of  nitrogen  following  these  treatments.  They 
conclude  that  this  is  at  least  partly  responsible  for  the 
greater  productiveness  of  the  soils  after  the  treatments 
mentioned. 

389.  Russell  and  Hutchinson's  theory.  —  The  next 
comprehensive  theory  to  be  brought  forward  was  one  by 
Russell  and  Hutchinson,  who  account  for  the  increased 
productiveness  of  soils  partially  sterilized,  either  by  heat 
or  by  volatile  antiseptics,  as  due  to  the  use  by  plants  of 
the  ammonia,  which,  as  had  been  shown  by  previous 
investigators,  accumulated  in  soils  so  treated  by  reason 
of  the  stimulation  given  to  the  process  of  ammonification 
and  the  depression  of  nitrification.  They  hold,  further- 
more, that  the  stimulation  of  ammonification  is  brought 
about  by  the  greatly  increased  numbers  of  bacteria  in 
the  soil  following  the  destruction  of  some  larger  organisms, 
probably  protozoa  or  allied  forms,  that  normally  interfere 
with  the  activities  of  the  ammonifying  bacteria.  Care- 
ful experiments  by  these  investigators  have  shown  that 
there  is  a  much  larger  quantity  of  nitrogen  in  the  combined 
forms   of   ammonia   and    nitrates   in   partially   sterilized 

1  Heinze,  B.  Eine  Weitere  Mitteilungen  uber  den  Schwefel- 
kohlenstofi3  und  die  CS2-Behandlung  des  Bodens.  Centrlb. 
f.  Bakt.,  II,  Band  18,  Seite  56-74,  246-264,  462-470,  624-634, 
790-798.     1907. 

2  Pfeiffer,  Th.,  Frank,  L.,  Friedlander,  K.,  and  Ehrenberg, 
P.  Der  Stickstoffhaushalt  des  Ackerbodens.  Mitt.  d.  Landw. 
Inst.  d.  Konigl.  Univ.  Breslau,  Band  4,  Seite  715-851.     1909. 


THE  NITROGEN   CYCLE  471 

soils  than  in  untreated  soils.  There  can  be  no  doubt, 
therefore,  that,  at  least  for  some  higher  plants,  the  quan- 
tity of  available  nitrogen  is  greater  in  the  treated  soils. 

The  relation  of  protozoa  to  the  ammonifying  bacteria 
is  somewhat  more  difficult  of  demonstration.  Methods 
for  the  enumeration  of  protozoa  in  the  soil  are  not  suffi- 
ciently well  worked  out  to  admit  of  an  entirely  satisfactory 
study  of  their  relation  to  the  ammonifying  bacteria. 
However,  Russell  and  Hutchinson  do  not  hold  that  pro- 
tozoa are  necessarily  the  limiting  factor  in  ammonia 
production  in  normal  soils,  but  grant  that  some  other 
organism  of  comparatively  large  size  may  be  responsible 
for  this.  They  intimate  also  that  not  only  the  available 
nitrogen,  but  also  the  quantities  of  other  plant  nutrients, 
are  limited  by  organisms  destroyed  by  partial  steriliza- 
tion ;  otherwise  increased  productiveness  induced  by 
partial  sterilization  would  be  confined  to  soils  in  which 
nitrogen  is  normally  the  limiting  factor.  The  theory 
does  imply,  however,  that  plant-food  is  the  limiting  factor 
in  all  soils  benefited  by  partial  sterilization  under  the 
conditions  of  the  experiment.1 

1  Russell,  E.  J.,  and  Darbishire,  F.  V.  Oxidation  in  soils 
and  its  relation  to  productiveness.  Part  2.  The  influence 
of  partial  sterilization.  Jour.  Agr.  Sci.,  Vol.  2,  pp.  305-326. 
1907. 

Russell,  E.  J.,  and  Hutchinson,  H.  B.  The  effect  of  partial 
sterilization  of  soil  on  the  production  of  plant  food.  Jour. 
Agr.  Sci.,  Vol.  3,  pp.  111-144.     1909. 

Russell,  E.  J.,  and  Hutchinson,  H.  B.  The  effect  of  partial 
sterilization  of  soil  on  the  production  of  plant  food.     Part  2. 

Russell,  E.  J.,  and  Hutchinson,  H.  B.  The  limitation  of  bac- 
terial numbers  in  normal  soils  and  its  consequences.  Jour.  Agr. 
Sci.,  Vol.  5,  pp.  152-221.     1903. 

Buddin,  W.  Partial  sterilization  of  soil  by  volatile  and 
non-volatile  antiseptics.  Jour.  Agr.  Sci.,  Vol.  6,  pp.  417-451. 
1914. 


472       SOILS:    PROPERTIES  AND  MANAGEMENT 

Some  typical  results  of  investigations  by  Russell  and 
Hutchinson  on  the  effect  of  partial  sterilization  on  bac- 
teria numbers,  ammonia  production,  and  presence  of 
protozoa  are  given  below :  — 


Bacteria 
after  Sixty- 
eight  Days, 
to  a  Gram  of 

Dry  Soil 

Nitrogen 

AS   NH3  AND 
NO3  AFTER 

Sixty-eight 
Days,  Parts 
to  a  Million 
of  Dry  Soil 

Detri- 
mental 
Factor 

Protozoa 
Found 

Untreated  soil    .     . 

Soil  heated  to  40°  C.j 
for  three  hours     J 

Soil  heated  to  56°  C.j 
for  three  hours     J 

11,100,000 

7,500,000 
37,500,000 

13.0 

14.4 
36.7 

Present 

Present 
Killed 

f  Ciliates 
I  Amoeba 
{ Monads 

f  Ciliates 
<  Amoeba 
( Monads 

AU  killed 

390.  Greig-Smith's  theory.  —  An  entirely  different 
explanation  of  the  effect  of  partial  sterilization  on  soils 
has  been  advanced  by  Greig-Smith.1  He  states  that 
when  disinfectants  are  applied  to  the  soil  their  action  is 
a  double  one.  They  kill  the  less  resistant  bacteria,  and 
dissolve  from  the  surfaces  of  the  soil  particles  a  waxy 
covering,  to  which  he  has  given  the  name  "  agricere." 
The  surviving  bacteria,  among  which  are  the  beneficial 
ones,  are  able  to  develop  more  rapidly  because  of  the 
greater  accessibility  of  the  food  supply  which  the  re- 
moval of  the  "  agricere  "  has  exposed. 

Greig-Smith  holds  that  heat  destroys  substances  toxic 

1  Greig-Smith,  R.  The  Bacteriotoxins  and  the  "Agricere" 
of  Soils.     Centrlb.  f.  Bakt.,  II,  Band  30,  Seite  154-156.     1911. 


THE  NITROGEN   CYCLE 


473 


to  bacteria,  and  also  certain  of  the  less  resistant  bacteria, 
thus  permitting  the  more  resistant  species  to  multiply 
very  rapidly  owing  to  the  absence  of  the  bacteriotoxins. 

In  order  to  ascertain  whether  chloroform  has  any  effect 
other  than  the  destruction  of  protozoa,  Gr'eig-Smith 
applied  it  to  soil  previously  heated  to  62°  C.  (which  he 
had  found  was  sufficient  to  kill  all  protozoa),  and  then 
determined  the  number  of  bacteria  in  untreated  soil,  in 
heated  soil,  and  in  soil  heated  and  treated  with  chloro- 
form. The  counts  to  a  gram  of  soil  were  made  at  inter- 
vals, and  are  shown  below  : 1  — 


At 

Start 

4  Days 

12  Days 

25  Days 

39  Days 

Untreated  soil 

52 

680,000 

2,700,000 

4,300,000 

5,400,000 

Soil     heated 

at  62°  C. 

16 

15,800,000 

11,800,000 

9,000,000 

8,000,000 

Soil     heated 

at   62°   C. 

and  treated 

with  chlo- 

roform 

13 

24,600,000 

45,400,000 

41,600,000 

90,000,000 

Greig-Smith  concludes  that  as  the  bacteria  developed 
more  rapidly  in  the  soil  treated  with  chloroform  after 
heating  than  in  the  soil  which  was  only  heated  and  in 
which  the  protozoa  were  presumably  dead,  the  chloro- 
form must  have  exerted  some  beneficial  effect  other  than 
the  destruction  of  protozoa,  and  assumes  that  this  is  due 
to  the  removal  of  "  agricere." 

Partial  or  complete  sterilization  of  soils  has  been  prac- 

1  Greig-Smith,  R.  Contributions  to  our  Knowledge  of  Soil 
Fertility.  Proe.  Linnsean  Soc.  New  South  Wales,  1912,  Part  II, 
pp.  238-243. 


474        SOILS:    PROPERTIES  AND  MANAGEMENT 

ticed  in  greenhouses  for  a  long  time,  principally  for  the 
purpose  of  combating  plant  diseases.  Its  value  in  in- 
creasing productiveness  has  been  a  consideration  since 
this  phase  of  the  subject  has  been  emphasized  by  investi- 
gations, and  the  treatment  of  "  sewage-sick  "  soils  has 
been  shown  by  Russell  and  Golding  !  to  be  a  practical 
matter.  It  is  as  a  means  of  studying  the  principles  of 
soil  fertility,  however,  that  the  investigation  of  the  sub- 
ject of  partial  sterilization  of  the  soil  is  of  greatest  im- 
portance. 

*  Russell,  E.  J.,  and  Golding,  J.  Investigations  on  "sick- 
ness" in  soil.  Part  1.  "Sewage  sickness."  Jour.  Agr.  Sci., 
Vol.  5,  pp.  27-47.     1912. 

Russell,  E.  J.,  and  Golding,  J.  Investigations  on  "sickness" 
in  soil.  Part  2.  "Sickness"  in  glasshouse  soils.  Jour.  Agr. 
Sci.,  Vol.  5,  pp.  86-111.     1912. 


CHAPTER  XXII 
THE  SOIL  AIR 

The  air  of  the  soil  is  merely  a  continuation  of  the 
atmospheric  air  into  the  interstitial  spaces  of  the  soil, 
when  these  are  not  filled  with  water.  As  it  is  more  or 
less  inclosed  by  the  soil,  movement  does  not  take  place 
so  readily  as  it  does  above  the  surface  of  the  ground  and 
hence  the  soil  air  is  more  greatly  influenced  by  its  sur- 
roundings than  is  atmospheric  air.  This  leads  to  impor- 
tant differences  in  composition  between  the  atmospheric 
air  and  soil  air,  the  composition  of  the  latter  depending 
on  a  variety  of  conditions  in  which  physical,  chemical, 
and  biological  properties  play  a  part. 

FACTORS   THAT   DETERMINE   VOLUME 

The  amount  of  air  that  soils  contain  varies  with  their 
properties,  and  in  any  one  soil  the  air  content  varies  with 
certain  changes  to  which  the  soil  is  subject  from  time  to 
time.  The  factors  that  influence  the  volume  of  air  in 
soils  are:  (1)  texture;  (2)  structure;  (3)  organic  matter ; 
(4)  moisture  content. 

391.  Texture. — The  size  of  the  soil  particles  affects 
the  air  capacity  of  the  soil  in  exactly  the  same  way 
as  it  does  the  pore  space,  since  in  dry  soil  they  are 
identical.  A  fine-textured  soil  in  a  dry  condition  would 
therefore  contain  as  large  a  volume  of  air  as  would  a 

475 


476       SOILS:    PROPERTIES  AND  MANAGEMENT 

coarse-textured  soil,  provided  the  particles  were  spherical 
and  all  of  the  same  size.  Under  the  conditions  actually 
existing  in  the  field,  the  soils  composed  of  small  particles 
generally  possess  the  larger  amount  of  air  space. 

392.  Structure.  —  The  volume  of  air  in  a  water-free 
soil  being  identical  with  the  pore  space,  the  formation 
of  aggregates  of  particles  is  favorable  to  a  large  air  volume. 
The  volume  of  air  in  any  soil,  therefore,  changes  from 
time  to  time;  and  particularly  is  this  true  of  a  fine- 
grained soil,  in  which  the  changes  in  structure  are  greater 
than  in  a  soil  with  large  particles.  A  change  in  soil 
structure  may  greatly  alter  the  volume  of  air  contained 
by  changing  the  pore  space,  thereby  influencing  the  pro- 
ductiveness. Clay  is  affected  to  the  greatest  extent  in 
this  way. 

393.  Organic  matter.  —  Since  organic  matter  is  more 
porous  than  mineral  particles  of  any  size  or  arrangement, 
the  effect  of  that  constituent  is  always  to  increase  the 
volume  of  air.  While  this  is  generally  beneficial  in  a 
humid  region,  it  is  often  very  injurious  in  an  arid  region. 
Unless  sufficient  water  falls  on  the  soil  to  wash  the  soil 
particles  around  the  organic  matter  and  to  maintain  a 
supply  sufficient  to  promote  decomposition,  the  presence 
of  vegetable  matter  leaves  the  soil  so  open  that  the  capil- 
lary rise  of  moisture  is  interfered  with,  and  the  consider- 
able movement  of  air  keeps  the  soil  dry,  with  the  result 
that  the  portion  of  the  soil  layer  mixed  with  and  lying 
above  the  organic  matter  is  too  dry  to  germinate  seeds 
or  to  support  plant  growth. 

394.  Moisture  content.  —  It  is  quite  evident  that  the 
larger  the  proportion  of  the  interstitial  space  filled  with 
water,  the  smaller  will  be  the  quantity  of  air  contained. 
This  does  not  necessarily  mean  that  the  higher  the  per- 


THE  SOIL   AIR  477 

centage  of  water  in  the  soil,  the  smaller  will  be  the  volume 
of  air,  since  the  amount  of  pore  space  determines  both  the 
water  and  the  air  capacity.  A  soil  with  30  per  cent 
moisture  may  contain  more  air  than  one  with  a  water 
content  of  20  per  cent,  because  of  the  tendency  of  mois- 
ture to  move  the  soil  particles  farther  apart. 

In  soils  in  the  field,  the  average  diameter  of  the  cross 
section  of  the  pore  space  is  the  most  potent  factor  in 
determining  the  volume  of  air.  Small  spaces  are  likely 
to  hold  water,  while  larger  spaces,  not  retaining  water 
against  gravity,  are  filled  with  air. 

In  a  clay  soil  the  volume  of  air  is  increased,  other 
things  being  equal,  by  the  formation  of  granules,  and  is 
decreased  by  deflocculation  or  compaction.  The  volume 
of  air  in  any  soil  may  be  calculated  from  the  following 
formula :  — 

%  air  space  =  %  pore  space  —  (%  H20  X  ap.  sp.  gr.) 

COMPOSITION   OF   SOIL   AIR 

The  air  of  the  soil  differs  from  that  of  the  outside 
atmosphere  in  that  it  contains  more  water  vapor,  a  much 
larger  proportion  of  carbon  dioxide,  a  correspondingly 
smaller  amount  of  oxygen,  and  slightly  larger  quantities 
of  other  gases,  including  ammonia,  methane,  hydrogen 
sulfide,  and  the  like,  formed  by  the  decomposition  of 
organic  matter. 

395.  Analyses  of  soil  air.  —  The  composition  of  the 
air  of  several  soils,  as  determined  by  Boussingault  and 
Lewy,  is  quoted  by  Johnson  l  in  the  table  following :  — 

1  Johnson,  S.  W.     How  Grops  Feed,  p.  219.     New  York,  1891. 


478       SOILS:    PROPERTIES   AND  MANAGEMENT 


Character 
of  Soil 


Volume  in  One 

Acre  of  Soil  to 

Depth  of  14 

Inches 


Air 
(Cu.  ft.) 


Carbon 
Dioxide 
(Cu.  ft.) 


Composition  of  100  Parts 
of  Soil  Air  by  Volume 


Carbon 
Dioxide 


Oxygen 


Nitrocen 


Sandy  subsoil  of  forest  . 
Loamy  subsoil  of  forest  . 
Surface  soil  of  forest    .     . 

Clay  soil 

Soil  of  asparagus  bed  not 

manured  for  one  year  . 
Soil     of    asparagus     bed 

freshly  manured  .  . 
Sandy  soil,  six  days  after 

manuring  ..... 
Sandy  soil,  ten  days  after 

manuring    (three    days 

of  rain) 

Vegetable  mold  compost 


4,416 

3,530 

5,891 

10,310 

11,182 

11,182 

11,783 


11,783 
21,049 


14 
28 
57 
71 

86 

172 

257 


1,144 

772 


0.24 
0.79 
0.87 
0.66 

0.74 

1.54 

2.21 


9.74 
3.64 


19.66 
19.61 
19.99      79.35 


79.55 
79.52 


19.02 
18.80 


10.35 
16.45 


80.24 
79.66 


79.91 
79.91 


There  are  several  factors  that  influence  the  composi- 
tion of  the  soil  air,  those  of  greatest  importance  being 
the  production  and  escape  of  carbon  dioxide. 

396.  Sources  of  carbon  dioxide  in  soil  air.  —  The 
presence  of  carbon  dioxide  in  soils  is  due  in  small  part 
to  infiltration  from  the  atmospheric  air,  there  being  a 
tendency  for  the  carbon  dioxide,  which  is  heavier  than 
oxygen  and  nitrogen,  to  settle  out.  It  may  also  have  a 
purely  chemical  origin.  But  in  much  greater  measure 
is  the  carbon  dioxide  a  product  of  biological  processes 
that  occur  in  the  soil.  At  one  time  it  was  believed  that 
the  formation  of  carbon  dioxide  in  soils  was  a  purely 
chemical  process  of  oxidation,  and  possibly  a  part  of  the 
gas  is  formed  in  that  way.  It  has  already  been  seen  that 
there  is  a  condensation  of  gases  in  the  manifold  pores 


THE  SOIL   AIR  479 

of  the  soil  (see  par.  268),  the  organic  portion  of  which  is 
especially  capable  of  condensing  gases.  Oxygen  con- 
densed on  the  surface  of  this  organic  matter  would,  in 
the  words  of  Johnson,1  "  spend  itself  in  chemical  action," 
of  which  carbon  dioxide  would  be  the  result. 

There  is  now  no  doubt,  however,  that  biological  pro- 
cesses are  largely  responsible  for  the  occurrence  of  the 
large  quantity  of  carbon  dioxide  in  the  soil  air.  There 
are  two  distinct  processes  involved  :  (1)  the  physiological 
action  of  bacteria  by  which  they  absorb  oxygen  and  give 
off  carbon  dioxide,  and  (2)  the  excretion  of  carbon  dioxide 
by  plant  roots.  The  extent  to  which  carbon  dioxide  is 
produced  in  normal  soils  in  these  two  ways  has  been  es- 
timated by  Stoklasa,2  who  has  done  much  work  on  the 
subject.  He  concludes  that  the  microorganisms  in 
an  acre  of  soil  to  a  depth  of  four  feet  may  produce  between 
sixty-five  and  seventy  pounds  of  carbon  dioxide  a  day 
for  two  hundred  days  in  the  year,  and  that  during  the 
growing  period  the  roots  of  oats  or  wheat  would  give 
off  nearly  as  much  to  an  acre. 

397.  Production  of  carbon  dioxide  as  affecting  com- 
position. —  Although  the  formation  of  carbon  dioxide 
in  the  soil  depends  on  the  decomposition  of  organic 
matter,  it  is  not  always  proportional  to  the  quantity 
of  organic  matter  present.  The  rate  of  decomposition 
varies  greatly,  and  where  this  is  depressed,  as  is  sometimes 
seen  in  muck  or  forest  soils,  the  content  of  carbon  dioxide 
is  relatively  low.     A  high  percentage  of  organic  matter 


1  Johnson,  S.  W.  How  Crops  Feed,  p.  218.  New  York, 
1891. 

2  Stoklasa,  J.  Ueber  den  Ursprung  die  Menge  und  die 
Bedeutung  des  Kohlendioxids  im  Boden.  Centrlb.  f.  Bakt.,  II, 
Band  14,  Seite  723-736.     1905. 


480       SOILS:    PROPERTIES  AND  MANAGEMENT 

is  in  itself  likely  to  prevent  a  proportional  formation  of 
carbon  dioxide,  since  the  accumulation  of  the  gas  may 
inhibit  further  activity  of  the  decomposing  organisms. 

Ramann  1  states  that  the  percentage  of  carbon  dioxide 
in  the  soil  air  has  the  following  relations :  — 

1.  The  carbon  dioxide  increases  with  the  depth. 

2.  In  general  the  percentage  of  carbon  dioxide  rises 
and  falls  with  the  temperature,  being  higher  in  the  warm 
months  and  lower  in  the  cold  months. 

3.  Changes  in  temperature  and  air  pressure  change  the 
percentage  of  carbon  dioxide. 

4.  In  the  same  soil  the  content  of  carbon  dioxide  varies 
greatly  from  year  to  year. 

5.  An  increase  of  moisture  in  the  soil  increases  the  per- 
centage of  carbon  dioxide. 

6.  The  amount  of  carbon  dioxide  varies  in  different 
parts  of  the  soil. 

The  movement  of  carbon  dioxide  from  the  soil  depends 
chiefly  on  diffusion  into  the  outside  atmosphere.  The 
conditions  governing  diffusion,  which  will  be  discussed 
elsewhere  (par.  400),  therefore  largely  determine  the 
rate  of  loss  of  carbon  dioxide  from  the  soil. 

FUNCTIONS   OF  THE   SOIL  AIR 

Both  oxygen  and  carbon  dioxide,  as  they  exist  in  the 
air  of  the  soil,  have  important  relations  to  the  processes 
by  which  the  soil  is  maintained  in  a  habitable  condition 
for  the  roots  of  plants.  Deprived  of  these  gases,  the  soil 
would  soon  become  sterile. 

398.  Oxygen.  —  An  all-important  process  in  the  soil 
is  that  of  oxidation,  because  by  it  the  organic  matter 

1  Ramann,  E.     Bodenkunde.     Seite  301.     Berlin,  1905. 


THE  SOIL  AIR  481 

that  would  soon  accumulate  to  the  exclusion  of  higher 
plant  life  is  disposed  of,  and  the  plant-food  materials 
are  brought  into  a  condition  in  which  they  may  be  ab- 
sorbed by  plant  roots.  The  presence  of  oxygen  is  essen- 
tial to  the  life  of  the  decomposing  organisms  and  to  the 
complete  decay  of  organic  matter.  Through  this  pro- 
cess, roots  of  past  crops,  as  well  as  other  organic  matter 
that  has  been  plowed  under,  are  removed  from  the  soil. 
The  process  of  decay  gives  rise  to  products,  chiefly  car- 
bon dioxide,  that  are  solvents  of  mineral  matter,  and  leaves 
the  nitrogen  and  ash  constituents  more  or  less  available 
for  plant  use. 

Oxygen  is  also  necessary  for  the  germination  of  seeds 
and  the  growth  of  plant  roots.  These  phenomena,  al- 
though not  involving  the  removal  of  large  quantities 
of  oxygen,  are  yet  entirely  dependent  on  its  presence  in 
considerable  amounts. 

399.  Carbon  dioxide.  —  The  solvent  action  of  carbon 
dioxide  is  its  most  important  function  in  the  soil.  By 
this  action  it  prepares  for  absorption  by  plant  roots  most 
of  the  mineral  substances  found  in  the  soil.  Although 
a  weak  acid  when  dissolved  in  water,  its  universal  pres- 
ence and  continuous  formation  during  the  growing  season 
results  in  a  large  total  effect. 

Carbonic  acid  dissolves  from  the  soil  more  or  less  of 
all  the  nutrients  required  by  plants.  The  amounts  so 
dissolved  are  appreciably  greater  than  those  dissolved 
in  pure  water.  The  constant  formation  of  -carbon  dioxide 
by  decomposition  of  organic  matter  keeps  this  solvent 
continually  in  contact  with  the  soil. 

Carbon  dioxide  serves  a  useful  purpose  in  combining 
with  certain  bases  to  form  compounds  beneficial  to  the 
soil.  Particularly  is  this  the  case  with  calcium  carbonate, 
2i 


482       SOILS;    PROPERTIES  AND  MANAGEMENT 

which  is  of  the  greatest  benefit  to  the  soil  in  maintaining 
a  slight  alkalinity  very  favorable  to  the  development  of 
many  beneficial  bacteria  and  to  the  maintenance  of  good 
tilth. 

Stoklasa  *  has  correlated  the  carbon  dioxide  production 
with  the  quantity  of  phosphates  found  in  the  drainage 
water  from  certain  soils.  Some  of  his  results  are  given 
below :  — 


P2O6  in  Drainage 

Water 

(Kilograms  to  a  hectare) 


Relative  Produc- 
tion of  COl 
(Milligrams  to  a  kilo- 
gram soil  in  24  hours) 


Loam    .     . 
Clay      .     . 
Lime  soil 
Humous  soil 


24 
15 
36 
56 


Stoklasa  considers  that  the  production  of  carbon  dioxide 
is  a  measure  of  the  intensity  of  bacterial  action  in  the 
soil,  and  that  in  consequence  of  this  activity  the  phos- 
phorus is  rendered  soluble. 

When  carbon  dioxide  is  combined  as  sodium  carbonate 
or  potassium  carbonate  in  considerable  quantity,  as  in 
certain  alkali  soils,  a  very  injurious  action  on  plant  roots 
and  on  soil  structure  results.  On  plants  the  carbonate 
acts  as  a  direct  poison  (see  par.  305).  The  effect  on 
soil  structure  is  to  deflocculate  the  particles  producing 
the  separate  grain  or  the  compact  arrangement  (see  par. 
420). 


1  Stoklasa,  J.  Methoden  zur  Bestimmung  der  Atmungs- 
intensitat  der  Bakterien  im  Boden.  Zeit.  f.  d.  Landw.  Versuchs- 
wesen  in  Oesterreieh,  Band  14,  Seite  1243-79.     1911. 


THE  SOIL  AIR  483 


MOVEMENT   OF   SOIL  AIR 


There  is  a  constant  movement  of  the  air  in  the  inter- 
stitial spaces  of  the  soil  and  an  exchange  of  gases  between 
the  soil  atmosphere  and  the  outside  atmosphere,  as  well 
as  a  more  general,  but  probably  less  effective,  movement 
of  the  air  out  of  or  into  the  soil,  as  the  controlling  condi- 
tions may  determine.  The  movement  may  be  produced 
by  any  one  or  more  of  the  following  phenomena :  (1) 
diffusion  of  gases ;  (2)  movement  of  water ;  (3)  changes 
in  atmospheric  pressure;  (4)  changes  of  temperature  in 
atmosphere  or  in  soil ;   (5)  suction  produced  by  wind. 

400.  Diffusion  of  gases.  —  The  wide  difference  in  the 
composition  of  soil  and  atmospheric  air  gives  rise  to  a 
movement  of  gases  due  to  a  tendency  for  the  external 
and  the  internal  gases  to  come  into  equilibrium.  Accord- 
ing to  Buckingham,1  the  interchange  of  atmospheric  and 
soil  air  is  due  in  large  measure  to  diffusion. 

The  rate  of  movement  of  the  soil  air  due  to  diffusion 
is  dependent  on  the  aggregate  volume  of  the  interstitial 
spaces,  not  on  their  average  size.  Thus,  it  is  the  porosity 
of  the  soil  that  influences  most  largely  the  diffusion  of 
the  air  from  it.  Consequently  the  size  of  the  particles 
is  not  a  factor,  but  good  tilth  permits  diffusion  to  take 
place  more  rapidly  than  does  a  compact  condition  of  soil, 
as  the  volume  of  the  pore  space  is  thereby  increased. 
Compacting  the  soil  in  any  way,  as  by  rolling  or  trampling, 
has  the  opposite  effect. 

401.  Movement  of  water.  —  As  water,  when  present 
in  a  soil,  fills  certain  of  the  interstitial  spaces,  it  decreases 
the  air  space  when  it  enters  the  soil  and  increases  it  when 

1  Buckingham,  E.  Contributions  to  Our  Knowledge  of 
the  Aeration  of  Soils.     U.  S.  D.  A.,  Bur.  Soils,  Bui.  25.     1904. 


484       SOILS:    PROPERTIES  AND  MANAGEMENT 

it  leaves.  The  downward  movement  of  rain  water  pro- 
duces a  movement  of  soil  air  by  forcing  it  out  through 
the  drainage  channel  below,  while  at  the  same  time  a 
fresh  supply  of  air  is  drawn  in  behind  the  wave  of  satura- 
tion as  the  water  passes  down  from  the  surface.  The 
movement  thus  occasioned  extends  to  a  depth  where 
the  soil  becomes  permanently  saturated  with  water. 
Twenty-five  per  cent  of  the  air  in  a  soil  may  be  driven  out 
by  a  normal  change  in  the  moisture  content  of  the  soil. 

402.  Changes  in  atmospheric  pressure.  —  Waves  of 
high  or  of  low  atmospheric  pressure,  frequently  involving 
a  change  of  0.5  inch  on  the  mercury  gauge,  cross  the  con- 
tinent alternately  every  few  days.  The  presence  of  a 
low  pressure  allows  the  soil  air  to  expand  and  issue  from 
the  soil,  while  a  high  pressure  following  causes  the  out- 
side air  to  enter  in  order  to  equalize  the  pressure.  An 
appreciable,  but  not  important,  movement  of  soil  air  is 
produced  in  this  way. 

The  size  of  the  interstitial  spaces  is  more  potent  than 
their  volume  in  effecting  soil  ventilation  by  this  and  the 
following  methods. 

403.  Changes  of  temperature  in  atmosphere  or  in  soil.  — 
A  movement  of  soil  air  may  be  induced  by  a  change  of 
temperature  in  the  atmosphere  or  in  the  soil  itself.  Changes 
in  atmospheric  temperature  act  in  the  same  way  as  do 
changes  in  atmospheric  pressure;  in  fact,  it  is  the  effect 
of  temperature  on  air  pressure  that  causes  the  movement. 
Like  the  movement  due  to  atmospheric  pressure,  it  is 
not  great ;  but  where  the  soil  immediately  at  the  surface 
of  the  ground'  attains  a  temperature  of  120°  F.  at  midday, 
as  is  the  case  in  the  Corn  Belt,  the  movement  must  be 
appreciable. 

The    diurnal    change    in    soil    temperature    decreases 


THE  SOIL  AIR  485 

rapidly  from  the  surface  downward,  due  to  the  absorp- 
tion and  slow  conduction  of  heat  (see  par.  227).  At 
the  Nebraska  Experiment  Station  x  the  average  diurnal 
range  for  the  month  of  August,  1891,  was  as  follows  :  — 

Diurnal  Range  of  Air  and  Soil  Temperatures 

Degrees  Fahrenheit 

Air  5  feet  above  ground 14.4 

Soil  1  inch  below  surface 17.9 

Soil  3  inches  below  surface 14.8 

Soil  6  inches  below  surface 9.2 

Soil  9  inches  below  surface 6.6 

Soil  12  inches  below  surface 4.3 

Soil  24  inches  below  surface 0.5 

Soil  36  inches  below  surface 0.0 

This  soil  contains  about  fifty  per  cent  of  pore  space,  in 
the  upper  foot  of  which  forty  per  cent  is  normally  filled 
with  water  during  the  summer  months.  This  leaves  518 
cubic  inches  of  air  in  the  upper  cubic  foot  of  soil.  With 
an  increase  in  temperature,  the  air  expands  ji>j  in  volume 
for  each  degree  Fahrenheit.  The  average  increase  of 
temperature  is,  in  this  case,  about  11  degrees  Fahrenheit 
for  the  first  foot.  The  air  exhaled  or  inhaled  by  each 
cubic  foot  of  soil  would  then  be 

=11.6  cubic  inches 

491 

As  this  is  slightly  over  two  per  cent  of  the  air  contained 
in  the  upper  foot  of  soil,  and  as  the  movement  below 
that  depth  is  negligible,  the  change  in  composition  at  any 

1  Swezey,  G.  D.  Soil  Temperatures  at  Lincoln,  Nebraska. 
Neb.  Agr.  Exp.  Sta.,  16th  Ann.  Rept.,  pp.  95-102.     1903. 


486       SOILS:    PROPERTIES  AND  MANAGEMENT 

one  time  is  not  great ;  but  this  pumping  effect  is  kept  up 
day  after  day,  although  less  energetically  in  the  coolef 
seasons  of  the  year.  In  proportion  as  poor  drainage 
equalizes  the  temperature  it  would  prevent  this  type  oJ 
circulation.  The  total  effect,  assisted  by  diffusion,  is  to 
aid  materially  in  ventilating  the  soil.  Owing  to  diffusion 
of  air  in  the  interstitial  spaces,  the  air  expelled  is  different 
in  composition  from  that  inhaled. 

404.  Suction  produced  by  wind.  —  The  movement  of 
wind,  being  almost  always  in  gusts,  alternately  increases 
and  decreases  the  atmospheric  pressure  at  the  surface 
of  the  soil.  There  is  a  tendency,  therefore,  for  the  soil 
air  to  escape  and  for  atmospheric  air  to  penetrate  the 
soil  with  each  change  in  pressure.  The  effect  presumably 
influences  only  the  superficial  air  spaces,  but  it  must  be 
very  frequent  in  its  action.  No  measurements  have 
been  made  and  no  definite  estimate  of  its  effect  can  be 
stated. 

METHODS    FOR    MODIFYING    THE    VOLUME    AND    THE    MOVE- 
MENT  OF   SOIL  AIR 

The  conditions  that  influence  the  ventilation  of  soils 
are :  (1)  volume  and  size  of  the  interstitial  spaces ;  (2) 
moisture  content;  (3)  daily  and  annual  range  in  tem- 
perature. 

Although  the  size  of  the  interstitial  spaces  does  not 
appear  to  greatly  influence  the  diffusion  of  gases  from 
a  soil,  it  has  a  marked  effect  on  certain  of  the  other  pro- 
cesses by  which  air  enters  and  leaves  the  soil.  A  sandy 
soil,  a  soil  in  good  tilth,  and,  particularly,  a  soil  composed 
of  clods,  permit  of  more  rapid  movement  of  air  than  does 
a  compact  soil. 


THE  SOIL  AIR  487 

While  a  certain  movement  of  air  through  the  soil  is 
desirable,  and  indeed  necessary,  for  the  reasons  already 
stated  a  very  considerable  movement  is  injurious  unless 
there  is  an  abundant  rainfall.  The  effect  of  air  move- 
ment through  the  soil  is  to  remove  soil  moisture.  In  a 
region  of  light  rainfall  and  low  atmospheric  humidity,  this 
may  be  disastrous  if  the  soil  is  not  kept  compact  by  care- 
ful tillage.  On  the  other  hand,  in  a  humid  region  and 
in  clay  soil  there  is  likely  to  be  too  small  a  supply  of  oxygen 
for  the  use  of  crops  and  lower  plant  life  unless  the  soil 
is  well  stirred. 

405.  Tillage.  —  The  ordinary  operations  of  tillage 
greatly  influence  the  ventilation  of  the  soiL  When  a  soil 
is  plowed,  the  soil  at  the  bottom  of  the  furrow  is  exposed 
directly  to  the  air  at  the  surface,  and,  by  the  separation 
of  adhering  particles  and  aggregates  of  particles,  air 
is  brought  into  contact  with  particles  that  may  previously 
have  been  completely  shut  off  from  air.  It  is  partly 
because  of  its  effect  on  soil  ventilation  that  plowing  is 
beneficial,  and  the  necessity  for  its  practice  is  greater 
in  a  humid  region  and  on  a  heavy  soil  than  in  a  region 
of  light  rainfall  and  on  a  light  soil.  'The  practice  of  list- 
ing corn,  by  which  the  soil  is  sometimes  left  unplowed 
for  a  number  of  years,  although  in  semiarid  regions  pro- 
ductive of  crops  of  sufficient  yield  to  make  them  profitable, 
would  fail  utterly  on  the  heavy  soils  of  a  humid  region. 
/kSubsoiling,  by  loosening  the  subsoil,  increases  the 
ventilation  to  a  greater  depth.  Rolling  and  subsurface 
packing  both  diminish  the  volume  and  the  movement  of 
air.  Their  essential  difference  is  in  their  effect  on  mois- 
ture rather  than  on  air(  Harrowing  and  cultivation  have 
the  opposite  effect,  and  both  increase  the  production  of 
nitrates  in  the  soil  by  promoting  aeratiom 


488       SOILS:    PROPERTIES  AND  MANAGEMENT 

406.  Manures.  —  Farm  manures,  lime,  and  those 
amendments  that  improve  the  structure  of  the  soil,  have 
for  that  reason  a  beneficial  action  on  soil  aeration.  By 
their  effect  on  the  physical  condition  of  the  soil  they 
increase  its  permeability,  and  by  their  action  in  con- 
tributing to  the  production  of  carbon  dioxide  they  stimu-. 
late  diffusion. 

It  is  chiefly  through  its  effect  in  increasing  the  volume 
of  air  space  in  soils  that  farm  manure  is  injurious  in  light 
soils  of  semiarid  regions.  It  may  thus  be  injurious  in- 
stead of  beneficial,  if  used  under  certain  conditions. 

407.  Underdrainage.  —  By  lowering  the  water  table, 
underdrainage  by  means  of  tiles  removes  from  the  soil 
the  water  from  all  but  the  small  capillary  spaces,  and 
leaves  free  to  the  air  the  remainder  of  the  interstitial 
spaces.  There  is  also  a  very  considerable  movement 
of  air  through  the  drains,  and  a  movement  of  air  upward 
from  the  drains  to  the  surface  of  the  soil,  which  serves 
to  aerate  to  some  extent  this  intervening  layer.  The 
aeration  of  the  soil  brought  about  by  underdrainage 
is  one  of  its  beneficial  features. 

408.  Irrigation.  —  The  influence  of  irrigation  on  the 
soil  is  much  like  that  of  rainfall.  The  alternate  filling 
and  emptying  of  the  interstitial  spaces  with  water  and 
air  causes  a  very  considerable  change  of  air. 

409.  Cropping.  —  The  roots  of  plants  left  in  the  soil 
after  a  crop  has  been  harvested  decay  and  leave  channels 
in  the  soil  through  which  air  penetrates.  Below  the  fur- 
row slice,  where  the  soil  is  not  stirred  and  where  it  is 
usually  more  dense  than  at  the  surface,  this  affords  an 
important  means  of  aeration.  The  absorption  of  moisture 
from  the  soil  by  roots  also  causes  the  air  to  penetrate,  in 
order  to  replace  the  water  withdrawn. 


CHAPTER   XXIII 
COMMERCIAL  FERTILIZERS 

As  treated  in  this  volume,  manures  include  all  those 
substances,  with  the  exception  of  water  (the  function  and 
application  of  which  is  discussed  in  par.  167),  that  are 
added  to  soils  to  make  them  more  productive.  There  are 
several  ways  in  which  manures  applied  to  soils  may  in- 
crease plant  growth  :  (1)  by  addition  of  the  nutrient  mate- 
rials utilized  by  plants,  which  is  the  chief  function  of 
most  of  the  so-called  commercial  fertilizers;  (2)  by  im- 
provement of  the  physical  condition  of  a  soil,  which 
usually  results  from  the  application  of  lime  and  the  in- 
corporation of  organic  matter ;  (3)  by  favoring  the  action 
of  useful  bacteria,  which  is  one  of  the  beneficial  results 
of  farm  manure  and  also  of  lime;  (4)  by  counteracting 
the  effects  of  toxic  substances  —  as,  for  instance,  the 
conversion  of  sodium  carbonate  into  sulfate  by  gypsum, 
or  the  neutralization  of  acidity,  or  possibly  the  destruc- 
tion of  toxic  organic  substances  by  certain  salts ;  (5)  by 
catalytic  action,  either  on  chemical  processes  in  the  soil 
or  by  its  influence  on  those  bacteria  that  exert  a  favorable 
influence  on  soil  fertility  or  by  direct  stimulation  of  the 
plant. 

410.  Early  ideas  of  the  function  of  manures.  — 
Manures  were  at  one  time  supposed  to  pulverize  the  soil, 
and  the  French  word  manoeuvrer,  from  which  the  word 
manure  comes,   implies  to  work  with  the  hand.     This 

489 


490       SOILS:    PROPERTIES  AND  MANAGEMENT 

idea  probably  originated  through  the  observation  that 
farm  manure,  which  was  the  only  manure  in  use  at  that 
time,  made  the  soil  less  cloddy. 

It  has  been  argued,  notably  by  Jethro  Tull,1  that  since 
tillage  pulverizes  the  soil  it  may  be  used  as  a  substitute 
for  manures.  There  are,  however,  conditions  aside  from 
tilth  that  are  influenced  by  manures,  and  good  tilth  alone 
will  not  suffice  to  maintain  a  permanently  intensive  agri- 
culture. It  is  true  in  the  United  States,  as  it  is  in  Europe, 
that  a  large  consumption  of  manures  goes  hand  in  hand 
with  a  highly  developed  and  intensive  system  of  farming. 

411.  Development  of  the  idea  of  the  nutrient  function 
of  manures.  —  While  the  use  of  animal  excrement  on  cul- 
tivated soils  was  practiced  as  far  back  as  systematic  agri- 
culture can  be  definitely  traced,  the  earliest  record  of 
the  use  of  mineral  salts  for  increasing  the  yield  of  crops 
was  published  in  1669  by  Sir  Kenelm  Digby.2  He  says : 
"  By  the  help  of  plain  salt  petre,  diluted  in  water,  and 
mingled  with  some  other  fit  earthly  substance,  that  may 
familiarize  it  a  little  with  the  corn  into  which  I  endeavored 
to  introduce  it,  I  have  made  the  barrenest  ground  far 
outgo  the  richest  in  giving  a  prodigiously  plentiful  har- 
vest." His  dissertation  does  not,  however,  show  any 
true  conception  of  the  reason  for  the  increase  in  the  crop 
through  the  use  of  this  fertilizer.  In  fact,  the  want  of 
any  real  knowledge  at  that  time  of  the  composition  of 
the  plant  would  have  made  this  impossible. 

In  1804,  Theodore  de  Saussure  3  published  his  chemical 


1  Tull,  Jethro.     Horse-Hoeing  Husbandry.     London.    1829. 

2  Digby,  Kenelm.     A  Discourse  Concerning  the  Vegetation 
of  Plants.     London.     1669. 

3  Saussure,    Theodore    de.     Recherches    Chimiques    sur    la 
Vegetation.     Paris.     1804. 


COMMERCIAL  FERTILIZERS  491 

researches  on  plants,  in  which  he,  for  the  first  time, 
called  attention  to  the  significance  of  the  ash  ingredients 
of  plants,  and  pointed  out  that  without  them  plant  life 
is  impossible  and,  further,  that  only  the  ash  of  the  plant 
tissue  is  derived  from  the  soil. 

Justus  von  Liebig,1  in  his  writings  published  about 
the  middle  of  the  nineteenth  century,  emphasized  still 
more  strongly  the  importance  of  mineral  matter  in  the 
plant  and  the  extraction  of  this  matter  from  the  soil. 
He  refuted  the  theory,  at  that  time  popular,  that  plants 
absorb  their  carbon  from  humus,  but  he  made  the  mis- 
take of  attaching  little  importance  to  the  presence  of 
humus  in  the  soil.  He  showed  the  importance  of  potas- 
sium and  phosphorus  in  manures,  but  in  his  later  expres- 
sions he  failed  to  appreciate  the  value  of  nitrogenous 
manures,  holding  that  a  sufficient  amount  is  washed 
from  the  atmosphere  in  the  form  of  ammonia. 

A  true  conception  of  the  necessity  for  a  supply  of 
combined  nitrogen  in  the  soil  was  even  at  that  time  enter- 
tained by  Boussingault  and  by  Sir  John  Lawes,  although 
the  elaborate  experiments  conducted  by  Lawes,  Gilbert, 
and  Pugh  2  in  1857  were  required  to  fully  demonstrate 
the  fact.  Their  care  in  conducting  the  experiments 
resulted  in  their  sterilizing  the  soil  with  which  they  ex- 
perimented, and  hence  their  failure  to  discover  the  utiliza- 
tion of  free  atmospheric  nitrogen  by  legumes. 

1  Liebig,  J.  Justus  von.  Principles  of  Agricultural  Chemistry 
with  Special  Reference  to  the  Late  Researches  Made  in  England. 
London.  1855.  Also,  Chemistry  in  its  Applications  to  Agri- 
culture and  Physiology.    New  York.     1556. 

2  Lawes,  J.  B.,  Gilbert,  J.  IT.,  and  Pugh,  E.  On  the  Sources 
of  the  Nitrogen  of  Vegetation,  with  Special  Reference  to  the 
Question  whether  Plants  Assimilate  Free  or  Uncombined  Nitro- 
gen.    Rothamsted  Memoirs,  Vol.  1,  No.  1.     1862. 


492       SOILS:    PROPERTIES  AND  MANAGEMENT 

Between  1840  and  1850,  Sir  John  Lawes  began  the 
manufacture  of  bone  superphosphate,  and  about  the 
same  time  Peruvian  guano  and  nitrate  of  soda  were  intro- 
duced into  Europe.  The  commercial  fertilizer  industry 
thus  dates  from  that  time. 

412.  Classes  of  manures.  —  While  manures  are  very 
numerous  as  to  kind  and  while  a  certain  manure  may  have 
a  number  of  distinct  functions,  they  may  yet  be  roughly 
divided  into  classes.  They  will  accordingly  be  treated 
here  under  the  following  heads  :  (1)  commercial  fertilizers ; 
(2)  soil  amendments;  (3)  farm  manures;  (4)  green 
manures. 

413.  Commercial  fertilizers.  —  Although  the  commer- 
cial fertilizer  industry  is  little  more  than  half  a  century 
old,  the  sale  of  fertilizers  in  this  country  amounts  to  more 
than  $110,000,000  annually.  Animal  refuse  and  phos- 
phate fertilizers  are  exported,  while  nitrate  of  soda  and 
potassium  salts  are  imported. 

Of  the  fertilizers  sold  in  the  United  States  in  1909, 
about  fifty  per  cent  was  consumed  in  the  South  Atlantic 
States,  in  an  area  lying  within  three  hundred  miles  of 
the  seaboard.  Nearly  one-half  of  the  remainder  was  pur- 
chased in  the  Middle  Atlantic  and  New  England  States. 
Only  five  per  cent  was  purchased  west  of  the  Mississippi 
River.1 

Primarily  the  function  of  commercial  fertilizers  is  to 
add  plant  nutrients  to  the  soil,  usually  in  a  -form  more 
readily  soluble  than  those  already  present  in  large  quan- 
tity. While  other  beneficial  effects  may  be  produced  by 
certain  fertilizers,  these  are  usually  of  secondary  impor- 
tance as  compared  with  the  addition  of  the  plant  nutrients. 

1  Statistics  from  Thirteenth  Census  of  the  United  States. 
Abstract  of  the  Census,  p.  372.     Washington.     1913. 


COMMERCIAL   FERTILIZERS  493 

414.  Fertilizer  constituents.  —  Prepared  fertilizers,  as 
found  on  the  market,  are  usually  composed  of  a  number 
of  ingredients.  Since  these  are  the  carriers  of  the  fertiliz- 
ing material,  and  since  it  is  on  their  composition  and  solu- 
bility that  the  value  of  a  fertilizer  depends,  a  knowledge 
of  the  properties  of  these  constituents  is  of  interest  to 
every  one  who  uses  fertilizers  and  is  a  valuable  aid  in  their 
purchase. 

FERTILIZERS   USED   FOR  THEIR  NITROGEN 

Nitrogen  is  the  most  expensive  constituent  of  manures 
and  is  of  great  importance,  since  it  is  very  likely  to  be 
deficient  in  soils.  A  commercial  fertilizer  may  have  its 
nitrogen  in  the  form  of  soluble  inorganic  salt,  or  combined 
as  organic  material.  On  the  form  of  combination  de- 
pends to  a  certain  extent  the  value  of  the  nitrogen,  as 
the  soluble  inorganic  salts  are  very  readily  available  to 
the  plant,  while  the  organic  forms  must  pass  through  the 
various  processes  leading  to  nitrification  before  the 
plant  can  use  the  nitrogen  so  contained.  The  inorganic 
nitrogen  fertilizers  are  sodium  nitrate,  ammonium  sulfate, 
calcium  nitrate,  and  calcium  cyanamide. 

415.  Forms  in  which  nitrogen  exists  in  soils.  —  There 
are  several  forms  in  which  nitrogen  exists  in  soils.  The 
uncombined  nitrogen  of  the  soil  air  constitutes  the  largest 
supply  because  of  its  diffusibility  with  the  atmospheric 
air.  Next  in  quantity  is  the  nitrogen  of  organic  com- 
pounds, ranging  from  0.05  to  0.3  per  cent  in  ordinary 
arable  land  and  slightly,  but  appreciably,  soluble  in  soil 
water.  In  upland  cultivated  soils  the  nitrogen  of  nitrate 
salts  forms  the  next  largest  supply,  but  rarely  exceeds 
20  per  cent  of  the  total  combined  nitrogen  of  the  soil. 


494       SOILS:    PROPERTIES  AND  MANAGEMENT 

In  swamp  and  inundated  soils  the  nitrogen  of  ammonium 
salts  and  nitrites  forms  a  larger  proportion  of  the  soil 
nitrogen  than  does  the  nitrate  nitrogen,  but  in  well 
aerated  soils  these  compounds  exist  in  very  small  quan- 
tities. 

416.  Forms  in  which  nitrogen  is  absorbed  by  plants.  — 
The  utilization  of  atmospheric  nitrogen  by  leguminous 
plants  and  by  a  few  others  that  have  nodule-bearing  roots 
.  has  been  established  beyond  question ;  but  the  extent 
to  which  this  form  of  nitrogen  may  be  utilized  by  other 
plants,  or  the  identity  of  the  plants  that  participate  in 
its  use,  are  subjects  on  which  opinions  differ,  and  which 
are  still  being  investigated. 

417.  Use  of  nitrates  by  plants.  —  Boussingault  first 
demonstrated  the  importance  of  nitrates  for  higher 
plants.  Previous  to  that  time  ammonia  had  been  con- 
sidered the  chief  source  of  nitrogen,  and  at  a  still  earlier 
time  humus  had  been  considered  the  source.  Liebig 
gave  the  weight  of  his  influence  in  favor  of  ammonia 
as  the  supply.  He  was  unaware,  of  course,  of  the  trans- 
formation of  ammonia  nitrogen  into  nitrates  in  the  soil. 
Since  the  publication  of  the  experiments  by  Boussin- 
gault and  the  later  work  on  nitrification,  there  has 
been  a  tendency  to  consider  nitrate  nitrogen  as  the 
only  available  supply  of  nitrogen  for  agricultural  plants. 
While  this  is  an  extreme  view  of  the  matter,  the 
fact  remains  that  all.  the  higher  plants,  including  the 
legumes,  appear  to  be  able  to  absorb  nitrates,  and  this 
form  of  nitrogen  has  frequently  proved  of  greater  benefit 
to  plants  than  other  forms  of  nitrogen  tested  at  the 
same  time. 

418.  Ammonia  as  a  plant-food.  —  That  rice  plants  on 
swamps  use  ammonia  nitrogen  rather  than  other  forms 


COMMERCIAL   FERTILIZERS  495 

has  been  demonstrated  by  KeJlner  *  and  later  by  Kelley.2 
On  upland  soils,  however,  it  is  presumable  that  rice  plants 
utilize  nitrate  nitrogen,  wjrich  would  indicate  that  some 
plants,  at  least,  may  adapt  themselves  to  the  use  of  the 
more  abundant  form  of  nitrogen. 

Hutchinson  and  Miller 3  found  that  peas  obtained 
nitrogen  from  ammonium  salts  as  readily  as  from  sodium 
nitrate,  but  that  wheat  plants,  although  able  to  obtain 
nitrogen  directly  from  ammonium  salts,  grew  much  better 
in  a  solution  containing  nitrates.  One  feature  brought 
out  by  the  numerous  experiments  with  ammonium  salts 
is  the  difference  between  plants  of  various  kinds  in  respect 
to  their  ability  to  absorb  nitrogen  in  this  form. 

419.  Utilization  of  humus  compounds  by  plants.  — 
One  of  the  early  beliefs  in  regard  to  plant  nutrition  was 
that  organic  matter  as  such  is  directly  absorbed  by  higher 
plants.  This  opinion  was  afterwards  entirely  replaced 
by  the  mineral  theory  propounded  by  Liebig;  and  still 
later  the  discovery  of  the  nitrifying  process  almost  dis- 
posed completely  of  the  belief  that  organic  matter,  is  a 
food  for  higher  plants.  It  is  quite  certain,  however,  that 
some  organic  nitrogenous  compounds  furnish  suitable 
nutrient  material  for  some  higher  plants  without  under- 
going bacterial  change. 

Hutchinson  and  Miller,  in  the  paper  just  referred  to, 
give  the  following  list  of  the  organic  substances  used  in 

1  Kellner,  0.  Agrikulturchemische  Studien  liber  die  Reis- 
kiiltur.     Landw.  Vers.  Stat.,  Band  30,  Seite  18-41.     1884. 

2  Kelley,  W.  P.  The  Assimilation  of  Nitrogen  by  Rice. 
Hawaii  Agr.  Exp.  Sta.,  Bui.  24,  pp.  5-20.     1911. 

3  Hutchinson,  H.  B.,  and  Miller,  N.  IT.  J.  The  Direct 
Assimilation  of  Inorganic  and  Organic  Forms  of  Nitrogen  by 
Higher  Plants.  Centrlb.  f.  Bakt.,  II,  Band  30,  Seite  513-547. 
1911. 


496       SOILS:'  PROPERTIES  AND  MANAGEMENT 

experiments  by  various   investigators,  and  their   avail- 
ability for  the  nutriment  of  higher  plants :  — 

Readily  Assimilated 

Ammonium  salts 
Acetamide    CH3 .  CO  .  NH2 

Urea    CO<wtt2 

Barbituric  acid  (with  calcium  carbonate) 

CO\NH  .  CO/CH5 

Alloxan     CO<^  '  <g>CO 

Humates 

Assimilated 

Formamide     H  .  CO  .  NH2 

Glycine     NH2  .  CH2  .  COOH 

Aminopropionic  acid     CH3  .  CH(XH2)  .  COOH 

/NH, 
Guanidine  hydrochloride     [C  =  NH     HC1 

\NH2. 

Cyanuric  acid     CO<^t  Ttj  '  p~/NH 

CO  .  NH2 
Oxamide 

CO  .  NH2 

CH(NH2)COOH 
Sodium  aspartate 

CH2 .  COOH 

Peptone 


COMMERCIAL   FERTILIZERS  497 

Doubtful 
Trimethylamine 

para-+Urazine    CCK^^.  '  xTtt/CO 

Hexamethylenetetramine 
Not  Assimilated 
Ethyl  nitrate  Hydroxylamine  hydrochloride 

Propionitrile  Methyl  carbonate 

Toxic 
Tetranitromethane 

This  list  comprises  only  those  substances  that  have  been 
used  in  experiments  with  peas.  Many  other  substances 
remain  to  be  tested,  and  those  already  tested  may  act 
differently  with  other  plants. 

One  of  the  organic  compounds  isolated  from  soils  by 
Shorey,1  called  creatinine,  has  been  shown  by  Skinner2 
to  be  used  directly  by  plants  as  a  source  of  nitrogen, 
and  to  have  produced  a  better  growth  of  wheat  seedlings 
than  did  an  equivalent  quantity  of  nitrogen  in  the  form 
of  sodium  nitrate.  Histidine,  arginine,  and  creatine 
have  also  been  found  in  soils  and  shown  to  be  a  direct 
source  of  nitrogen  for  wheat  seedlings  (par.  92) . 

These  and  numerous  other  investigations  of  this  subject 
show  that  amine  as  well  as  amide  nitrogen  is  assimilated 
by  at  least  some  agricultural  plants,  but  to  what  extent 
most  of  these  compounds  may  successfully  replace  the 

1  Shorey,  E.  C.  I.  The  Isolation  of  Creatinine  from  Soils. 
U.  S.  D.  A.,  Bur.  Soils,  Bui.  83,  pp.  11-22.     1911. 

2  Skinner,  J.  J.  III.  Effects  of  Creatinine  on  Plant  Growth. 
U.  S.  D.  A.,  Bur.  Soils,  Bui.  83,  pp.  33-44.     1911. 

2k 


498       SOILS:    PROPERTIES  AND  MANAGEMENT 

inorganic  forms  of  nitrogen  has  not  been  definitely  worked 
out.  Certain  organic  nitrogenous  fertilizers  —  as,  for  ex- 
ample, dried  blood  —  have  a  high  commercial  value,  the 
nitrogen  in  this  form  selling  for  more  a  pound  than  the  nitro- 
gen in  any  of  the  inorganic  salts.  Many  crops,  especially 
among  garden  vegetables,  are  most  successfully  grown  only 
when  supplied  with  organic  nitrogenous  material.  Some 
nitrate  nitrogen  is  always  present  under  natural  soil  con- 
ditions, so  that  crops  are  never  limited  to  organic  nitro- 
gen alone ;  and  it  may  be  that  the  latter  form  of  nitrogen 
is  most  useful  when  it  supplements  the  nitrate  nitrogen. 

420.  Sodium  nitrate.  —  This  now  constitutes  the  prin- 
cipal source  of  inorganic  nitrogen  in  commercial  fertilizers. 
The  salt  exists  in  the  crude  condition  in  northern  Chili. 
The  crude  salt  is  purified  by  crystallization,  and  as  put 
on  the  market  it  contains  about  96  per  cent  sodium 
nitrate,  or  about  16  per  cent  of  nitrogen,  2  per  cent  of 
water,  and  small  amounts  of  chlorides,  sulfates,  and  in- 
soluble matter.  The  cost  of  nitrogen  in  this  form  is 
from  fifteen  to  eighteen  cents  a  pound. 

Because  of  its  easy  availability,  sodium  nitrate  acts 
quickly  in  inducing  growth.  For  this  reason  it  is  used 
much  by  market  gardeners,  and  for  other  purposes  when 
a  rapid  growth  is  desired.  It  is  the  most  active  form  of 
nitrogen.  A  light  dressing  on  meadowland  in  early 
spring  assists  greatly  in  hastening  growth  by  furnishing 
available  nitrogen  before  the  conditions  are  favorable 
for  the  process  of  nitrification.  On  small  grain  a  similarly 
useful  purpose  is  served  where  the  soil  is  not  rich, 
y  Owing  to  the  fact  that  nitrate  is  not  absorbed  by  the 
soil  in  large  quantities,  it  is  easily  lost  in  the  drainage 
water ;  for  this  reason  it  should  be  applied  only  when  crops 
are  growing  on  the  soil,  and  then  only  in  moderate  quantity. 


COMMERCIAL   FERTILIZERS  499 

The  continued  and  abundant  use  of  sodium  nitrate 
on  the  soil  may  result,  through  its  deflocculating  action, 
in  breaking  down  aggregates  of  soil  particles,  thus  com- 
pacting and  injuring  the  structure.  This  effect  is  attrib- 
uted to  the  accumulation  of  sodium  salts,  particularly 
the  carbonate,  as  the  sodium  is  not  utilized  by  the  plant 
to  the  same  extent  as  is  the  nitrogen. 

421.  Ammonium  sulfate.  —  When  coal  is  distilled,  a 
portion  of  the  nitrogen  is  liberated  as  ammonia  and  is 
collected  by  passing  the  products  of  distillation  through 
water  in  which  the  ammonia  is  soluble,  forming  the  am- 
moniacal  liquor.  The  ammonia  thus  held  is  distilled  into 
sulfuric  acid,  with  the  formation  of  ammonium  sulfate 
and  the  removal  of  impure  gases. 

Commercial  ammonium  sulfate  contains  about  twenty 
per  cent  of  nitrogen.  It  is  the  most  concentrated  form  in 
which  nitrogen  can  be  purchased  as  a  fertilizer,  having 
from  sixty  to  eighty  pounds  more  of  nitrogen  to  a  ton 
than  sodium  nitrate.  It  is  therefore  economical  to 
handle.  Its  effect  on  crops  is  not  so  rapid  as  that  of  sodium 
nitrate,  but  it  is  not  so  quickly  carried  from  the  soil  by 
drainage  water,  as  the  ammonium  salts  are  readily  ab- 
sorbed by  the  soil.  A  pound  of  nitrogen  in  the  form  of 
ammonium  sulfate  has  about  the  same  agricultural  value 
as  the  same  amount  in  the  form  of  nitrate  if  the  soil  on 
which  it  is  used  is  abundantly  supplied  with  lime;  but 
on  an  acid  soil  ammonium  sulfate  has  less  value. 

The  long  and  extensive  use  of  ammonium  sulfate  on  a 
soil  has  a  tendency  to  produce  an  acid  condition,  through 
the  accumulation  of  sulfates  which  are  not  largely  taken 
up  by  plants. 

Ammonium  sulfate,  like  sodium  nitrate,  should  not  be 
applied  in  autumn,  as  the  ammonia  is  converted  into 


500       SOILS:    PROPERTIES  AND   MANAGEMENT 

nitrates  and  leached  from  the  soil  in  sufficient  quantities  to 
entail  a  very  decided  loss  of  nitrogen.  There  is  not  likely 
to  be  so  large  a  loss  of  nitrogen  from  ammonium  salts  as 
from  nitrates,  and,  as  would  naturally  be  expected,  there  is 
greater  loss  of  nitrogen  when  these  salts  are  used  alone  than 
when  they  are  combined  with  other  fertilizing  ingredients. 
Hall x  has  estimated  the  loss  of  nitrogen  from  certain 
drained  plats  at  the  Rothamsted  Experiment  Station. 
This  estimate  is  based  on  the  concentration  of  the  drain- 
age from  the  different  plats,  of  which  there  was  no  record 
of  total  flow,  but  for  which  the  measurements  of  flow  from 
the  lysimeter  draining  60  inches  of  soil  were  taken  and  the 
total  loss  of  nitrates  was  calculated  on  this  basis.  Esti- 
mated in  this  way  the  effects  of  several  different  methods 
of  manuring  are  shown  in  the  accompanying  table :  — 

Pounds  to  the  Acre  of  Nitric  Nitrogen  in  Drainage  Water 


1879-80 

1880-81 

Treatment 

Spring 
sowing 

to 
harvest 

Harvest 

to 
spring 
sowing 

Spring 
sowing 

to 
harvest 

Harvest 

to 
spring 
sowing 

Unmanured 

1.7 

10.8 
13.3 

12.6 

15.6 

59.9 
14.3 

16.4 

0.6 
0.7 

4.3 

15.0 

3.4 
7.4 

3.7 

17.1 

Mineral  fertilizers  only 

Minerals  +  400   pounds  ammonium 
salts      .     .    • 

1.6 
18.3 

17.7 
21.4 

Minerals  +  550    pounds   nitrate   of 
soda 

45.0 

41.0 

Minerals  +  400   pounds  ammonium 
salts  applied  in  autumn  .... 

400  pounds  ammonium  salts  alone  . 

400  pounds  ammonium   salts  +  sul- 
phate of  potash 

9.6 
42.9 

19.0 

74.9 
35.2 

25.3 

Estimated  drainage  in  inches     . 

11.1 

4.7 

1.8 

18.8 

1  Hall,  A.  D.     The  Book  of  the  Rothamsted  Experiments, 
p.  235.     New  York,  1905. 


COMMERCIAL  FERTILIZERS  501 


This  table,  in  addition  to  confirming  the  statements 
already  made  in  regard  to  the  loss  of  nitrogen  in  drainage 
water,  also  shows  how  closely  the  supply  of  available 
nitrogen  was  used  by  the  crops  on  those  plats,  which  were 
evidently  in  need  of  nitrogen  fertilization  as  the  plats 
lost  very  little  nitrogen  during  the  growing  season,  while 
during  the  remainder  of  the  year  they  lost  nearly  as 
much  as  did  some  of  the  nitrogen-manured  plats.  The 
table  also  indicates  that  the  loss  when  nitrate  is  used  is 
greater  than  when  ammonium  salts  are  applied,  as  the 
amount  of  nitrogen  in  the  550  pounds  of  nitrate  is  really 
eight  pounds  to  the  acre  more  than  in  the  400  pounds  of 
ammonium  sulfate,  which  is  not  sufficient  to  account  for 
the  difference  in  the  loss.  However,  half  of  the  nitrate- 
treated  plat  received  no  other  manure  and  produced  only 
a  small  crop,  which  would  naturally  result  in  a  greater 
loss  by  drainage. 

422.  Fertilizers  containing  atmospheric  nitrogen.  — 
The  vast  store  of  atmospheric  nitrogen,  chemically  un- 
combined  but  very  inert,  will  furnish  an  inexhaustible 
supply  of  this  highly  valuable  fertilizing  element,  when  it 
can  with  reasonable  economy  be  combined  in  some  manner 
resulting  in  a  product  that  will  be  commercially  trans- 
portable and  that  will,  when  placed  in  the  soil,  be  or  be- 
come soluble  without  liberating  substances  toxic  to  plants. 
The  importance  of  the  nitrogen  supply  for  agriculture  may 
be  appreciated  when  it  is  considered  that  nitrates  are 
being  carried  off  in  the  drainage  water  of  all  cultivated 
soils  at  the  rate  of  twenty-five  to  fifty  pounds,  and  even 
more,  to  the  acre  annually,  and  that  nearly  as  much 
more  is  removed  in  crops. 

The  exhaustion  of  the  supply  of  nitrogen  in  most  soils 
may  be  accomplished  within  one  or  two  generations  of 


502       SOILS:    PROPERTIES  AND  MANAGEMENT 

men,  unless  a  renewal  of  the  supply  is  brought  about  in 
some  way.  Natural  processes  provide  for  an  annual  ac- 
cretion through  the  washing-down  of  ammonia  and 
nitrates  by  rain  water  from  the  atmosphere,  and  through 
the  fixation  of  free  atmospheric  nitrogen  by  bacteria  ;  but 
without  the  frequent  use  of  leguminous  crops,  the  supply 
could  not  be  maintained.  Farm  practice  of  the  present 
day  requires  the  application  of  nitrogen  in  some  form  of 
manure,  and,  as  the  end  of  the  commercial  supply  of  com- 
bined nitrogen  is  easily  in  sight,  there  is  urgent  need  of 
discovering  a  new  source.  This  has  been  done  by  com- 
bining calcium  with  atmospheric  nitrogen  in  the  forms  of 
calcium  cyanamide  and  calcium  nitrate. 

423.  Cyanamid.  —  The  trade  name  for  calcium  cyana- 
mide is  "  cyanamid  "  and  that  name  is  therefore  used 
in  this  volume.  One  process  for  the  production  of  cyana- 
mid consists  in  passing  nitrogen  into  closed  retorts  con- 
taining powdered  calcium  carbide  heated  to  a  high  tem- 
perature ;  the  product  being  calcium  cyanamide  and  free 
carbon :  — 

CaC2  +  2N  =  CaCN2  +  C 

The  free  carbon  remains  distributed  in  the  cyanamide 
and  gives  the  fertilizer  a  black  color.  The  nitrogen  re- 
quired for  the  process  is  obtained  either  by  passing  air 
over  heated  copper,  or  by  the  fractional  distillation  of 
liquid  air. 

The  fertilizer,  as  placed  on  the  market,  is  a  heavy, 
black  powder  or  granulated  material  with  a  somewhat  dis- 
agreeable odor. 

424.  Composition  of  cyanamid.1  —  Cyanamid  as  manu- 

1  Cyanamid  is  a  trade  name ;  the  chemical  compound  is 
spelled  cyanamide. 


COMMERCIAL   FERTILIZERS  503 

factured  in  this  country  has  about  the  following  composi- 

tion:1—  Percent 

Calcium  cyanamide    ....     CaCN2  45.92 

Calcium  carbonate     ....     CaC03  4.04 

Calcium  sulfide CaS  1.73 

Calcium  phosphide     ....     Ca3P2  0.04 

Calcium  hydroxide     ....     Ca(OH)2  26.60 

Free  carbon C  13.14 

Iron  and  alumina R2O3  1.98 

Silica Si02  1.62 

Magnesia      .......     MgO  0.15 

Combined  moisture    ....  3.12 

Free  moisture H20  0.35 

Undetermined 1.31 

100.00 

According  to  this  composition  the  material  would  con- 
tain 16  per  cent  of  nitrogen.  Lime  in  the  forms  of  carbo- 
nate and  hydroxide  would  add  somewhat  to  its  value, 
and  the  residue  of  the  calcium  cyanamide,  which  upon 
decomposition  is  also  calcium  hydroxide,  is  likewise  ben- 
eficial to  the  soil. 

425.  Changes  of  calcium  cyanamide  in  the  soil.  — 
Calcium  cyanamide  must  be  decomposed  in  the  soil  be- 
fore its  nitrogen  becomes  available  to  plants.  There 
are  several  steps  in  the  decomposition  process  by  which 
the  nitrogen  finally  emerges  in  the  form  of  ammonia. 
These,  according  to  Pranke  in  the  work  just  cited,  con- 
sist first  of  hydrolysis,  by  which  acid  calcium  cyanamide 
and  calcium  hydroxide  are  formed  :  — 

2  CN  .  NCa  +  2  H20  =  (CN  .  NH)2Ca  +  Ca(OH)2 

calcium  water  acid  calcium  calcium 

cyanamide  cyanamide  hydroxide 

1  Pranke,  E.  J.    Cyanamid,  p.  8.    Eastern,  Pennsylvania.    1913. 


504     soils:  properties  and  management 

The  acid  calcium  cyanamide  quickly  loses  its  calcium, 
leaving  free  cyanamide.  Investigators  differ  as  to  the 
process  involved  in  this  change,  but  the  ultimate  condi- 
tion of  the  calcium  is  carbonate.  The  three  explanations 
of  the  process  may  be  represented  by  the  following  re- 
actions :  — 

1.  (CN  .  NH)2Ca+C02+H20  =  2  CN  .  NH2+CaC03 

In  this  reaction  the  carbon  dioxide  of  the  soil  water  is 
supposed  to  cause  precipitation  of  the  calcium. 

2.  (CN  .  NH)2Ca  +  2  H20  =  2  CN  .  NH2  +  Ca(OH)2 

In  this  case  hydrolysis  occasions  the  reaction.  The 
hydroxide  would,  of  course,  be  converted  into  carbonate 
in  the  soil. 

3.  (CN  .  NH)2Ca  +  C02  =  CN  .  NH2  +  CaCN2C02 

acid  calcium  carbon  free  calcium 

cyanamide  dioxide  cyanamide  cyanamide 

carbonate 

CaCN2C02  +  H20  =  CN  .  NH2  +  CaCO, 

free  calcium 

cyanamide  carbonate 

By  this  reaction  calcium  cyanamide  carbonate  is  an  in- 
termediate product,  but  is  at  once  hydrolyzed  and  free 
cyanamide  produced. 

The  next  step  in  the  process  is  the  formation  of  urea  by 
hydrolysis  of  the  free  cyanamide  :  — 

CN  .  NH2  +  H20  -  CO(NH2)2 

free  cyanamide  water  urea 

The  changes  up  to  the  production  of  urea  are  independent 
of  bacterial  action.  The  urea  is  converted  through  bac- 
terial action  into  ammonium  carbonate  :  — 


COMMERCIAL   FERTILIZERS  505 

CO(NH2)2  +  2  H20  =  (NH4)2C03 

urea  ammonium 

carbonate 

This  may  be  converted  into  nitrates  in  the  usual  manner. 

426.  The  use  of  cyanamid.  —  The  changes  as  here 
described  are  those  that  proceed  under  favorable  condi- 
tions in  the  soil.  When  conditions  are  not  favorable  — 
as,  for  example,  when  a  soil  is  saturated  with  water  or 
when  it  is  acid  —  some  more  or  less  injurious  products 
may  be  formed.  For  this  reason  cyanamid  is  not  likely 
to  be  so  satisfactory  on  soils  of  this  nature  as  on  better 
soils.  To  very  sandy  soils  it  is  not  well  suited.  Ordi- 
narily its  fertilizing  value  is  not  greatly  below  that  of 
sodium  nitrate,  and  is  about  equal  to  that  of  ammonium 
sulfate  when  not  used  in  heavy  applications. 

It  should  be  incorporated  with  the  soil  at  least  a  week 
before  planting,  as  it  may  injure  the  young  plants  if  de- 
composition has  not  proceeded  far  enough  to  remove  its 
somewhat  toxic  properties.  As  it  must  undergo  this 
decomposition  before  its  nitrogen  becomes  available  to 
the  young  plants,  there  is  an  added  reason  for  this  pre- 
caution. It  does  not  give  its  best  results  as  a  top-dressing 
because  it  requires  incorporation  with  the  soil  for  its 
proper  decomposition. 

427.  Calcium  nitrate.  —  The  other  process  for  com- 
bining atmospheric  nitrogen  is  of  more  recent  invention 
than  that  for  the  manufacture  of  calcium  cyanamid  but 
is  not  conducted  on  a  commercial  scale  in  this  country; 
however,  with  the  vast  opportunities  for  developing  elec- 
tric power  which  are  offered  in  certain  localities,  factories 
for  the  manufacture  of  calcium  nitrate  will  some  day  be 
established. 

The  process  employs  an  electric  arc  to  produce  nitric 


506       SOILS:    PROPERTIES  AND  MANAGEMENT 

oxide  by  the  combustion  of  atmospheric  nitrogen,  accord- 
ing to  the  simple  equation  :  — 

N2  +  02  =  2NO 
NO  +  O  =  N02 

A  very  high  power  is  required  for  this  synthesis,  in- 
volving a  temperature  of  2500°  to  3000°  C,  and  the 
expense  of  the  operation  is  determined  almost  entirely  by 
the  cost  of  the  electricity. 

The  nitric  oxide  gas  is  passed  through  milk  of  lime, 
giving  basic  calcium  nitrate :  — 

Ca(OH)2  +  2  HN03  =  Ca(N03)2  +  2  H20 

The  calcium  nitrate  resulting  from  this  process  has  a 
yellowish  white  color,  and  is  easily  soluble  in  water  but 
deliquesces  very  rapidly  in  the  air.  This  last  property 
can  be  overcome  by  adding  an  excess  of  lime  in  the  manu- 
facture, thus  producing  a  basic  calcium  nitrate  which 
contains  only  8.9  per  cent  of  nitrogen.  Another  way  of 
avoiding  the  difficulties  involved  by  the  deliquescent 
property  of  the  nitrate  is  practiced  by  the  factory  at 
Nottoden,  Norway.  This  consists  in  first  melting  the 
product,  then  grinding  it  fine  and  packing  it  in  air-tight 
casks.  The  fertilizer  thus  prepared  contains  from  11  to 
13  per  cent  of  nitrogen. 

Calcium  nitrate  contains  its  nitrogen  in  a  form  directly 
available  to  plants.  It  resembles  sodium  nitrate  in  its 
solubility,  availability,  and  lack  of  absorption  by  the  soil. 
It  may  be  spread  on  the  surface  of  the  ground,  as  it  exerts 
no  poisonous  action  and  does  not  tend  to  form  a  crust,  as 
does  sodium  nitrate. 

The  relative  values  of  the  different  soluble  nitrogen 
fertilizers  vary  with  a  great  many  conditions  and  can  be 


COMMERCIAL   FERTILIZERS  507 

accurately  judged  only  by  a  large  number  of  tests.  At 
present,  both  calcium  nitrate  and  cyanamid  are  being 
produced  at  less  cost  per  pound  of  nitrogen  than  is  sodium 
nitrate,  when  laid  down  in  the  neighborhood  of  the  fac- 
tories in  Europe.  It  seems  fairly  certain  that,  when  the 
processes  have  been  further  improved,  the  result  will  be 
to  greatly  reduce  the  cost  of  available  nitrogen. 

428.  Organic  nitrogen  in  fertilizers. — The  commercial 
fertilizers  containing  organic  nitrogen  include  cottonseed 
meal,  which  contains  7  per  cent  of  nitrogen  when  free 
from  hulls;  linseed  meal,  with  5.5  per  cent  of  nitrogen; 
castor  pomace,  with  6  per  cent  of  nitrogen ;  and  a  num- 
ber of  refuse  products  from  packing  houses,  among  which 
are  red  dried  blood  and  black  dried  blood,  the  former 
having  about  13  per  cent  of  nitrogen  and  the  latter  from 
6  to  12  per  cent ;  dried  meat  and  hoof  meal,  with  12  to 
13  per  cent  of  nitrogen;  ground  fish,  with  8  per  cent  of 
nitrogen ;  and  tankage,  of  which  the  concentrated  product 
has  a  nitrogen  content  of  from  10  to  12  per  cent  and  the 
crushed  tankage  from  4  to  9  per  cent;  also  leather  meal 
and  wool-and-hair  waste,  but  .these,  because  of  their 
mechanical  condition,  are  of  very  little  value. 

The  meals  made  from  seeds  are  primarily  stock  foods 
but  are  sometimes  used  as  manures.  They  decompose 
rather  slowly  in  the  soil,  owing  to  their  high  oil  content, 
and  are  much  more  profitably  fed  to  live  stock  than  ap- 
plied as  farm  manure.  They  contain  some  phosphorus 
and  potash  as  well  as  nitrogen. 

Guano  consists  of  the  excrement  and  carcasses  of  sea 
fowl.  The  composition  of  guano  depends  on  the  climate 
of  the  region  in  which  it  is  found.  Guano  from  an  arid 
region  contains  nitrogen,  phosphorus,  and  potassium,  while 
that  from  a  region  where  rains  occur  contains  only  phos- 


508       SOILS:    PROPERTIES  AND  MANAGEMENT 

phorus —  the  nitrogen  and  potassium  having  been  largely 
leached  out.  In  a  dry  guano  the  nitrogen  exists  as  uric 
acid,  urates,  and,  in  small  quantities,  ammonium  salts. 
A  damp  guano  contains  more  ammonia.  The  phosphorus 
is  present  as  calcium  phosphate,  ammonium  phosphate4, 
and  the  phosphates  of  other  alkalies.  A  portion  of 
the  phosphate  is  readily  soluble  in  water.  Thus  all 
the  plant-food  either  is  directly  soluble  or  becomes  so 
soon  after  admixture  with  the  soil.  The  composition 
is  extremely  variable.  The  best  Peruvian  guano  con- 
tains from  10  to  12  per  cent  of  nitrogen,  from  12  to  15 
per  cent  of  phosphoric  acid,  and  from  3  to  4  per  cent  of 
potash. 

Guano  was  formerly  a  very  important  fertilizing  ma- 
terial, but  the  supply  has  become  so  nearly  exhausted 
that  it  is  relatively  unimportant  at  the  present  time. 

Of  the  abattoir  products,  dried  blood  is  the  most  readily 
decomposed,  and  therefore  has  its  nitrogen  in  the  most 
available  form.  In  fact,  it  produces  results  more  quickly 
than  any  other  form  of  organic  nitrogen.  It  requires  a 
condition  of  soil  favorable  to  decomposition  and  nitrifica- 
tion, which  prevents  its  exerting  a  strong  action  in  early 
spring.  It  should  be  applied  to  the  soil  before  the  crop 
is  planted.  *  The  black  dried  blood  contains  from  2  to  4 
per  cent  of  phosphoric  acid. 

Dried  meat  contains  a  high  percentage  of  nitrogen,  but 
does  not  decompose  so  easily  as  dried  blood,  and  is  not  so 
desirable  a  form  of  nitrogen.  It  can  be  fed  to  hogs  or 
poultry  to  advantage,  and  the  resulting  manure  is  very 
high  in  nitrogen. 

Hoof  meal,  while  high  in  nitrogen,  decomposes  slowlv, 
being  less  active  than  dried  blood.  It  is  of  use  in  increas- 
ing the  store  of  nitrogen  in  a  depleted  soil. 


COMMERCIAL   FERTILIZERS  509 

Ground  fish  is  an  excellent  form  of  nitrogen,  and  is  as 
readily  available  as  blood  but  has  a  lower  nitrogen  content. 

Tankage  is  highly  variable  in  composition,  and  the  con- 
centrated tankage,  being  more  finely  ground,  undergoes 
more  readily  the  decomposition  necessary  for  the  utiliza- 
tion of  the  nitrogen.  Crushed  tankage  contains  from  3  to 
12  per  cent  of  phosphoric  acid,  in  addition  to  its  nitrogen. 

Leather  meal  and  wool-and-hair  waste  when  untreated 
are  in  such  a  tough  and  undecomposable  condition  that 
they  may  remain  in  the  soil  for  years  without  losing  their 
structure.      They  are  not  to  be  recommended  as  manures. 

429.  Availability  of  organic  nitrogenous  fertilizers.  — 
The  forms  in  which  combined  nitrogen  is  available  to 
most  agricultural  plants  has  already  been  stated  to  be 
nitrates,  ammonium  salts,  and  certain  organic  compounds. 
Of  the  latter  the  simple  compounds,  as  urea,  appear  to  be 
most  readily  taken  up  by  plants.  Decomposition  is  there- 
fore a  necessary  process  for  most  of  these  fertilizers,  and 
their  usefulness  is,  in  general,  proportional  to  the  readi- 
ness with  which  aerobic  decomposition  proceeds,  or  to  the 
proportion  of  available  compounds  that  they  contain  in 
their  original  condition.  Guano,  for  instance  apparently, 
contains  much  nitrogen  that  is  available  without  further 
decomposition.  Dried  blood  quickly  decomposes  and 
soon  forms  available  substances,  consisting  of  the  simpler 
organic  nitrogenous  compounds,  ammonia  and  nitrates. 
The  decomposition  process  is  a  biological  one,  arising  from 
the  action  of.  microorganisms  that  first  break  down  the 
complicated  organic  compounds,  forming  simpler  ones, 
and  finally  carry  the  nitrogen  into  the  form  of  ammonia, 
then  to  nitrous  acid,  and  at  last  to  nitric  acid. 

Numerous  attempts  have  been  made  to  determine  the 
relative  availability  of  the  nitrogen  in  various  organic 


510       SOILS:    PROPERTIES  AND  MANAGEMENT 


nitrogenous  fertilizers.  A  few  such  tests,  in  which  nitrate 
of  soda  and  ammonium  sulfate  are  used  as  a  basis  for  com- 
parison, are  given  in  the  table  below,  the  statement  being 
in  terms  of  percentage  availability  when  nitrate  of  soda 
is  taken  as  one  hundred.  The  experiments  quoted  were 
conducted  by  Wagner  and  Dorsch,1  by  Johnson,  Jenkins, 
and  Britton,2  and  by  Voorhees  and  Lipman.3 

Percentage  Availability  of  Fertilizer  Nitrogen 


Wagner 

Johnson 

Voorhees 

and 

and 

and 

Dorsch 

OTHBR8 

Lipman 

Nitrate  of  soda       

100 

100 

100 

Sulfate  of  ammonia 

90 

70 

Dried  blood        

70 

73 

64 

Bone  meal 

60 

17 

Stable  manure 

45 

53 

Tankage    

49 

Horn  and  hoof  meal 

70 

68 

Linseed  meal 

69 

Cottonseed  meal 

65 

Castor  pomace        

65 

Wool  waste 

30 

Leather  meal      .     .     . 

20 

Dry  ground  fish 

64 

One  difficulty  in  drawing  conclusions  from  these  experi- 
ments is  that  the  substances  grouped  under  the  same 
name  are  not  always  identical  in  the  method  of  their 

1  Wagner,  P.,  and  Dorsch,  F.  Die  Stickstoffdiingung  der 
Landw.  Kulturpflanzen,  Erstes  Teil.     Berlin.     1892. 

2  Johnson,  S.  W.,  Jenkins,  E.  H.,  and  Britton,  W.  E.  Experi- 
ments on  the  Availability  of  Fertilizer-Nitrogen.  Connecticut 
Agr.  Exp.  Sta.,  21st  Annual  Rept.,  Part  4,  pp.  257-277.     1897. 

3  Voorhees,  E.  B.,  and  Lipman,  J.  G.  Investigations  Rela- 
tive to  the  Use  of  Nitrogenous  Materials,  1898-1907.  New 
Jersey  Agr.  Exp.  Sta.,  Bui.  221.     1909. 


COMMERCIAL   FERTILIZERS  511 

preparation  or  in  their  composition.  Another  discrep- 
ancy arises  from  the  fact  that  all  soils  do  not  respond  in 
the  same  relative  degree  to  any  one  fertilizer.  Thus, 
Sackett 1  found  that  in  some  soils  dried  blood  was  am- 
monified more  rapidly  than  was  cottonseed  meal,  while 
in  other  soils  the  reverse  was  true;  and  that  a  similar 
difference  obtained  in  soils  with  respect  to  the  ammoni- 
fication  of  alfalfa  meal  and  flaxseed  meal.  It  would 
therefore  appear  to  be  impossible  to  make  any  close  dis- 
tinctions in  the  relative  availability  of  the  nitrogen  in 
various  organic  nitrogenous  fertilizers.  A  considerable 
number  of  these  experiments  are,  in  the  aggregate,  useful 
in  pointing  out  the  probable  relative  availabilities  of  the 
more  widely  differing  nitrogen-bearing  substances. 

FERTILIZERS   USED   FOR  THEIR  PHOSPHORUS 

Phosphorus  is  generally  present  in  combination  with 
lime,  iron,  or  alumina.  Some  of  the  phosphates  contain 
also  organic  matter,  in  which  case  they  generally  carry 
some  nitrogen.  Phosphates  associated  with  organic 
matter  decompose  more  quickly  in  the  soil  than  do  un- 
treated mineral  phosphates. 

430.  Bone  phosphate.  —  Formerly  bones  were  used 
entirely  in  the  raw  condition,  ground  or  unground.-  When 
ground  they  act  as  a  fertilizer  more  quickly  than  when 
unground.  Raw  bones  contain  about  22  per  cent  of  phos- 
phoric acid  and  4  per  cent  of  nitrogen.  The  phosphorus 
is  in  the  form  of  tricalcic  phosphate  (CasCPO^). 

Most  of  the  bone  now  on  the  market  is  first  boiled  or 

1  Sackett,  W.  G.  The  Ammonifying  Efficiency  of  Certain 
Colorado  Soils.  Colorado  Agr.  Exp.  Sta.,  Bui.  184,  pp.  3-23. 
1912. 


512       SOILS:    PROPERTIES  AND  MANAGEMENT 

steamed.  This  frees  it  from  fat  and  nitrogenous  matter, 
both  of  which  are  used  in  other  ways.  Steamed  bone  is 
more  valuable  as  a  fertilizer  than  raw  bone,  because  the 
fat  in  the  latter  retards  decomposition  and  also  because 
steamed  bone  is  in  a  better  mechanical  condition.  The 
form  of  the  phosphoric  acid  is  the  same  as  in  raw  bone 
and  constitutes  from  28  to  30  per  cent  of  the  product, 
while  the  nitrogen  is  reduced  to  l|  per  cent. 

Bone  tankage,  which  has  already  been  spoken  of  as  a 
nitrogenous  fertilizer,  contains  from  7  to  9  per  cent  of 
phosphoric  acid,  largely  in  the  form  of  tricalcium  phos- 
phate. All  these  bone  phosphates  are  slow-acting  ma- 
nures, and  should  be  used  in  a  finely  ground  form  and  for 
the  permanent  benefit  of  the  soil  rather  than  as  an  imme- 
diate source  of  nitrogen  or  phosphorus. 

431.  Mineral  phosphates.  —  There  are  many  natural 
deposits  of  mineral  phosphates  in  different  parts  of  the 
world,  some  of  the  most  important  of  which  are  in  North 
America.  The  phosphorus  in  all  these  is  in  the  form  of 
tricalcium  phosphate,  but  the  materials  associated  with 
it  vary  greatly. 

Apatite  is  found  in  large  quantities  in  the  provinces  of 
Ontario  and  Quebec,  Canada.  It  exists  chiefly  in  crys- 
talline form.  The  tricalcium  phosphate  of  which  it  is 
composed  is  in  one  form  associated  with  calcium  fluoride 
and  in  the  other  with  calcium  chloride.  The  Canadian 
apatite  contains  about  40  per  cent  of  phosphoric  acid, 
being  richer  than  that  found  elsewhere.  Phosphorite  is 
another  name  for  apatite,  but  is  chiefly  applied  to  the 
impure  amorphous  form. 

Coprolites  are  concretionary  nodules  found  in  the 
chalk  or  other  deposits  in  the  south  of  England  and  in 
France.     They  contain  from  25  to  30  per  cent  of  phos- 


COMMERCIAL  FERTILIZERS  513 

phoric  acid,  the  other  constituents  being  calcium  carbonate 
and  silica. 

South  Carolina  phosphate  contains  from  26  to  28  per 
cent  of  phosphoric  acid  and  a  very  small  amount  of  iron 
and  alumina.  As  these  substances  interfere  with  the 
manufacture  of  superphosphate  from  rock,  their  presence 
is  very  undesirable  —  rock  containing  more  than  from 
3  to  6  per  cent  being  unsuitable  for  that  purpose. 

Florida  phosphates  exist  in  the  form  of  soft  phosphate, 
pebble  phosphate,  and  bowlder  phosphate.  Soft  phos- 
phate contains  from  18  to  30  per  cent  of  phosphoric  acid, 
and  because  of  its  being  more  easily  ground  than  most 
.  of  these  rocks  it  is  often  applied  to  the  land  without  being 
first  converted  into  a  superphosphate.  The  other  two 
forms,  pebble  phosphate  and  bowlder  phosphate,  are 
highly  variable  in  composition,  ranging  from  20  to  40  per 
cent  in  phosphoric  acid  content.  Tennessee  phosphate 
contains  from  30  to  35  per  cent  of  phosphoric  acid. 

Basic  slag,  or,  as  it  is  also  called,  phosphate  slag  or 
Thomas  phosphate,  is  a  by-product  in  the  manufacture 
of  steel  from  pig-iron  rich  in  phosphorus.  The  phos- 
phorus present  is  usually  considered  to  be  in  the  form  of 
tetracalcium  phosphate,  (CaO)4P205,  or  possibly  a  double 
silicate  and  phosphate  of  lime  having  the  composition 
(CaO)5P205Si02.  It  contains  also  calcium,  magnesium, 
aluminium,  iron,  manganese  silica,  and  sulfur.  Because 
of  the  presence  of  iron  and  aluminium,  and  because  its 
phosphorus  is  more  readily  soluble  than  tricalcium  phos- 
phate, the  ground  slag  is  applied  directly  to  the  soil  with- 
out treatment  with  acid. 

The  degree  of  fineness  to  which  the  slag  is  ground  is 
supposed  to  be  an  important  factor  in  determining  its 
solubility  in  the  soil.  It  is  much  more  soluble  in  water 
2l 


514       SOILS:    PROPERTIES  AND  MANAGEMENT 

charged  with  carbon  dioxide  than  in  pure  water,  a  property 
that  greatly  increases  its  value  because  of  the  fact  that 
soil  water  always  contains  more  or  less  of  this  gas.  It  is 
also  readily  acted  upon  by  organic  acids.  For  this  reason 
it  is  particularly  effective  in  a  peat  soil,  and  likewise  in 
most  soils  deficient  in  lime.  As  it  contains  a  considerable 
quantity  of  free  lime  it  has  another  beneficial  effect  on 
such  soils. 

432.  Superphosphate  fertilizers.  —  In  order  to  render 
more  readily  available  to  plants  the  phosphorus  contained 
in  bone  and  mineral  phosphates,  the  raw  material,  purified 
by  being  washed  and  finely  ground,  is  treated  with  sulfuric 
acid.  This  results  in  a  replacement  of  phosphoric  acid  by 
sulfuric  acid,  with  the  formation  of  monocalcium  phos- 
phate and  calcium  sulfate,  and  a  smaller  amount  of  dical- 
cium  phosphate,  according  to  the  reactions :  — 

Ca3(P04)2+  2  H2S04  =  CaH4(P04)2  +  2  CaS04 
Ca3(P04)2+  H2S04=  Ca2H2(P04)2  +  CaS04 

The  tricalcium  phosphate  being  in  excess  of  the  sul- 
furic acid  used,  some  of  it  remains  unchanged. 

In  the  treatment  of  phosphate  rock  some  of  the  sul- 
furic acid  is  consumed  in  acting  on  the  impurities  present, 
which  usually  consist  of  calcium  and  magnesium  carbo- 
nates, iron  and  aluminium  phosphates,  and  calcium  chlo- 
ride or  fluoride,  converting  the  bases  into  sulfates  and 
freeing  carbon  dioxide,  water,  hydrochloric  acid,  and 
hydrofluoric  acid.  The  resulting  superphosphate  is  there- 
fore a  mixture  of  monocalcium  phosphate,  dicalcium  phos- 
phate, tricalcium  phosphate,  calcium  sulfate,  and  iron  and 
aluminium  sulfates. 

In  the  superphosphates  made  from  bone,  the  iron  and 
aluminium    sulfates   do    not    exist    in    anv    considerable 


COMMERCIAL    FERTILIZERS  515- 

quantities.  However,  as  long  as  the  phosphorus  remains 
in  the  form  of  monocalcium  phosphate,  the  value  of  a 
pound  of  available  phosphorus  in  the  two  kinds  of  fertilizer 
is  the  same ;  but  the  remaining  tricalcium  phosphate  has 
a  greater  value  in  the  bone  than  in  the  rock  superphosphate. 

The  superphosphates  made  from  animal  bone  contain 
about  12  per  cent  of  available  phosphoric  acid  and  from 
3  to  4  per  cent  of  insoluble  phosphoric  acid.  They  also 
contain  some  nitrogen.  Bone  ash  and  bone  black  super- 
phosphates contain  practically  all  their  phosphorus  in  an 
available  form,  but  they  contain  little  or  no  nitrogen. 
South  Carolina  rock  superphosphate  contains  from  12  to 
14  per  cent  of  available  phosphoric  acid,  including  from 
1  to  3  per  cent  of  reverted  phosphoric  acid.  The  best 
Florida  rock  superphosphates  contain  from  17  per  cent 
downward  of  available  phosphoric  acid,  some  of  which  is 
reverted.  The  Tennessee  superphosphates  contain  from 
14  to  18  per  cent  of  available  phosphoric  acid. 

Double  superphosphates.  —  In  making  superphosphates 
a  material  rich  in  phosphorus  must  be  used,  not  less  than 
60  per  cent  of  tricalcium  phosphate  being  necessary  for 
their  profitable  production.  The  poorer  materials  are 
sometimes  used  in  making  what  is  known  as  double  super- 
phosphates. For  this  purpose  they  are  treated  with  an 
excess  of  dilute  sulfuric  acid ;  the  dissolved  phosphorus 
and  the  excess  of  sulfuric  acid  are  separated  from  the  mass 
by  filtering,  and  are  then  used  for  treating  phosphates 
rich  in  tricalcium  phosphate  and  thus  forming  superphos- 
phates. The  superphosphates  so  formed  contain  more 
than  twice  as  much  phosphorus  as  those  made  in  the 
ordinary  way. 

433.  Reverted  phosphoric  acid.  —  A  change  sometimes 
occurs  in  superphosphates  on  standing  by  which  some  of 


$ 


516       SOILS:    PROPERTIES  AND  MANAGEMENT 

the  phosphoric  acid  becomes  less  easily  soluble,  and  to 
that  extent  the  value  of  the  fertilizer  is  decreased.  This 
change,  known  as  reversion,  is  much  more  likely  to  occur 
in  superphosphates  made  from  rock  than  in  those  derived 
from  bone.  It  will  also  vary  in  different  samples,  a  well- 
made  article  usually  undergoing  little  change  even  after 
long  standing.  It  is  supposed  to  be  caused  by  the  presence 
of  undecomposed  tricalcium  phosphate  and  of  iron  and 
aluminium  sulfates. 

434.  Relative  availability  of  phosphate  fertilizers.  — 
Superphosphates  and  double  superphosphates  contain 
their  phosphorus  in  a  form  in  which  it  can  be  taken  up 
by  the  plant  at  once.  They  are  therefore  best  applied 
at  the  time  when  the  crop  is  planted,  or  shortly  before, 
or  they  may  be  applied  when  the  crop  is  growing.  Crude 
phosphates,  on  the  other  hand,  become  available  only 
through  the  natural  processes  in  the  soil.  They  should 
be  applied  in  quantity  sufficient  to  meet  the  needs  of  the 
crops  for  a  number  of  years. 

Reverted  phosphorus,  although  not  soluble  in  water, 
is  readily  soluble  in  dilute  acids.  It  is  now  generally 
believed  that  in  this  form  an  available  supply  of  phos- 
phorus is  furnished  to  the  plant.  In  a  statement  of  fer- 
tilizer analyses  reverted  phosphorus  is  termed  citrate- 
soluble,  and  this  and  the  water-soluble  are  termed  available. 

The  degree  of  fineness  to  which  the  material  is  ground 
makes  a  great  difference  in  the  availability  of  the  less 
soluble  phosphate  fertilizers,  especially  in  the  ground-rock 
phosphates  and  in  ground  bone.  This  material  should  be 
ground  fine  enough  to  pass  through  a  sieve  having  meshes 
at  least  one-fiftieth  of  an  inch  in  diameter. 

435.  Changes  that  occur  when  superphosphate  is  added 
to  soils.  —  When  incorporated  with  soils  superphosphate 


COMMERCIAL   FERTILIZERS  517 

undergoes  changes,  the  nature  of  which  depends  more  or 
less  on  the  properties  of  the  particular  soil  with  which  it 
is  mixed.  No  matter  how  readily  soluble  the  phosphorus 
may  be  in  the  fertilizer,  it  soon  becomes  insoluble  in  the 
soil,  only  a  fractional  proportion  of  it  being  recoverable  in 
water  extracts.  Absorption  by  colloidal  complexes  is  the 
fate  of  a  part  of  the  phosphorus,  in  which  condition  it  is 
still  available  to  plants,  especially  when  the  colloidal 
matter  becomes  coagulated.  The  excess  phosphorus  en- 
ters into  combination  with  the  calcium  of  the  soil,  form- 
ing tricalcium  phosphate  and  some  dicalcium  phosphate, 
and  with  the  iron  or  the  aluminium,  forming  phosphates  of 
those  metals.  The  latter  compounds  are  less  readily  soluble 
than  the  former,  and  probably  do  not  serve  as  a  direct 
source  of  phosphorus  for  plants ;  while  tricalcium  phosphate, 
although  acted  upon  by  plant  roots,  is  not  so  readily  avail- 
able as  is  the  phosphorus  held  by  the  colloidal  matter. 

It  is  desirable  that  there  should  be  an  abundant  supply 
of  calcium  in  a  soil  to  which  a  superphosphate  is  added,  be- 
cause the  phosphorus  not  absorbed  by  the  colloidal  matter 
of  the  soil  will,  under  such  circumstances,  form  more  cal- 
cium phosphate  than  if  only  a  small  supply  of  lime  is  pres- 
ent, according  to  the  law  of  mass  action.  The  great  loss 
of  availability  through  the  conversion  of  phosphorus  into 
iron  and  aluminium  phosphates  may  thus  be  mitigated. 

436.  Other  factors  influencing  the  availability  of  tri- 
calcium phosphate.  —  As  this  is  the  form  in  which  phos- 
phorus is  probably  most  extensively  held  in  the  ordinary 
soil,  and  as  it  is  also  a  cheap  form  of  phosphorus  in  manures, 
it  is  a  matter  of  some  importance  to  know  the  most  favor- 
able conditions  for  its  utilization  by  agricultural  plants. 
Experimentation  by  numerous  investigators  has  estab- 
lished at  least  four  factors  that  influence  the  availability 


518      SOILS:    PROPERTIES  AND  MANAGEMENT 

of  this  substance  :  (1)  kind  of  plant  grown;  (2)  degree  of 
basicity  of  soil;  (3)  fermentation  of  organic  matter; 
(4)  character  of  the  accompanying  salts. 

437.  Effect  of  plants  on  the  availability  of  tricalcium 
phosphate.  —  It  is  to  be  expected  that  the  various  kinds 
of  plants  should  not  all  exert  an  equal  influence  on  the 
availability  of  the  phosphorus  of  tricalcium  phosphate. 
Prhnischnikov x  found  that  lupines,  mustard,  peas, 
buckwheat,  and  vetch  responded  to  fertilization  with  raw 
rock  phosphate  in  the  order  named,  while  the  cereals  did 
not  respond  at  all.  He  did  not  include  maize  in  his 
experiments,  but  that  crop  is  said  to  respond  well  to  diffi- 
cultly soluble  phosphates.  It  is  generally  considered 
that  those  plants  which  have  a  long  growing  season  are 
better  able  to  utilize  tricalcium  phosphate  than  are  more 
rapidly  growing  plants.  An  explanation  for  the  ability 
of  some  plants  to  utilize  the  phosphorus  of  difficultly 
soluble  phosphates  more  successfully  than  do  other  plants 
has  been  sought  in  the  rate  of  excretion  of  carbon  dioxide 
by  plant  roots.  It  has  already  been  stated  (par.  324)  that 
Stoklasa  and  Ernst  found  that  the  capacity  of  a  plant  to 
absorb  phosphorus  from  difficultly  soluble  phosphates  is 
proportional  to  the  rate  at  which  carbon  dioxide  is  given 
off  by  the  roots,  but  that  the  experiments  of  Kossowitch 
and  Barakoff  failed  to  confirm  these  results.  This  ques- 
tion is  bound  up  with  the  larger  one  involving  the  solvent 
action  of  plant  roots,  regarding  which  little  is  now  known. 

438.  Effect  of  basicity  on  the  availability  of  tricalcium 
phosphate.  —  It  is  recognized  that  raw  rock  phosphate  is 
more  available  to  the  same  plant  in  some  soils  than  in 
others,  and  a  number  of  persons  have  stated,  as  the  result 

1  Prianischnikov,  D.  Bericht  liber  Verschiedene  Versuehe 
mit  Rohphosphaten  unter  Reduction.     Moscow.     1910. 


COMMERCIAL   FERTILIZERS  519 

of  experimentation,  that  the  availability  is  greater  in  acid 
soils  than  in  those  strongly  basic.  If  acidity  of  the  soil 
is  due  to  the  presenca  of  free  acid  (positive  acidity),  it  is 
conceivable  that  the  availability  may  be  due  to  the  sol- 
vent action  of  the  soil  acid  on  the  calcium  of  the  trical- 
cium  phosphate,  producing  the  dicalcium  salt  which  ap- 
pears to  be  fairly  readily  available  to  plants.  When, 
however,  soil  acidity  is  due  to  a  lack  of  basicity  (apparent 
acidity),  the  case  is  different.  Gedroiz 1  explains  this 
on  the  basis  of  the  absorptive  properties  of  the  apparently 
acid  soil.  He  regards  rock  phosphate,  not  as  a  chemical 
compound,  but  as  a  solid  solution  of  dicalcium  phosphate 
with  lime.  It  is  this  excessive  basicity  of  the  phosphate 
which  is  responsible  for  its  unavailability.  Absorption  of 
the  excess  calcium  would  leave  the  phosphate  in  a  more 
readily  available  condition  by  forming  the  dicalcium  salt, 
and  this  is  brought  about  in  an  apparently  acid  soil. 

Gedroiz  experimented  with  a  highly  basic  soil  that  did 
not  respond  to  fertilization  with  rock  phosphate.  He 
subjected  this  soil  to  repeated  washings  with  distilled 
water  charged  with  carbon  dioxide.  After  such  treatment 
the  soil  gave  a  marked  increase  in  crop  with  rock  phos- 
phate as  compared  with  the  same  soil  untreated.  Accord- 
ing to  Gedroiz  the  greater  availability  of  the  phosphate 
after  treatment  with  carbonic  acid  was  due  to  the  removal 
of  bases  and  the  greater  absorptive  power  of  the  soil 
brought  about  thereby.  This  was  further  corroborated 
by  the  fact  that  the  treated  soil  responded  to  a  test  for 
unsaturation  while  the  untreated  soil  did  not.     Without 


1  Gedroiz,  K.  K.  Soils  to  which  Rock  Phosphates  may 
be  Applied  with  Advantage.  Jour.  Exp.  Agronomy  (Russian), 
Vol.  12,  pp.  529-539,  811-816.  1911.  The  authors  are  in- 
debted to  Dr.  J.  Davidson  for  the  translation. 


520       SOILS:    PROPERTIES  AND  MANAGEMENT 

necessarily  accepting  all  of  Gedroiz's  explanation  of  the 
phenomenon,  there  can  be  little  doubt  that  lack  of  basicity 
is  a  factor  in  the  availability  of  raw  rock  phosphates  in 
some  soils. 

439.  Influence  of  fermenting  organic  matter.  —  There 
has  been  great  difference  of  opinion  among  investigators 
as  to  the  effect  of  fermentation  of  organic  matter  on  the 
availability  of  the  phosphorus  of  tricalcium  phosphate. 
The  contention  that  the  availability  is  increased  probably 
originated  with  Stoklasa,1  the  results  of  whose  experi- 
ments with  bone  meal  indicated  that  the  availability  is 
increased  by  fermentation.  A  large  number  of  experi- 
ments have  been  conducted  with  raw  rock  phosphate 
composted  with  stable  manure,  among  which  may  be 
mentioned  those  by  Hartwell  and  Pember2  and  also  by 
Tottingham  and  Hoffman3  who  in  carefully  conducted 
experiments  failed  to  find  that  the  availability  of  the  raw 
phosphate  was  increased  by  fermentation  with  stable 
manure.  Opposing  results  have  also  been  obtained,  how- 
ever, and  the  evidence  is  somewhat  conflicting.  Krober,4 
who  thinks  that  the  action  of  bacteria  is  due  to  the  acids 
they  produce,  explains  the  contradictions  in  the  various 

1  Stoklasa,  J.,  Duchacek,  F.,  and  Pitra,  J.  Ueber  den  Ein- 
fluss  der  Bakterien  auf  die  Knochenzersetzung.  Centrlb.  f. 
Bakt.,  II,  Band  6,  Seite  526-535,  554-558.     1900. 

2  Hartwell,  B.  L.,  and  Pember,  F.  R.  The  effect  of  cow  dung 
on  the  availability  of  rock  phosphate.  Rhode  Island  Agr. 
Exp.  Sta.,  Bui.  151.      1912. 

3  Tottingham,  W.  E.,  and  Hoffman,  C.  The  Nature  of  the 
Changes  in  Solubility  and  Availability  of  Phosphorus  in  Fer- 
menting Mixtures.  Wisconsin  Agr.  Exp.  Sta.,  Research  Bui. 
29.     1913.    . 

4  Krober,  E.  Ueber  das  Loslichwerden  der  Phosphorsaure 
aus  Wasserunloslichen  Verbindungen  unter  der  Einwirkung 
von  Bakterien  und  Hefen.  Jour,  f,  Landw.,  Band  57,  Seite 
5-80.     1909-1910. 


COMMERCIAL   FERTILIZERS  521 

experiments  as  arising  from  the  different  kinds  of  fer- 
mentation that  the  organic  matter  undergoes.  He  thinks 
that  acid  fermentation  renders  the  phosphate  more  readily 
soluble,  while  fermentation  that  does  not  give  rise  to  acids 
leaves  it  in  an  insoluble  condition. 

Parallel  with  the  biological  process  that  results  in  the 
transformation  of  insoluble  phosphates  into  soluble,  there 
is,  according  to  Stoklasa  and  others,  a  reverse  biological 
process  resulting  in  the  transformation  of  soluble  phos- 
phates into  insoluble. 

Whatever  may  be  the  conditions  under  which  raw  rock 
phosphate  is  rendered  more  readily  soluble  or  available 
by  fermentation  of  organic  matter,  it  does  not  appear 
that  composting  with  stable  manure  produces  this  change, 
at  least  from  results  of  numerous  experiments  including 
those  mentioned  above.  These  have  been  mainly  opposed 
to  any  such  conclusion. 

440.  Influence  of  other  salts.  —  The  presence  of  cer- 
tain salts  has  been  found  to  influence  the  availability  of 
difficultly  soluble  phosphates.  The  subject  has  been  in- 
vestigated by  a  large  number  of  experimenters  and  it  will 
be  possible  to  summarize  their  results  only  in  part  and 
very  briefly.  It  has  been  found,  for  instance,  that  cal- 
cium carbonate  decreases  the  availability  of  raw  rock 
phosphate  and  bone-meal.  Sodium  nitrate  reduces  the 
availability  of  the  tricalcium  phosphates,  while  the  am- 
monium salts  increase  their  availability.  Iron  salts 
decrease  availability.  The  influence  of  other  salts  has  not 
been  so  well  worked  out.  Prianischnikov,1  as  the  result 
of  his  extended  experiments  on  the  subject,  holds  that 

1  Prianischnikov,  D.  Ueber  den  Einfluss  von  Kohlensiinren 
Kalk  auf  die  Wirkung  von  Verschiedenen  Phosphaten.  Landw. 
Vers.  Stat.,  Band  75,  Seite  357-376.     1911. 


522      SOILS:    PROPERTIES  AND  MANAGEMENT 

salts  from  which  plants  absorb  acid  in  larger  amounts 
than  they  do  bases  decrease  availability,  or  at  least  do  not 
affect  it,  while  salts  from  which  plants  absorb  the  bases  in 
$ij|£er  quantity  than  the  acids  have  a  tendency  to  render 
the  pfrosfhaje  more  available,  because  of  the  solvent 
action  of  theffc^d. 

FERTILIZERS   USED   FOR  THEIR   POTASSIUM 

The  production  of  potassium  fertilizers  is  largely  con- 
fined to  Germany,  where  there  are  extensive  beds  varying 
from  50  to  150  feet  in  thickness,  lying  under  a  region  of 
country  extending  from  the  Harz  Mountains  to  the  Elbe 
River  ancL  known  as  the  Stassfurt  deposits.  Deposits 
have  lately  been  discovered  in  other  parts  of  Germany. 

441.  Stassfurt  salts.  —  The  Stassfurt  salts  contain 
their  potassium  either  as  a  chloride  or  as  a  sulfate.  The 
chloride  has  the  advantage  of  being  more  diffusible  in  the 
soil,  but  in  most  respects  the  sulfate  is  preferable.  Potas- 
sium chloride  in  large  applications  has  an  injurious  effect 
on  certain  crops,  among  which  are  tobacco,  sugar  beets, 
and  potatoes.  On  cereals,  legumes,  and  grasses,  the 
muriate  appears  to  have  no  injurious  effect. 

The  mineral  produced  in  largest  quantities  by  the 
Stassfurt  mines  is  kainit.  Chemically  it  consists  of  mag- 
nesium and  potassium  sulfate  and  magnesium  chloride, 
or  of  magnesium  sulfate  and  potassium  chloride.  Kainit 
has  the  same  effect  on  plants  as  has  potassium  chloride. 
It  contains  from  12  to  20  per  cent  of  potash  and  from  25 
to  45  per  cent  of  sodium  chloride,  with  some  chloride  and 
sulfate  of  magnesium. 

Kainit  should  be  applied  to  the  soil  a  considerable 
time  before  the  crop  for  which  it  is  intended  is  planted. 


COMMERCIAL   FERTILIZERS  523 

It  should  not  be  drilled  in  with  the  seed,  as  the  action  of 
the  chlorides  in  direct  contact  with  the  seed  may  injure 
its  viability.  In  addition  to  the  potassium  added  to  the 
soil  by  kainit,  there  are  also  in  this  fertilizer  magnesi 
and  sodium.  The  magnesium  may  be  objec^oiit^P  if 
there  is  much  already  present  in  the  soiL^pe  par.  458). 
Sodium  may  to  some  extent  replace  potassium  in  the  soil 
economy,  and  in  that  way  may  be  beneficial. 

Silvinit  contains  its  potassium  both  as  chloride  and  as 
sulfate.  It  also  contains  sodium  and  magnesium  chlorides. 
Potash  constitutes  about  16  per  cent  offrfrfae  material. 
Owing  to  the  presence  of  chlorides,  it^ftlP^iiFsame  effect 
on  plants  as  has  kainit. 

The  commercial  form  of  potassium  chloridffigeneralH' 
contains  about  80  per  cent  of  potassium  chloffae  or  50 
per  cent  of  potash.  The  impurities  are  largely  sodium 
chloride  and  insoluble  mineral  matter.  The  possible 
injury  to  certain  crops  from  the  use  of  the  chloride  has 
already  been  mentioned.  For  crops  not  so  affected,  potas- 
sium chloride  is  a  quickly  acting  and  effective  carrier  of 
potassium,  and  one  of  the  cheapest  forms. 

High-grade  sulfate  of  potassium  contains  from  48  to 
50  per  cent  of  potash.  Unlike  the  muriate  it  is  not  in- 
jurious to  crops,  but  is  more  expensive. 

There  are  a  number  of  other  Stassfurt  salts,  consisting 
of  mixtures  of  potassium,  sodium,  and  magnesium  in  the 
form  of  chlorides  and  sulfates.  They  are  not  so  widely 
used  for  fertilizers  as  are  those  mentioned  above. 

442.  Wood  ashes.  —  For  some  time  after  the  use  of 
fertilizers  became  an  important  farm  practice,  wood 
ashes  constituted  a  large  proportion  of  the  source  of  supply 
of  potassium.  They  also  contain  a  considerable  quantity 
of  lime  and  a  small  amount  of  phosphorus.     The  product 


524       SOILS:    PROPERTIES  AND  MANAGEMENT 

known  as  unleached  wood  ashes  contains  from  5  to  6  per 
cent  of  potash,  2  per  cent  of  phosphoric  acid,  and  30  per 
cent  of  lime.  Leached  wood  ashes  contain  about  1  per  cent 
of  potash,  lj  per  cent  of  phosphoric  acid,  and  from  28  to  29 
per  cent  of  lime.  They  contain  the  potassium  in  the  form 
of  a  carbonate,  which  is  alkaline  in  its  reaction  and  in  large 
amount  may  be  injurious  to  seeds.  They  are  beneficial 
to  acid  soils  through  the  action  of  both  the  potassium  and 
calcium  salts.  The  lime  is  valuable  for  the  other  effects  it 
has  on  the  properties  of  the  soil.     (See  pars.  454-457.) 

443.  Insoluble  potassium  fertilizers.  —  Insoluble  forms 
of  potassium,  existing  in  many  rocks  usually  in  the  form 
of  a  silicate,  are  not  regarded  as  having  any  manurial 
value.  Experiments  with  finely  ground  feldspar  have  been 
conducted  by  a  number  of  investigators,  but  have,  in  the 
main,  given  little  encouragement  for  the  successful  use  of 
this  material.  An  insoluble  form  of  potassium  is  not 
given  any  value  in  the  rating  of  a  fertilizer  based  on  the 
results  of  its  analysis. 

SULFUR  AND  SULFATES  AS  FERTILIZERS 

The  use  of  these  substances  as  a  means  of  increasing 
plant  growth  when  applied  to  soils  has  recently  received 
revived  attention.  The  use  of  free  sulfur  has  been  in- 
vestigated to  some  extent  in  France  and  Germany.  There 
have  been  suggested  three  ways  in  which  it  may  be  bene- 
ficial to  plants  (1)  as  a  direct  stimulant;  (2)  by  its  in- 
fluence on  the  activities  of  microorganisms;  (3)  as  a 
source  of  plant-food,  which  might  otherwise  be  deficient. 

444.  The  use  of  free  sulfur.  —  Boullanger  *  added 
flowers  of  sulfur  to  a  soil  at  the  rate  of  23  parts  to  a  million 

»  Boullanger  E.  Action  du  soufre  en  fleur  sur  la  vegetation. 
Compt.  Rend.  Acad.  Sci.  Paris,  T.  154,  pp.  369-370.     1912. 


COMMERCIAL   FERTILIZERS  525 

of  soil.  He  obtained  increased  growth  in  all  treated  soils 
on  which  carrots,  beans,  celery,  lettuce,  sorrel,  chicory, 
potatoes,  onions,  and  spinach  were  grown,  the  weight  of 
the  crops  on  the  treated  soil  being  from  10  per  cent  to  40 
per  cent  greater  than  those  on  the  untreated  soil.  On 
soils  that  had  been  sterilized  before  applying  sulfur  the 
effect  was  much  less,  from  which  he  concludes  that  the 
beneficial  effects  were  due  to  the  influence  of  the  sulfur 
on  the  microorganisms  of  the  soil.  There  may  be  some 
question,  however,  whether  this  conclusion  is  justifiable. 
Sulfur  was  found  by  Boullanger  and  Dugardin *  to  favor 
ammonification  in  soils.  Beneficial  effects  from  the  use 
of  free  sulfur  have  also  been  obtained  by  Demelon,2  and 
by  Bernhard 3  among  others,  while  von  Feilitzen 4  found 
it  to  be  ineffective  as  a  fertilizer. 

That  free  sulfur  may,  under  some  conditions,  exert  a 
beneficial  influence  on  plant  growth  must  undoubtedly  be 
conceded,  but  how  the  action  is  brought  about  remains  to 
be  conclusively  demonstrated.  Free  sulfur  is  insoluble  and 
cannot  be  absorbed  by  plant  roots.  However  it  is  readily 
oxidized  in  soils 5  eventually  producing  sulfates  with  bases  in 
the  soil  and  in  this  form  may  readily  be  taken  up  by  plants. 

boullanger,  E.,  and  Dugardin,  M.  Meeanisme  de  Taction 
fertilisante  du  soufre.  Compt.  Rend.  Acad.  Sci.  Paris,  T.  155, 
pp.  327-329.     1912. 

2  Demelon,  A.  Sur  Faction  fertilisante  du  soufre.  Compt. 
Rend.  Acad.  Sci.  Paris,  T.  154,  pp.  524-526.     1912. 

3  Bernhard,  A.  Versuche  liber  die  Wirkung  des  Schwefels  als 
Dung  im  Jahre  1911.  Deutsche  Landw.  Presse.  Band  39,  p.  275. 
1912. 

4  von  Feilitzen,  H.  Ueber  die  Verwendung  der  Schwefel- 
blute  zur  Bekampfung  des  Kartoffelschorfes  und  als  indirektes 
Dungemittel.     Fuhling's  Landw.  Zeit.  Band  62,  Seite  7.     1913. 

5  Mares,  M.  N.  Des  transformations  que  subit  le  soufre 
en  poudre  quand  il  es  reponds  sur  le  sol.  Compt.  Rend.  Acad. 
Sci.  Paris,  T.  69,  pp.  974-979.     1869. 


526       SOILS:    PROPERTIES  AND  MANAGEMENT 

445.  Sulfur  as  sulfate.  —  There  is  less  experimental 
evidence  regarding  the  effect  of  sulfur  in  the  form  of 
sulfate  on  plant  growth  than  there  is  for  the  free  sulfur. 
The  fact  that  the  bases  with  which  the  sulfate  is  com- 
bined are  likely  to  have  an  effect  on  plant  growth,  makes 
the  accumulation  of  proof  by  experimentation  a  somewhat 
more  difficult  matter.  That  there  may  be  a  possible  de- 
ficiency of  sulfur  in  arable  soils  has  been  pointed  out  by 
several  investigators,  including  Hart  and  Peterson l  in 
this  country.  They  point  out  that  crops  remove  more 
sulfur  from  the  soil  than  was  shown  by  the  early  deter- 
minations of  sulfur  in  plant  ash,  from  which  a  large  part 
of  the  sulfur  was  vojatilized  during  the  process.  They 
then  proceed  to  calculate  the  sulfur  removed  by  a  num- 
ber of  crops  on  the  basis  of  their  own  methods  and 
compare  this  with  the  phosphorus  in  similar  crops. 

Pounds  Sulfur  Trioxide  and  Phosphorus  Pentoxide 
Removed  to  the  Acre  by  Average  Crops 


Crop  and  Yield  to  the  Acre 


Content 

in  Pounds  to  the 

Acre 

80s 

P2O5 

15.7 

21.1 

14.3 

20.7 

19.7 

19.7 

12.0 

18.0 

64.8 

39.9 

92.2 

33.1 

98.0 

61.0 

11.5 

21.5 

11.3 

12.3 

Wheat  (30  bu. 

Barley  (40  bu.) 

Oats  (45  bu.) 

Corn  (30  bu.) 

Alfalfa  (9000  lb.  dry  wt.)  .  . 
Turnips  (4657  lb.  dry  wt.)  .  . 
Cabbage  (4800  lb.  dry  wt.)  .  . 
Potatoes  (3360  lb.  dry  wt.)  .  . 
Meadow  hay  (2822  lb.  dry  wt.) 


1  Hart,  E.  B.,  and  Peterson,  W.  H.  Sulphur  requirements 
of  farm  crops  in  relation  to  the  soil  and  air  supply.  Wise. 
Agr.  Exp.  Sta.,  Research  Bui.  No.  14.     1911. 


COMMERCIAL   FERTILIZERS 


527 


They  then  call  attention  to  the  quantities  of  sulfur 
trioxide  contained  in  average  soils  which,  as  shown  by 
Hilgard,  are  less  than  the  quantities  of  phosphorus  pen- 
toxide. 


Content  in  Pounds  to  the 
Acre 


S03 


P205 


Sandy  soils 
Clay  soils  . 


1650 
2250 


2610 
4230 


To  ascertain  whether  the  supply  of  sulfur  in  the  soil 
is  really  depleted  by  cropping,  the  same  authors  made 
parallel  determinations  of  sulfur  in  five  virgin  soils  and 
in  five  soils  of  the  same  respective  types  that  had  been 
cropped  for  sixty  years.  In  each  type  the  cropped  soil 
contained  less  sulfur  than  the  virgin  soil,  the  average  for 
the  cropped  soils  being  .053  per  cent  S03  and  for  the 
virgin  soils  .085  per  cent  S03. 

There  is  no  doubt  that  the  quantity  of  sulfur  carried 
down  by  rain  and  snow  is  much  less  than  that  removed 
in  drainage  water.  There  can  be  no  question  therefore 
that  most  soils,  and  especially  cultivated  soils,  are  losing 
more  sulfur  than  they  receive  by  natural  processes. 

It  has  been  customary  to  add  to  soils  manures  of  one 
kind  or  another  that  contain  more  or  less  sulfur.  Among 
these  are  farm  manure  and  other  animal  or  bird  excre- 
ments, residues  of  crops,  animal  offal,  gypsum  or  land 
plaster,  superphosphate,  ammonium  sulfate,  potassium 
sulfate,  kainit,  and  the  like,  all  of  which  contain  conse- 
quential  quantities   of   sulfur.     It   seems   probable   that 


528       SOILS:    PROPERTIES  AND  MANAGEMENT 

any  system  of  soil  management  that  does  not  include 
one  or  more  of  these  substances  would  probably,  on  some 
soils  at  least,  be  improved  by  making  provision  for  the 
application  of  sulfur  in  some  form. 


CATALYTIC   FERTILIZERS 

The  term  catalytic  fertilizers  has  been  used  rather 
loosely  to  designate  a  class  of  substances  that,  when  added 
to  a  soil,  increase  plant  growth  by  apparently  accelerating 
the  processes  that  normally  take  place  in  soils.  They 
do  not  function  as  fertilizers  because  their  value  does  not 
lie  in  the  nutrients  that  they  possess,  but  they  may 
properly  be  classed  as  soil  amendments.  However, 
substances  not  classed  as  catalyzers,  such  as  t  lime,  have 
such  action,  and  in  all  probability  most  of  the  fertilizers 
do  also,  so  that  it  is  difficult  to  draw  any  definite  distinc- 
tion and  the  term  will  doubtless  be  used  only  temporarily. 

446.  Nature  of  catalytic  action.  —  The  term  catalysis 
is  employed  in  a  chemical  sense  to  mean  a  change  brought 
about  in  a  compound  by  an  agent  that  itself  remains 
stable.  As  an  example  of  this  may  be  cited  the  part  that 
hydrochloric  acid  plays  in  the  inversion  of  cane  sugar, 
the  acid  not  entering  into  the  reaction  but  by  its  presence 
greatly  accelerating  it.  When  an  attempt  is  made  to 
study  these  phenomena  in  soils,  it  becomes  difficult, 
owing  to  the  multiplicity  of  factors  and  reactions,  to 
determine  whether  the  agent  is  acting  in  a  purely  cata- 
lytic manner. 

447.  Catalytic  action  of  soils.  —  Most  soils  themselves 
act  as  catalyzers  in  so  far  as  they  hasten  the  decomposition 
of  hydrogen  peroxide.  Many  substances,  both  organic 
and  inorganic,  have  this  property,  and  it  is  not  necessarily 


COMMERCIAL   FERTILIZERS  529 

entirely  lost  to  the  soil  after  the  organic  matter  has  been 
destroyed  by  ignition.  It  is  therefore  not  due  to  an 
enzyme,  as  stated  by  Konig,  Hasenbaumer,  and  Coppen- 
rath,1 who  first  investigated  the  subject,  nor  entirely  to 
organic  substances  in  the  soil.  Doubtless  there  are  several, 
or  perhaps  many,  activating  substances  any  of  which  have 
this  property.  It  is  altogether  likely  that  other  catalyzers 
exist  in  soils,  and  that  they  affect  various  reactions  that  are 
concerned  in  plant  production.  Among  these  substances, 
as  pointed  out  by  Konig,  Hasenbaumer,  and  Coppenrath,2 
are  manganese  and  iron  oxides,  which  are  well  known  to 
exert  catalytic  action  on  certain  reactions.  While  soils 
naturally  possess  certain  catalytic  powers,  it  seems  possible 
to  still  further  activate  some  soils  by  proper  applications 
of  so-called  catalytic  fertilizers. 

Organic  matter  is  doubtless  concerned  in  the  catalytic 
properties  of  soils,  and  the  investigators  just  mentioned 
found  that  in  six  soils  the  catalytic  action  stood  in  almost 
direct  relation  to  the  humus  content ;  Sullivan  and  Reid,3 
however,  did  not  find  this  correlation  to  hold.  Both 
organic  and  inorganic  substances  are  involved  in  this 
property  of  soils,  but  the  forms  in  which  they  operate 
are  not  well  understood.  In  the  main  productive  soils 
have  a  strong  catalytic  effect  and  very  poor  soils  are  weak 
in  this  respect,  but  this  correlation  also  is  not  constant. 

1  Konig,  J.,  Hasenbaumer,  J.,  and  Coppenrath,  E.  Einige 
Neue  Eigenschaften  des  Aekerbodens.  Landw.  Vers.  Stat., 
Band  63,  Seite  471-478.     1905-1906. 

2  Konig,  J.,  Hasenbaumer,  J.,  and  Coppenrath,  E.  Bezieh- 
ungen  zwischen  den  Eigenschaften  des  Bodens  und  der  Nahr- 
stoffaufnahme  durch  die  pflanzen.  Landw.  Vers.  Stat.,  Band 
66,  Seite  401-461.  %  1907. 

3  Sullivan,  M.  X.,  and  Reid,  F.  R.  Studies  in  Soil  Catalysis, 
U.  S.  D.  A.,  Bur.  Soils,  Bui.  86.     1912. 

2m 


530       SOILS:    PROPERTIES  AND  MANAGEMENT 

448.  Substances  used  as  catalytic  fertilizers.  —  A 
large  number  of  substances  have  been  found  to  act  as 
catalytic  fertilizers.  Among  these  are  various  salts  of 
manganese,  iron,  aluminium,  zinc,  lead,  copper,  nickel, 
cobalt,  uranium,  boron,  cerium,  lanthanum,  and  the  like. 
These  substances  stimulate  plant  growth  when  used  in 
small  quantities,  and  are  toxic  in  large  amounts.  In 
water  cultures  a  much  less  quantity  of  any  of  them  is 
required  to  produce  an  injurious  action  on  plant  growth 
than  when  applied  to  an  equal  volume  of  soil.  The 
absorptive  properties  of  the  soil  and  the  less  ready  diffusi- 
bility  serve  to  mitigate  the  toxic  action. 

Different  kinds  of  plants  respond  differently  to  the  same 
concentration  of  any  of  these  substances.  For  instance, 
Montemartini 1  found  that  uranium,  copper,  zinc,  alumin- 
ium, and  cadmium  oxides  retard  the  germination  of  beans 
and  accelerate  the  germination  of  maize  when  used  in  equal 
concentrations. 

Of  the  various  plant  stimulants  mentioned,  manganese 
is  the  only  one  that  gives  promise,  at  the  present  time, 
of  usefulness  on  a  commercial  basis,  and  it  is  the  only  one 
that  will  receive  separate  treatment  in  this  book. 

449.  Manganese.  —  It  seems  probable  that  all  soils 
contain  manganese,  but  the  quantity  present  in  some 
soils  is  very  small,  often  being  less  than  0.01  per  cent; 
in  other  soils,  however,  more  than  1  per  cent  is  found, 
and  Kelly2  reports  an  Hawaiian  soil  containing  9.74  per 


1  Montemartini,  L.  Quoted  with  other  experiments  on  this 
subject  by  N.  H.  J.  Miller,  in  Annual  Reports  on  the  Progress 
of  Chemistry,  Vol.  10,  pp.  229-230.     1914. 

2  Kelly,  M.  P.  The  Influence  of  Manganese  on  the  Growth 
of  Pineapples.  Jour.  Indus,  and  Eng.  Chem.,  Vol.  1,  p.  533. 
1909. 


COMMERCIAL   FERTILIZERS  531 

cent  of  M113O4.  Sullivan  and  Robinson1  examined 
twenty-six  American  soils  and  found  the  content  of  MnO 
to  vary  from  0.01  to  0.51  per  cent,  the  average  being 
0.071  per  cent. 

Manganese  is  a  universal  constituent  not  only  of  soils, 
but  likewise  of  plants  grown  under  natural  conditions; 
in  plants  the  quantities  present  vary  much  more  than  in 
soils,  and  range  from  a  few  tenths  of  one  per  cent  to  nearly 
one-half  of  the  total  ash.  However,  plants  may  be  pro- 
duced in  water  cultures  or  other  media  in  which  apparently 
no  manganese  is  present  and  a  normal  growth  and  fructi- 
fication will  follow.  It  is  evident,  therefore,  that  any 
benefit  to  plant  growth  that  may  accrue  through  the 
addition  of  manganese  to  the  soil  is  not  due  to  its  function 
as  a  nutrient  material  in  the  sense  in  which  nitrogen, 
potassium,  and  phosphorus  act  in  that  capacity. 

450.  Physiological  role  of  manganese.  —  It  was  the 
discovery  by  Bertrand  2  of  the  existence  of  manganese  in 
the  oxidizing  enzymes  of  plants  and  of  its  function  in 
stimulating  the  oxygen-carrying  power  of  these  catalytic 
agents  that  suggested  its  use  as  a  stimulating  agent  in 
crop  production.  In  water  cultures  a  very  dilute  solution 
of  manganese  salts  increases  plant  growth,  but  beyond  a 
very  low  concentration  its  effect  is  toxic.  Plants  differ 
widely  in  their  response  to  manganese,  with  respect  both 
to  stimulation  and  to  injury.  A  certain  concentration 
may  be  stimulating  to  one  plant  and  toxic  to  another. 

Experiments    in   the    application   of   manganese    salts 

1  Sullivan,  M.  X.,  and  Robinson,  W.  O.  Manganese  as  a 
Fertilizer,  U.  S.  D.  A.,  Bur.  Soils,  Circ.  75.     1912. 

2  Bertrand,  G.  Sur  l'Action  Oxydante  des  Sels  Manganeux 
et  sur  la  Constitution  Chemique  des  Oxydases.  Compt.  Rend. 
Acad.  Sci.  Paris,  Tome  124,  pp.  1355-1358.     1897. 


532       SOILS:    PROPERTIES  AND  MANAGEMENT 

to  soils  have  not  afforded  as  satisfactory  results  as  have 
the  trials  with  water  cultures.  Applications  of  a  certain 
salt  of  manganese,  when  applied  at  the  same  rates  to 
different  soils,  have  in  some  cases  produced  increased 
growth,  have  in  other  cases  had  no  apparent  effect,  and 
have  in  still  other  cases  proved  injurious  to  plants.  The 
reason  for  this  is  doubtless  to  be  found  in  the  inherent 
properties  of  the  particular  soil  to  which  the  application 
is  made. 

451.  Action  of  manganese  as  a  fertilizer.  —  The  fact 
that  manganese  stimulates  plant  growth  in  water  cultures 
is  very  good  evidence  that  it  has  at  least  a  direct  action 
on  the  plant.  Whether  it  has  a  further  influence  through 
reactions  brought  about  in  the  soil  is  less  evident,  although 
it  seems  likely  that  such  is  the  case.  Thus,  Skinner  and 
Sullivan  x  conclude  from  some  of  their  experiments  that 
oxidation  in  some  soils  is  increased  by  the  application  of 
manganese  salts.  It  also  seems  probable  that  manganese 
may  have  some  influence  on  the  activity  of  the  microor- 
ganisms of  the  soil,  but  this  has  not  been  definitely  demon- 
strated. 

452.  Forms  of  manganese  and  response  of  soils.  — 
The  manganese  salts  that  have  been  found  to  be  effective 
as  fertilizers  are  the  sulfate,  the  chloride,  the  nitrate,  the 
carbonate,  and  the  dioxide.  Of  these  the  first  has  been 
most  generally  used,  and  in  quantities  up  to  50  pounds 
an  acre  it  has  in  most  cases  not  been  toxic.  On  acid 
soils  it  is  not  supposed  to  exercise  any  beneficial  action, 
and  on  very  productive  soils  Skinner  and  Sullivan,  in 
the  experiments  cited  above,  found  it  to  be  ineffective; 


1  Skinner,  J.  J.,  and  Sullivan,  M.  X.     The  Action  of   Man- 
ganese in  Soils.     U.  S.  D.  A.,  Bui.  42.     1914. 


COMMERCIAL  FERTILIZERS  533 

while  they  obtained  appreciable  benefit  from  its  use  on 
poor  soils.  They  argue  that  since  very  productive  soils 
have  great  oxidative  power  the  use  of  manganese  is  un- 
necessary, but  since  poor  soils  undergo  insufficient  oxida- 
tion the  stimulation  that  this  process  receives  by  the 
application  of  manganese  is  productive  of  much  good. 
Accordingly  manganese  is  most  profitably  used  on  poor 
soils  not  deficient  in  lime. 


CHAPTER   XXIV 
SOIL  AMENDMENTS 

Certain  substances  are  sometimes  added  to  soils  for 
the  purpose  of  increasing  productiveness  through  their 
influence  on  the  physical  structure  of  the  soil,  and  thereby 
on  the  chemical  and  bacteriological  properties.  These 
substances  are  called  soil  amendments.  It  is  true  that 
they  may  add  essential  plant  ingredients  to  the  soil,  but 
that  function  is  of  minor  importance. 

453.  Salts  of  calcium.  —  Calcium,  although  essential 
to  plant  growth,  seldom  needs  to  be  added  to  the  soil  to 
supply  the  plant  directly ;  but  because  of  its  effect  on  the 
soil  properties,  its  use  is  beneficial  to  a  great  number  of 
soils. 

454.  Effect  on  tilth  and  bacterial  action.  —  On  clay 
soils  the  effect  of  lime  is  to  bring  the  fine  particles  into 
aggregates  which  are  loosely  cemented  by  calcium  carbon- 
ate. The  effect  of  this  structure  on  tilth  has  already  been 
explained  (par.  120).  On  sandy  soils  the  carbonate  of 
calcium  serves  to  bind  some  of  the  particles  together, 
making  the  structure  somewhat  firmer  and  increasing  the 
water-holding  power.  It  should  be  used  only  in  small 
quantities  on  sandy  soils. 

There  is  a  tendency  for  most  cultivated  soils  to  become 

acid,  as  has  already  been  explained  (par.  283).     Acidity 

may  reach  a  point  where  it  becomes  directly  injurious  to 

certain  plants,  but  it  becomes  indirectly  injurious  before 

•      534 


SOIL  AMENDMENTS  535 

that  point  is  reached.  One  way  in  which  this  occurs  is 
by  curtailing  the  quantity  of  calcium  carbonate  in  the 
soil.  An  easily  available  base  to  combine  with  the 
organic  acids  affords  the  most  favorable  condition  for  the 
decomposition  processes  due  to  bacterial  action,  and 
hence  the  best  results  cannot  be  obtained  where  carbonate 
of  lime  is  not  present.  Its  action  in  improving  tilth  also 
facilitates  desirable  forms  of  bacteriological  activity  by 
increasing  the  permeability  of  the  soil  for  air. 

455.  Liberation  of  plant-food  materials.  —  It  has  been 
stated  that  the  alkalies  and  the  alkaline  earths  are  more 
or  less  interchangeable  in  certain  compounds  in  the  soil. 
The  addition  of  lime  may  in  this  way  liberate  potassium, 
when  otherwise  it  would  be  difficult  for  crops  to  obtain 
a  sufficient  supply  from  a  particular  soil.  The  substitu- 
tion of  bases  has  been  discussed  (par.  251)  and  the 
liberation  of  potassium  is  in  accord  with  these  phenomena. 
Magnesium,  although  rarely  deficient,  may  also  be  made 
available  in  this  way.  The  use  of  calcium  salts  may, 
under  some  soil  conditions,  render  phosphorus  more  use- 
ful, probably  by  supplying  a  base  more  soluble  than  iron 
or  alumina,  with  which,  in  soils  deficient  in  calcium,  the 
phosphorus  might  otherwise  be  combined.  Experiments 
by  Prianischnikov,1  in  which  plants  were  grown  in  washed 
sand  containing  Hellriegel's  nutrient  solution  to  which 
mono-,  di-,  and  tri-calcium  phosphate  respectively  were 
added,  both  with  and  without  calcium  carbonate,  showed 
a  decreased  availability  of  the  tricalcium  phosphate  due 
to  the  presence  of  the  carbonate,  but  neither  a  reduced 
nor  an  increased  availability  of  the  other  forms  of  phos- 

1  Prianischnikov,  D.  Ueber  den  Einfluss  von  Kohlensauren 
Kalk  auf  die  Wirkung  von  Verschiedenen  Phosphaten.  Landw. 
Vers.  Stat.,  Band  75,  Seite  357-376.     1911. 


536      SOILS:    PROPERTIES  AND  MANAGEMENT 

phorus  arising  from  the  presence  of  carbonate.  Neither 
did  the  availability  of  iron  or  aluminium  phosphate 
appear  to  be  influenced  by  calcium  carbonate. 

These  and  recent  experiments  by  Simmermacher  l  and 
others  tend  to  discredit  the  earlier  conclusions  as  quoted 
above  and  as  set  forth  by  Deherain  2  regarding  the  favor- 
able influence  of  lime  on  the  availability  of  phosphorus. 
However,  the  preponderating  evidence  is  still  with  the 
earlier  experimenters.  The  principles  that  underlie  the 
effect  of  lime  on  availability  of  phosphorus  are  discussed 
in  paragraphs  259  and  260. 

456.  Influence  of  lime  on  the  formation  of  nitrates  in 
soil.  —  It  has  already  been  remarked  that  nitrification 
proceeds  very  slowly  in  acid  soils.  A  soluble  base 
must  be  present  with  which  the  nitric  acid  may  com- 
bine, otherwise  the  process  will  be  inhibited  by  the  toxic 
effect  of  the  acid  on  the  bacteria  concerned  in  the  forma- 
tion of  the  acid.  The  addition  of  lime  is  the  most 
economical  method  of  providing  the  base.  This  is 
often  a  matter  of  great  moment  for  crops  that  respond 
readily  to  nitrate  nitrogen,  and  is  one  of  the  important 
reasons  for  applying  lime  to  sour  soils.  The  fact  that 
some  plants  grow  better  in  some  soils  than  in  strongly 
basic  ones  is  also  an  indication  that  such  plants  absorb 
a  considerable  part  of  their  nitrogen  in  forms  other  than 
nitrates. 

Many  investigators  have  found  that  the  presence  of 
calcium  carbonate  promotes  the  ammonifying  and  nitrify- 
ing process.     The  addition  of  calcium   carbonate  to  a 

1  Simmermacher,  W.  Einwirkung  der  Kohlensauren  Kalkes 
bei  der  Dungung  von  Haferkulturen  rait  Mono-  und  Dicalrium 
Phosphat.     Laridw.  Vers.  Stat.,  Band  77,  Seite  441-471.     1912. 

2  Deherain,  P.  P.     Traite  de  Chemie  Agricole,  p.  525.     1892. 


SOIL  AMENDMENTS  537 

sandy  loam  soil  was  found  by  Kellerman  and  Robinson  1 
to  favor  the  formation  of  nitrates  up  to  an  application 
of  2  per  cent,  which  is  much  more  than  would  ever  be 
applied  in  practice.  It  must  be  kept  in  mind,  however, 
that  this  limit  does  not  apply  to  all  soils,  as  the  absorp- 
tive properties  of  the  soil  for  lime  will  determine  the 
maximum  application  that  may  profitably  be  made. 
Kellerman  and  Robinson  found  also  that  the  application 
of  magnesium  carbonate  in  excess  of  0.25  per  cent  in- 
hibited the  formation  of  nitrates.  Kelly 2  also  has 
recently  reported  that  the  addition  of  magnesium  car- 
bonate to  the  soils  with  which  he  experimented  resulted 
in  a  marked  depression  of  both  ammonification  and  nitri- 
fication, and  that  the  addition  of  calcium  carbonate  did 
not  overcome  this  depressing  influence. 

457.  Effect  on  toxic  substances  and  plant  diseases.  — 
Free  acids  are  toxic  to  most  agricultural  plants.  Some 
plants  are  much  more  sensitive  than  others.  Alfalfa, 
for  example,  should  have  a  slightly  alkaline  medium  for 
its  best  growth,  and  any  acid  is  very  injurious.  Calcium 
salts,  in  neutralizing  acidity,  remove  this  toxic  condition. 
A  liberal  application  of  lime  is  therefore  a  precaution 
against  injury  of  this  kind. 

The  presence  of  soluble  calcium,  with  its  effects  on  the 
soil,  retards  the  development  of  certain  plant  diseases, 
such  as  the- "finger  and  toe"  disease  of  the  Gruciferse. 
On  the  other  hand,  it  may  promote  some  diseases,  as,  for 
example,  potato  scab. 

1  Kellerman,  K.  F.,  and  Robinson,  F.  R.  Lime  and  Legume 
Inoculation.     Science,  n.  s.,  Vol.  32,  pp.  159-160.     1910. 

2  Kelly,  W.  P.  The  Effect  of  Calcium  and  Magnesium  Car- 
bonates on  Some  Biological  Transformations  of  Nitrogen  in 
Soils.  Univ.  of  Calif.  Pub.,  Agr.  Sci.,  Vol.  1,  No.  3,  pp.  39-49. 
1912. 


538      SOILS:    PROPERTIES  AND  MANAGEMENT 

458.  The  lime-magnesia  ratio.  —  The  physiological 
balancing  of  magnesium  by  calcium  was  first  worked  out 
by  Loew,1  and  the  ratio  in  which  these  two  cations  should 
exist  in  nutrient  solutions  in  order  to  secure  the  best 
growth  of  certain  agricultural  plants  has  been  very  satis- 
factorily demonstrated  by  the  experiments  of  many 
investigators.  The  optimum  ratio  varies  with  different 
kinds  of  plants,  and  in  general  the  calcium  must  exceed 
the  magnesium  in  amount,  but  there  is  a  limit  beyond 
which  it  should  not  be  present.  If  calcium  alone  is 
present,  it  acts  as  a  toxic  agent  on  the  plant,  and  mag- 
nesium acts  in  a  similar  way.  It  is  only  when  the  ratio 
between  these  cations  falls  within  certain  limits  that 
they  exert  no  toxic  action.  This  ratio  varies  between 
one  part  of  calcium  oxide  to  one  part  of  magnesium 
oxide,  and  seven  parts  of  calcium  oxide  to  one  part  of 
magnesium  oxide. 

In  the  soil  the  relations  of  calcium  and  magnesium  to 
plant  growth  are  not  so  simple.  It  is  impossible  to 
determine  the  actual  or  the  relative  quantities  of  these 
cations  that  are  available  for  absorption  by  the  plant. 
This  is  mainly  because  of  the  absorptive  properties  of 
soils,  by  which  they  remove  the  bases  from  solution  and 
hold  them  in  a  somewhat  difficultly  soluble  form.  The 
ratio  of  calcium  to  magnesium  is  not  likely  to  disturb 
crop  yields  in  soils  unless  the  quantity  of-  magnesium 
present  happens  to  be  very  large.  Gile  and  Ageton 2 
have  found  ordinarily  fertile  soils  having  ratios  as  high 

1  Loew,  O.  The  Physiological  Role  of  the  Mineral  Nutrients 
of  Plants.     U.  S.  D.  A.,  Bur.  Plant  Indus.,  Bui.  1,  p.  53.     1901. 

2  Gile,  P.  P.,  and  Ageton,  C.  U.  The  Significance  of  the 
Lime-Magnesia  Ratio  in  Soil  Analyses.  Journ.  Indus,  and 
Eng.  Chem.,  Vol.  5,  pp.  33-35.     1913. 


SOIL   AMENDMENTS  539 

as  500  CaO  to  1  MgO  by  weight.  On  the  other  hand, 
excessive  applications  of  magnesium  compounds  have 
been  found  to  be  injurious  on  some  soils.  Even  on  a 
very  heavy  clay  soil,  at  Cornell  University,  an  applica- 
tion of  1333  pounds  to  the  acre  of  magnesite  markedly 
decreased  the  yields  of  sorghum  and  oats.  The  soil 
originally  contained  about  equal  parts  of  calcium  and 
magnesium. 

459.  Forms  of  calcium.  —  Calcium  is  used  on  the 
soil  in  the  form  of  calcium  oxide,  or  quicklime  (CaO), 
water-slaked  lime  (Ca(OH)2),  air-slaked  lime  (CaC03), 
ground  limestone,  marl  (also  a  carbonate),  and  calcium 
sulfate,  or  gypsum  (CaS04  .  2  H20).  The  application  of 
any  of  these  is  usually  called  liming  the  soil,  although 
gypsum  does  not  serve  exactly  the  same  purpose  as  do 
the  other  forms.  Owing  to  differences  in  the  molecular 
weights  of  these  compounds  of  calcium,  it  requires  more 
of  some  forms  than  of  others  to  furnish  the  same  amount 
of  calcium.  Approximately  equivalent  quantities  of  some 
of  the  common  forms  when  fairly  pure  are :  — 

Quicklime 56  pounds 

Water-slaked  lime 74  pounds 

Air-slaked  lime,  marl,  and  ground  limestone     100  pounds 

Quicklime,  and  the  hydrate,  when  added  to  the  soil,  even- 
tually assume  some  of  the  more  insoluble  forms  of  com- 
bination or  remain  as  the  carbonate,  never  being  present 
as  the  oxide.  It  is  always  desirable  to  have  present  in  the 
soil  at  least  a  small  amount  of  calcium  carbonate. 

460.  Caustic  limes.  —  Quicklime  and  water-slaked  lime 
have  a  markedly  alkaline  reaction,  and  hence  neutralize 
quickly  any  active  acidity  that  may  exist  in  the  soil. 
They  act  quickly  also  in  liberating  plant-food,  particularly 


540      SOILS:    PROPERTIES  AND  MANAGEMENT 

nitrogen.  Some  soils  respond  more  rapidly  to  quicklime 
or  water-slaked  lime  than  to  carbonate  of  lime,  especially 
when  the  carbonate  is  in  the  form  of  marl  or  ground  lime- 
stone, these  substances  never  being  in  such  a  finely  pul- 
verized condition  as  is  caustic  lime.  The  use  of  the 
caustic  forms  of  lime  has  been  said  to  result  in  the  loss 
of  nitrogen  by  the  too  rapid  decomposition  of  organic 
compounds. 

On  clays  the  granulating  effect  of  caustic  lime  is  more 
marked  than  that  of  the  carbonate,  and  for  this  reason 
the  former  has  a  distinct  advantage  for  use  on  heavy  clay. 
For  the  same  reason  an  occasional  moderate  dressing  is 
better  than  a  heavy  dressing  given  less  frequently. 

461.  Carbonate  of  lime.  —  Air-slaked  lime  has  the 
advantage  of  being  in  a  finely  divided  condition,  and 
does  not  produce  the  injurious  action  on  organic  matter 
that  is  sometimes  attributed  to  caustic  lime.  Its  effect 
on  the  granulation  of  clay  soils  is  probably  less  pro- 
nounced than  that  of  caustic  lime. 

Marl  (par.  27)  differs  from  air-slaked  lime  principally 
in  its  property  of  being  in  a  less  finely  pulverized  condi- 
tion. It  acts  less  quickly  than  does  caustic  lime.  Owing 
to  the  fact  that  marl  deposits  differ  greatly  in  the  com- 
position of  their  products,  it  is  well  to  know  the  quality 
of  the  material  before  buying  it.  The  carbonate  of  lime 
in  marl  may  vary  from  5  or  10  to  90  or  95  per  cent  in 
different  samples. 

Ground  limestone  has  been  used  extensively  in  recent 
years.  It  is  very  important  that  it  be  finely  ground,  as 
on  the  comminution  of  the  material  much  of  its  efficiency 
depends.  However,  it  is  doubtful  whether  there  is  any 
advantage  in  making  it  finer  than  is  required  to  pass 
through  a  sieve  with  50  meshes  to  the  inch. 


SOIL  AMENDMENTS  541 

462.  Relative  effectiveness  of  caustic  lime  and  car- 
bonate. —  In  order  to  test  the  value  of  ground  limestone 
and  other  forms  of  calcium  carbonate,  experiments  in 
which  it  was  compared  with  caustic  lime  have  been  con- 
ducted at  some  of  the  experiment  stations.  Reports  of 
tests  at  the  Pennsylvania  Experiment  Station,1  in  which 
plats  treated  with  slaked  lime  at  the  rate  of  two  tons  per 
acre  once  in  four  years  were  compared  with  plats  treated 
with  ground  limestone  at  the  rate  of  two  tons  to  the  acre 
every  two  years,  show  that  at  the  end  of  twenty  years, 
in  every  case,  the  total  yields  were  greater  on  the  plats 
receiving  ground  limestone.  After  the  treatment  on 
these  plats  had  been  continued  for  sixteen  years,  a  de- 
termination of  nitrogen  showed  the  upper  nine  inches  of 
soil  on  the  limestone-treated  plats  to  contain  2979  pounds 
of  nitrogen  to  the  acre,  and  the  slaked-lime  plats  to  con- 
tain 2604  pounds.  It  may  be  inferred  from  these  figures 
that  the  slaked  lime  caused  a  slightly  greater  destruction 
of  organic  matter  than  did  the  limestone. 

Patterson 2  also  conducted  experiments  for  eleven 
years  with  caustic  lime  produced  by  burning  both  stone 
and  shells,  and  the  carbonate  of  lime  in  ground  shells  and 
shell  marl.  The  average  crops  of  maize,  wheat,  and  hay 
were  all  larger  on  the  plats  treated  with  carbonate  of 
lime. 

While  these  experiments  show,  at  first  glance,  results 

1  Waters,  II.  J.,  and  Hess,  E.  H.  General  Fertilizer  Experi- 
ments. Pennsylvania  State  College,  Rept.  1894,  Part  2,  pp. 
258-281.  Also,  Hunt,  T.  F.  Soil  Fertility.  Pennsylvania 
Agr.  Exp.  Sta.,  Bui.  93.     1909. 

2  Patterson,  H.  J.  Lime,  Sources  and  Relation  to  Agri- 
culture. Maryland  Agr.  Exp.  Sta.,  Bui.  66,  pp.  127-130.  1900. 
Alsa,  Investigations  on  the  Liming  of  Soils,  Maryland  Agr.  Exp. 
Sta.,  Bui.  110,  pp.   16-21.     1906. 


542       SOILS:    PROPERTIES  AND   MANAGEMENT 

rather  favorable  to  the  use  of  carbonate  of  lime,  a  careful 
analysis  of  them  by  Wheeler1  raises  sonic  doubt  as  to 
the  legitimacy  of  this  interpretation.  He  points  out,  for 
instance,  that  in  the  Pennsylvania  experiments  excessive 
quantities  of  lime  were  used,  and  that  no  farm  manure 
nor  commercial  fertilizers  were  applied  to  the  plats  be- 
tween which  comparisons  were  made. 

There  is,  unfortunately,  a  paucity  of  definite  and  con- 
clusive data  that  may  be  applied  to  the  solution  of  the 
question  as  to  the  relative  values  of  these  different  forms 
of  lime  for  use  as  soil  amendments,  but  some  information 
has  accumulated  through  experience  and  practice  that 
may  be  taken  as  a  fairly  safe  guide  in  their  use.  It  is 
well  known,  for  instance,  that  burned  lime  has  a  more 
pronounced  effect  on  soil  granulation  than  has  the  car- 
bonate, and  may  therefore  be  expected  to  be  more  bene- 
ficial to  heavy  clay  soils.  On  the  other  hand,  burned 
lime  is  not  so  desirable  a  form  to  apply  to  very  sandy 
soils,  especially  when  they  are  likely  to  be  dry,  as  there 
is  danger  that  organic  matter  will  be  destroyed. 

463.  Sulfate  of  calcium.  —  Gypsum,  in  which  form 
calcium  sulfate  is  usually  applied  to  soils,  has  been  used 
for  many  years  and  was  a  popular  soil  amendment  in 
this  country  before  the  common  commercial  fertilizers 
were  used  to  any  great  extent.  It  frequently  went  by 
the  name  of  land  plaster,  and,  as  it  was  rather  widely 
distributed  in  nature  and  not  difficult  to  obtain,  it  was 
ground  and  largely  used  in  many  localities  throughout 
the  eastern  states.     Its  popularity  has  waned  in  recent 

1  Wheeler,  H.  J.  Is  the  Recommendation  that  Only  Ground 
Limestone  Should  be  Used  for  Agricultural  Purposes'  a  Sound 
and  Rational  One?  National  Lime  Manufacturers'  Assoc, 
Bui.  4.     1912. 


SOIL   AMENDMENTS  543 

years,  and  its  effectiveness  has  apparently  decreased  as 
the  soils  on  which  it  was  used  have  been  longer  under 
cultivation.  Possibly  this  is  due  to  the  tendency  of  these 
soils  to  become  more  acid,  which  has  caused  the  gypsum 
to  be  less  effective  in  liberating  potassium  —  a  property 
with  which  it  has  generally  been  credited.  At  present 
gypsum  is  not  very  generally  used  on  soils.  It  must  be  re- 
membered, however,  that  superphosphates  always  contain 
a  considerable  proportion  of  this  material,  and  it  may  add 
appreciably  to  the  beneficial  effects  of  that  fertilizer. 

Aside  from  its  action  in  liberating  potassium  (the  actual 
extent  of  which  has  never  been  very  clearly  demon- 
strated), gypsum  serves  to  supply  sulfur  to  the  soil.  The 
sulfur,  while  it  may  be  needed  in  some  soils,  has  the  dis- 
advantage of  being  present  as  an  acid ;  and  if  the  acid 
is  added  in  larger  quantity  than  is  removed  by  plants, 
there  is  a  resulting  loss  of  basic  material  in  the  drainage 
water  and  a  tendency  for  the  soil  to  become  sour. 

The  action  of  gypsum  in  improving  tilth  is  less  marked 
than  that  of  caustic  lime  or  of  the  carbonate.  As  a  source 
of  calcium  it  is  of  no  moment,  as,  if  applied  in  such  quan- 
tities as  those  in  which  the  other  forms  are  used,  the 
sulfate  would  be  very  injurious.  Ordinarily  it  is  applied 
at  the  rate  of  only  a  few  hundred  pounds  to  the  acre  at  the 
most.  On  the  whole,  gypsum  is  not  an  adequate  substi- 
tute for,  nor  so  desirable  a  form  of,  calcium  as  the  oxide, 
the  hydroxide,  or  the  carbonate. 

464.  Common  salt.  —  Sodium  chloride  has  a  marked 
effect  on  some  soils,  but  wherein  its  effectiveness  lies 
is  not  well  understood.  The  addition  of  sodium  and  of 
chlorine  as  plant  constituents  is  clearly  not  the  reason, 
as  these  substances  are  always  present  in  soils  in  avail- 
able form  far  in  excess  of  their  requirements. 


544       SOILS:    PROPERTIES  AND  MANAGEMENT 

The  effect  of  sodium  chloride  on  clay-bearing  soils 
is  to  liberate  certain  plant  nutrients,  among  which  arc 
calcium,  magnesium,  potassium,  and  phosphorus.  This 
action,  although  limited  in  amount,  is  probably,  in  some 
cases  at  least,  partly  responsible  for  the  beneficial  action 
of  common  salt. 

The  structure  of  the  soil  is  improved  by  the  applica- 
tion of  sodium  chloride,  just  as  it  is  by  lime,  although 
usually  not  to  the  same  extent. 

Another  effect  of  salt  is  to  conserve  and  distribute 
soil  moisture.  Its  conserving  action  is  probably  due  to 
an  increase  in  the  density  of  the  soil-water  solution,  thus 
retarding  transpiration.  The  film  movement  of  water  is 
likewise  increased  by  the  presence  of  salt  in  the  solution, 
and  in  this  way  the  upward  movement  of  bottom  water 
is  facilitated  and  the  supply  within  reach  of  the  roots 
maintained  in  time  of  drought. 

It  has  been  seen  that  sodium  is  not  one  of  the  substances 
essential  to  the  growth  of  plants.  But  that  sodium  may 
be  substituted,  in  part,  for  the  potassium  absorbed  by 
agricultural  plants  in  their  normal  growth,  has  been 
shown  in  this  country  by  the  experiments  of  Wheeler 
and  Adams;1  and  the  more  ready  availability  of  the 
sodium  applied  as  a  chloride  than  of  the  potassium  in 
its  natural  condition  in  some  soils  probably  accounts  in 
part  for  the  beneficial  effects  of  this  salt. 

It  is  not  all  soils,  however,  that  are  benefited  by  salt, 
its  usefulness  not  being  of  such  wide  application  as  that 
of  lime.  Certain  crops,  as  previously  mentioned,  are 
injured  by  the  presence  of  chlorine. 

1  Wheeler,  H.  J.,  and  Adams,  G.  E.  Concerning  the  Agri- 
cultural Value  of  Sodium  Salts.  Rhode  Island  Agr.  Exp.  Sta., 
Bui.  106.     1905. 


SOIL  AMENDMENTS  545 

465.  Muck.  —  The  effect  of  muck  (par.  72)  is  to 
change  the  structure  of  soils,  making  a  heavy  clay  soil 
lighter  and  more  porous,  and  binding  together  the  par- 
ticles of  a  sandy  soil.  Both  classes  of  soils,  but  particu- 
larly the  sandy  type,  have  a  greater  water-holding 
capacity  after  treatment  with  muck,  owing  to  its  great 
absorptive  power  which  amounts  to  70  per  cent  or  more 
of  its  own  weight.  It  is  to  its  content  of  organic  matter 
that  the  physical  effects  of  muck  are  due. 

Muck  contains  1  to  2  per  cent  of  organic  nitrogen, 
calculated  to  dry  matter,  which  does  not  readily  undergo 
ammonification.  The  addition  of  farm  manure  (which 
ferments  readily)  and  of  lime  serves  to  hasten  ammoni- 
fication. Its  use  as  an  absorbent  in  the  stable  fits  it  well 
for  use  on  the  land. 

Very  large  applications  of  muck,  are  necessary  when 
it  is  used  to  improve  the  structure  of  the  soil.  From 
ten  to  forty  or  fifty  tons  per  acre  are  frequently  applied. 

Muck  has  been  used  successfully  as  a  carrier  of  Bacillus 
radicicola;  for  this  it  is  eminently  adapted  by  its  absorbent 
qualities,  which  prevent  it  from  drying  out  and  thus  caus- 
ing injury  to  the  bacteria.  At  the  rate  of  thirty  pounds 
to  the  acre  it  has  served  as  a  highly  effective  medium  for 
inoculating  soil  for  alfalfa.1 

Muck  is  also  used  as  a  filler  in  certain  commercial 
fertilizers. 

1  Lyon,  T.  L.,  and  Bizzell,  J.  A.  Some  Experiments  in  Top- 
Dressing  Timothy  and  Alfalfa.  Cornell  Univ.  Agr.  Exp.  Sta., 
Bui.  339.     1913. 


2n 


CHAPTER  XXV 
FERTILIZER  PRACTICE 

The  purchase  and  use  of  commercial  fertilizers  in  an 
economical  way  requires  not  only  specific  technical 
knowledge  of  the  various  materials,  as  already  set  forth, 
but  also  a  certain  amount  of  general  knowledge  both 
practical  and  theoretical.  There  are  at  present  so  many 
fertilizing  materials  on  the  market  under  various  trade 
names,  that  the  question  as  to  the  best  one  to  buy  for  a 
certain  crop  growing  under  definite  soil  and  climatic 
conditions  becomes  a  difficult  one.  The  greater  the 
general  knowledge,  therefore,  that  a  person  possesses 
as  to  the  effects  of  the  different  elements  on  plant  growth, 
as  to  fertilizer  inspection  and  control,  as  to  methods  of 
buying,  as  to  home  mixing,  as  to  methods  and  time  of 
application,  and  as  to  mixtures  for  special  crops,  the 
better  he  is  able  to  utilize  fertilizers  that  will  result  in 
financial  gain.  That  a  fertilizer  shall  be  profitable  is 
the  ultimate  desideratum.  Moreover,  as  all  fertilizers 
exert,  either  directly  on  indirectly,  a  residual  effect,  the 
problem  necessarily  broadens  into  a  study  of  the  systems 
of  applying  fertilizers  to  a  series  of  crops  or  to  a  rotation, 
rather  than  a  study  of  the  effects  of  one  particular  ferti- 
lizer application  on  one  particular  crop. 

Note.  —  For  discussions  of  fertilizer  practice  see  Halligan,  J.  E., 
Soil  Fertility  and  Fertilizers,  Chapters  13-17.  Easton,  Penn- 
sylvania. 1912.  Also,  Van  Slyke,  L.  L.  Fertilizers  and  Crops, 
Chapters  21-25,  and  27-35.  New  York,  1912.  Also,  Fraps, 
G.  S.  Principles  of  Agricultural  Chemistry,  Chapter  16.  Eas- 
ton, Pennsylvania.     1913. 

546 


FERTILIZER   PRACTICE  547 

466.   Effects  of  nitrogen  on  plant  growth.1  —  Of  the 

three  primary  elements  of  a  fertilizer,  nitrogen 2  seems 
to  have  the  quickest  and  most  pronounced  effect,  not 
only  when  present  in  excess  of  the  other  constituents, 
but  also  when  moderately  used.  It  tends  primarily  to 
encourage  aboveground  vegetative  growth  and  to  impart 
to  the  leaves  a  deep  green  color,  a  lack  of  which  is  usually 
due  to  insufficient  nitrogen.  It  tends  in  cereals  to  in- 
crease the  plumpness  of  the  grain,  and  with  all  plants  it 
is  a  regulator  in  that  it  governs  to  a  certain  extent  the 
utilization  of  potash  and  phosphoric  acid.  Its  application 
tends  to  produce  succulence,  a  quality  particularly  de- 
sirable in  certain  crops.  In  its  general  effects  it  is  very 
similar  to  moisture,  especially  when  supplied  in  excessive 
quantities. 

The  peculiarity  of  nitrogen  lies  not  only  in  its  absolute 
necessity  for  plant  growth,  its  stimulation  of  the  vegeta- 
tive parts,  and  its  close  relationship  to  the  general  tone 
and  vigor  of  the  crop,  but  also  in  the  fact  that  it  was  not 
one  of  the  original  elements  of  the  earth's  crust.  During 
the  formation  of  the  soil  it  slowly  and  gradually  became 
present,  brought  down  by  rains  and  fixed  naturally  in  the 
soil  itself  mostly  through  the  agency  of  bacterial  action. 
Even  now  it  exists  largely  locked  up  in  complex  nitrog- 
enous compounds  of  the  humus  and  the  less  decayed 
organic  matter,  and  becomes  slowly  available  to  plants 

1  Discussions  of  the  effects  of  the  various  elements  on  plants 
may  be  found  as  follows :  Russell,  E.  J.  Soil  Conditions  and 
Plant  Growth,  Chapter  IT,  pp.  19-50.  London,  1912.  Also, 
Hall,  A.  D.  Fertilizers  and  Manures,  Chapters  III,  V,  and  VI. 
New  York,  1910. 

2  For  a  discussion  of  nitrogen  in  relation  to  crop  yield,  see 
Hunt,  T.  F.  The  Importance  of  Nitrogen  in  the  Growth  of 
Plants.     Cornell  Univ.  Agr.  Exp.  Sta.,  Bui.  247.     1907. 


548      SOILS:    PROPERTIES  AND  MANAGEMENT 

largely  through  bacterial  activity.  It  may  be  stated  with 
certainty  that  one  of  the  possible  limiting  factors  to 
crop  growth  is  a  lack  of  water-soluble  nitrogen  at  critical 
periods  in  amounts  necessary  for  normal  crop  develop- 
ment. Since  soluble  nitrogen  may  be  very  readily  lost 
from  the  soil  by  leaching,  the  problem  of  proper  plant 
nutrition  becomes  a  serious  one.  Not  only  must  the 
farmer  be  able  to  so  regulate  its  addition  in  fertilizers 
as  to  obtain  the  highest  efficiency,  but  he  must  understand 
the  control  and  encouragement  of  the  natural  fixation 
as  well.  The  emphasis  placed  on  all  phases  of  the  nitr<  >gen 
problem  serves  to  reveal  its  great  importance  in  fertility 
practices. 

Because  of  the  immediately  visible  effect  from  the  ap- 
plication of  soluble  nitrogen,  the  average  farmer  is  prone 
to  ascribe  too  much  importance  to  its  influence  in  proper 
crop  development.  This  attitude  is  unfortunate,  since 
nitrogen  is  the  highest-priced  constituent  of  ordinary 
fertilizers.  Moreover,  of  the  three  primary  elements 
it  is  the  only  one  which  added  in  excess  will  result  in 
harmful  after  effects  on  the  crop.  Its  general  influences, 
besides  its  functions  in  the  metabolic  and  synthetic 
processes  of  plant  development,  may  be  listed  briefly  as 
follows : 

1.  Nitrogen  tends  to  increase  the  growth  of  the  above- 

ground  parts. 

2.  It  delays  maturity  by  encouraging  vegetative  growth. 

This  oftentimes  endangers  the  crop  to  frost,  or 
may  cause  trees  to  winter  badly. 

3.  It  increases  the  ratio  of  straw  to  grain  in  cereals,  and 

the  ratio  of  leaves  to  underground  parts  in  root 
crops. 


FERTILIZER  PRACTICE  549 

4.  It  weakens  the  straw  and  causes  lodging  in  grain.     This 

is  due  to  an  extreme  lengthening  of  the  internodes, 
and  as  the  head  fills  the  stem  is  no  longer  able  to 
support  the  increased  weight. 

5.  It  lowers  quality.     This  is  especially  noticeable  in 

certain  grains  and  fruits,  as  barley  and  peaches. 
The  shipping  qualities  of  fruit  and  vegetables  are 
also  impaired. 

6.  It  increases  the  percentage  of  nitrogen  in  the  crop, 

particularly  in  the  straw  of  cereals  and  in  timothy 
hay. 

7.  It  decreases  resistance  to  disease.     This  is  probably  due 

to  a  change  in  the  physiological  resistance  to  disease 
within  the  plant,  and  also  to  a  thinning  of  the  cell 
wall,  allowing  a  more  ready  infection  from  without. 

While  certain  plants,  as  the  grasses,  lettuce,  radishes, 
and  the  like,  depend  for  their  usefulness  on  plenty  of 
nitrogen,  for  the  average  crop  it  is  generally  better  to 
limit  the  amount  of  nitrogen  so  that  growth  may  be 
normal.  This  results  in  a  better  utilization  of  the  nitro- 
gen and  in  a  marked  reduction  of  the  fertilizer  cost  for  a 
unit  of  crop  growth.  This  is  a  vital  factor  in  all  fertil- 
izer practice,  and  shows  immediately  whether  fertilization 
is  or  is  not  an  economic  success. 

467.  Effects  of  phosphorus  on  plant  growth.  —  It  is 
difficult  to  determine  exactly  the  functions  of  phosphoric 
acid  in  the  economy  of  even  the  simplest  plants.  Neither 
cell  division  nor  the  formation  of  fat  and  albumen  go  on 
to  a  sufficient  extent  without  it.  Starch  may  be  pro- 
duced when  it  is  lacking,  but  will  not  change  to  sugar. 
As  grain  does  not  form  without  its  presence,  it  very 
probably  is  concerned  in  the  production  of  nucleoproteid 


550       SOILS:    PROPERTIES  AND  MANAGEMENT 

materials.  Its  close  relationship  to  cell  division  may 
account  for  its  presence  in  seeds.  Its  general  effects  on 
plant  growth  may  be  listed  as  follows : 

1.  Phosphorus  hastens  maturity  by  its  effect  on  rate  of 

ripening.  This  makes  phosphorus  especially  valu- 
able in  wet  years,  and  in  cold  climates  where  the 
season  is  short. 

2.  It  increases  root  development,  especially  of  lateral  and 

fibrous  rootlets.  This  renders  it  valuable  with 
such  soils  as  do  not  encourage  root  extension  and 
to  such  crops  as  naturally  have  a  restricted  root 
development.  Phosphorus  is  therefore  valuable  in 
fall-sown  crops,  in  years  of  drought,  and  for  farm- 
ing on  arid  land. 

3.  It  decreases  the  ratio  of  straw  to  grain  by  hastening  the 

filling  of  the  grain  and  by  promoting  maturity. 

4.  It  strengthens  the  straw,  due  to  its  balancing  effect  on 

the  nitrogen. 

5.  It  improves  the  quality  of  the  crop.     This  has  been 

recognized  in  the  handling  of  pastures  in  England 
and  France.  The  effect  on  vegetables  is  also 
marked. 

6.  It  increases  percentage  of  phosphorus  in  the  crop. 

With  cereals  this  is  particularly  noticeable  in  the 
straw. 

7.  It  increases  resistance  to  disease,   due  probably  to 

more  normal  cell  development. 

Excessive  phosphorus  ordinarily  has  no  bad  effect, 
as  it  does  not  stimulate  any  part  excessively  as  does  ni- 
trogen, nor  does  it  lead  to  a  development  which  is  detri- 
mental.    Its  lack  is  not  quickly  apparent,  as  in  the  case 


FERTILIZER   PRACTICE  551 

of  nitrogen,  and  as  a  consequence  phosphorus  starvation 
may  occur  without  any  suspicion  thereof  being  enter- 
tained by  the  farmer. 

One  of  the  most  important  phases  to  be  noted  from  this 
comparison  of  the  effects  of  nitrogen  and  phosphorus  is 
the  balancing  powers  of  the  latter  on  the  unfavorable  in- 
fluences generated  by  the  presence  of  an  undue  quantity 
of  the  former.  This  is  a  vital  factor  in  fertilizer  practice, 
since  normal  fertilizer  stimulation  always  results  in  the 
most  economic  gains.  Such  a  normal  increase  is  obtained 
only  when  the  plant  functions  of  the  several  fertilizer 
constituents  are  in  proper  accord. 

468.  Effects  of  potassium  on  plant  growth.  —  The 
effects  of  potash  are  more  localized  than  those  of  nitrogen 
and  phosphorus.  Potash  is  essential  to  starch  formation, 
either  in  photosynthesis  or  in  translocation,  and  is  a 
necessary  component  of  chlorophyll.  It  is  important 
in  grain  formation,  giving  plump,  heavy  kernels.  In 
general  it  tends  to  impart  tone  and  vigor  to  a  plant.  In 
increasing  resistance  to  disease  it  tends  to  counteract 
the  ill  effects  of  too  much  nitrogen,  while  in  delaying 
maturity  it  works  against  the  ripening  influences  of 
phosphoric  acid.  In  a  general  way  it  exerts  a  balancing 
effect  on  both  nitrogen  and  phosphate  fertilizer  materials, 
and  consequently  is  necessary  in  a  mixed  fertilizer,  es- 
pecially if  the  potash  of  the  soil  is  lacking  or  unavail- 
able. As  with  phosphorus,  it  may  be  present  in  large 
quantities  in  the  soil  and  yet  exert  no  harmful  effect  on 
the  crop. 

469.  Law  of  the  minimum.  —  In  connection  with  the 
obvious  importance  of  utilizing,  for  any  particular  soil 
and  crop,  a  fertilizer  well  balanced  as  to  the  three  primary 
elements,     two     queries     naturally     arise.     These     are: 


552       SOILS:    PROPERTIES  AND  MANAGEMENT 

(1)  What  are  the  right  proportions  of  nitrogen,  phos- 
phorus,  and   potash  to  apply   under  given   conditions  ? 

(2)  What  would  be  the  effect  if  any  one  of  these  should 
not  be  present  in  such  a  quantity  as  to  make  it  equal  in 
function  to  the  others?  The  first  query  cannot  be  dis- 
posed of  until  the  question  of  fertilizer  mixtures  has  been 
considered.  The  second,  however,  is  not  affected  by  so 
many  factors,  and  is  more  clearly  a  question  of  the  func- 
tion of  the  elements  concerned. 

Any  element  that  exists  in  relatively  small  amounts  as 
compared  with  the  other  important  constituents  natu- 
rally becomes  the  controlling  factor  in  crop  development. 
Any  reduction  or  increase  in  this  element  will  cause  a 
corresponding  reduction  or  increase  in  the  crop  yield. 
This  element,  then,  is  said  to  be  "  in  the  minimum." 
In  fertilizer  practice,  ideal  conditions  would  exist  if  no 
constituent  functioned  as  a  decided  minimum  and  the 
entire  influence  of  each  single  element  were  fully  utilized. 
In  other  words,  the  fertilizer  would  be  balanced  as  to  its 
relationship  to  normal  plant  growth.  That  such  a  con- 
dition is  more  or  less  ideal  and  theoretical  is  obvious, 
from  the  fact  that  the  various  fertilizer  carriers  undergo 
more  or  less  radical  changes  after  being  applied  to  the 
soil.  The  composition  of  the  soil  itself  is  also  a  disturb- 
ing factor.  Nevertheless,  the  nearer  an  approach  can  be 
made  to  such  conditions,  the  greater  will  be  the  economy 
of  fertilizer  practice. 

Numerous  persons  have  investigated  the  question  as 
to  what  effect  an  increase  of  an  element  in  the  minimum 
may  have  on  crop  yield,  and  various  ideas  have  been 
advanced  thereon.  The  idea  of  a  definite  law  governing 
the  increase  of  plant  growth  according  as  the  element 
in  the  minimum   is   increased,   was  first   suggested   by 


FERTILIZER  PRACTICE  553 

Liebig.  Wagner 1  later  stated  definitely  that  up  to  a 
certain  point  the  increase  yield  was  proportional  to  the 
increase  in  the  application.  This,  however,  evidently 
cannot  apply  except  over  a  very  limited  field,  since  it  is 
a  matter  of  common  observation  that  increased  crop 
yield  becomes  lower  as  the  lacking  element  is  supplied. 
Recently  Mitscherlich  2  has  formulated  a  law 3  which  is  a 
logarithmic,  rather  than  a  direct,  function  of  the  increase 
in  the  element  occupying  the  position  of  the  minimum. 
Mitscherlich's  law  may  be  stated  concisely  as  follows: 
the  increased  growth  produced  by  a  unit  increase  of  the 
element  in  the  minimum  is  proportional  to  the  decre- 
ment from  the  maximum.  The  following  curve  (see 
Fig.  62)  constructed  from  data  obtained  by  Mitscherlich,4 
shows  the  trend  of  the  increased  growth  curve  as  governed 
by  increased  applications  of  an  element  in  the  minimum, 
other  factors  being,  of  course,  under  control.  This  curve 
is  maintained  by  Mitscherlich  to  approximate  a  theoretical 
curve  of  a  definite  mathematical  formula. 


1  Wagner,  H.  Beitrage  zur  Dungerlehre.  Landw.  Jahr., 
Band  12,  Seite  691  ft7.     1883. 

2  Mitscherlich,  A.  E.  Das  Gesetz  des  Minimums  und  das 
Gesetz  des  Abnehmen  den  Bodenertrages.  Landw.  Jahr., 
Band  38,  Seite  537-552.     1909. 

Also,  Ein  Beitrage  znr  Erforschung  der  Ausnutzung  des 
im  Minimum  Vorhandenen  Nahrstoffes  durch  die  Pflanze. 
Landw.  Jahr.,  Band  39,  Seite  133-156.     1910. 

3     fly 

—  =  (a  —  y)k.     Integrating,  log  (a  —  y)=  c  —  kx. 
'    dx 

y  =  total  yield  from  any  number  of  increments. 

x  =  amount  of  any  particular  fertilizer  constituent  utilized. 

a  =  maximum  yield  and  is  a  constant. 

k  =  a  constant  depending  on  y  and  x,  variables. 
4  Mitscherlich,  A.  E.     Uber  das  Gesetz  des  Minimums  und 
die  sich  aus  diesem  Ergebenden  Schlussfolgerungen.     Landw. 
Ver.  Stat.,  Band  75,  Seite  231-263.     1911. 


554      SOILS:    PROPERTIES  AND  MANAGEMENT 


60 

• 

SO 

OyT 

1 

5 

o 

44          J 

1 

301 

1° 

/O 

0 

/ 

0 

2-0 

GRAMS 


P£f2     POT 


Fig.  62. — Curve  showing  the  increased  growth  of  oats  under  the  in- 
fluence of  constantly  increasing  amounts  of  phosphorus,  that  ele- 
ment being  in  the  minimum. 

The  formula  as  proposed  by  Mitscherlich  has  been 
questioned  by  several  investigators,1  who  have  shown 
that  a  number  of  conditions,   such  as  light,   heat,   and 

1 Pfeiffer,  Th.,  Rlanek,  E.,  and  Flugel,  M.  Wasserund  Licht 
als  Vegetationsfaktoren  und  ihre  Beziehungen  zum  Gesetze  vom 
Minimum.     Landw.  Ver.  Stat.,  Band  76,  Seite  211-223.     1912. 

Also,  Maze,  P.     Recherches  sur  les  Relations  de  la  Plante 


FERTILIZER   PRACTICE  555 

moisture,  tend  to  disturb  the  application  of  such  a  law. 
The  fact  that  crop  yield  is  the  summation  of  so  many 
varying  factors  seems  to  argue  in  favor  of  no  hard  and 
fast  rule  regarding  the  increased  growth  due  to  the  added 
increments  of  an  element  in  the  minimum.  It  is  enough, 
in  the  practical  utilization  of  fertilizers,  to  remember  that 
this  curve  in  general  approximates  the  one  already  cited, 
and  that  in  order  to  obtain  the  best  results  from  a  com- 
plete fertilizer  a  mixture  should  be  used  that  is  approxi- 
mately balanced  so  far  as  the  effects  of  the  elements  are 
concerned,  the  crop  as  well  as  the  chemical  constitution 
of  the  soil  being  considered. 

470.  Fertilizer  brands.  —  In  an  attempt  to  meet  the 
demands  for  well-balanced  fertilizers  suited  to  various 
crops  and  soils,  manufacturers  have  placed  on  the  market 
numberless  brands  of  materials  containing  usually  at 
least  two  of  the  important  elements,  and  nearly  always 
the  three;  the  former  being  designated  as  incomplete 
fertilizers,  while  the  latter  are  spoken  of  as  complete 
fertilizers.  These  various  brands  usually  have  some 
catchy  name,  such  as  "  The  Ureka  Corn  Special/ '  "  Far- 
mers' Potato  and  Corn  Fertilizer,"  "  The  Golden  Har- 
vest," or  "  The  Empire  State  Sure  Crop  Phosphate." 
Such  a  name  frequently  implies  the  usefulness  of  the 
material  for  some  particular  crop,  but  oftener  it  has  no 
relation  either  to  crop  or  to  soil.  Ordinarily  the  name 
should  be  ignored  in  the  purchase  of  fertilizers. 

A  brand  of  fertilizer  is  usually  made  up  of  a  number 
of  materials  containing  the  important  ingredients.  These 
materials,  already  described,  are  called  carriers.  The 
making-up  of  a  commercial  fertilizer  consists,  then,  in 

avec  les  Elements  Nutritifs  du  Sol.  Compt.  Rend.,  Vol.  154, 
pp.  1711-1714.     1912. 


556       SOILS:    PROPERTIES  AND  MANAGEMENT 

merely  mixing  the  various  carriers  together  so  that  the 
required  percentages  of  nitrogen,  potash,  and  phosphoric 
acid  are  obtained,  care  being  taken  that  no  detrimental 
reaction  shall  occur  and  that  a  physical  condition  con- 
sistent with  easy  distribution  shall  be  maintained.  If 
the  substances  used  are  difficultly  soluble,  the  fertilizer 
is  not  so  valuable  as  one  composed  of  easily  soluble  con- 
stituents. The  general  solubility  of  the  various  in- 
gredients should  be  known  by  a  prospective  purchaser. 

The  various  brands  on  the  market,  besides  being 
complete  or  incomplete,  may  be  designated  as  high-grade 
or  low-grade.  These  terms  may  be  used  in  two  ways  — 
high-grade  or  low-grade  as  to  availability,  or  high-grade 
or  low-grade  as  to  amount  of  plant-food  constituents 
carried.  A  low-grade  fertilizer  in  the  percentages  of 
nitrogen,  phosphoric  acid,  and  potash  is  always  encum- 
bered with  a  large  amount  of  inert  material,  which  adds 
to  the  cost  of  mixing,  transportation,  and  handling.  It 
is  thus  usually  a  more  expensive  fertilizer  to  a  unit  of 
plant-food  obtained  than  one  of  higher  grade.  Except 
for  special  purposes,  a  low-grade  fertilizer  as  to  avail- 
ability should  be  bought  sparingly  or  not  at  all. 

471.  Fertilizer  inspection  and  control.— With  the 
many  different  materials  available  for  mixing  commercial 
fertilizers,  and  from  the  fact  that  so  many  opportunities 
are  open  for  fraud  either  as  to  availability  or  as  to  guaran- 
tee, laws  have  been  found  necessary  for  controlling  the 
sale  of  fertilizers.  Most  states  have  such  a  law,  the 
western  laws  generally  being  superior  to  those  in  force 
in  eastern  states,  where  the  fertilizer  sale  is  heavier. 
This  is  because  the  western  regulations  are  more  recent 
and  the  legislators  have  had  the  advantage  of  the  ex- 
perience gained  where  fertilizers  have  long  been  used. 


FERTILIZER  PRACTICE  551 

Moreover,  the  legislators  in  such  states  have  not  been  so 
strongly  confronted  with  fertilizer  lobbying,  and  have 
consequently  been  free  to  enact  stricter  laws  than  were 
possible  where  fertilizers  are  such  an  important  com- 
mercial commodity. 

Usually  certain  provisions  are  common  to  all  fertilizer 
laws.  In  general,  all  fertilizers  selling  for  a  certain  price 
or  over  (usually  $5  a  ton)  must  pay  a  state  license  fee  and 
print  the  following'data  on  the  bag  or  an  authorized  tag  :  — 

1.  Number  of  net  pounds  of  fertilizer  to  a  package. 

2.  Name,  brand,  or  trade-mark. 

3.  Name  and  address  of  manufacturer. 

4.  Chemical  composition  or  guarantee. 

The  composition  of  a  commercial  fertilizer  is  ordinarily 
expressed  simply;  for  example,  as  a  3-6-10,  meaning  3 
per  cent  of  nitrogen,  6  per  cent  of  phosphoric  acid,  and 
10  per  cent  of  potash.  This,  however,  is  too  brief  for  a 
guaranteed  analysis  on  goods  exposed  for  sale,  as  it  gives 
no  idea  whatsoever  regarding  the  solubilfty  of  the  ma- 
terials. As  might  be  expected,  there  is  a  wide  range  in 
the  character  of  the  guarantee  required  by  the  various 
states.  For  example,  some  states  insist  on  the  statement 
of  the  percentage  of  both  nitrogen  and  ammonia,  while 
others  insist  only  on  the  percentage  of  nitrogen.  Some 
require  the  soluble,  the  reverted,  and  the  total  phosphoric 
acid,  while  others  require  only  the  soluble  and  the  re- 
verted. As  to  potash,  in  some  cases  the  soluble  must  be 
stated,  while  in  other  cases  the  total  must  be  given.  In 
general,  a  guarantee  should  show  not  only  the  amount 
of  the  various  constituents,  but  also  their  form  or  avail- 
ability. The  guarantee  required  by  North  Dakota  is 
excellent  in  this  respect :  — 


558       SOILS:    PROPERTIES  AND   MANAGEMENT 

Guarantee  required  by  the  State  of  North  Dakota 

Percentage  of  N  in  nitrates        Percentage  of  P206  soluble 
Percentage  of  N  as  ammonia  in  water 

Percentage  of  N  total  Percentage     of     P205    re- 

verted 
Percentage     of     P2O5   in- 
Percentage  of  K20  soluble  soluble 

Percentage  of  K20  as  chloride     Percentage  of  P2()6  total 

Since  a  fertilizer  law  is  designed  primarily  to  protect 
not  only  the  purchasers  but  also  the  manufacturers,  a 
certain  amount  of  variation  is  allowed  below  a  guarantee. 
This  is  a  matter  of  extreme  variation  in  the  different  states. 
Ordinarily,  also,  the  offering  for  sale  of  any  leather  matter 
or  its  products,  either  separately  or  in  mixtures,  is  pro- 
hibited, unless  so  stated  specifically  on  the  package. 

For  the  enforcement  of  such  laws,  the  states  usually 
provide  adequate  machinery.  The  inspection  and  analyses 
may  be  in  the  hands  of  the  state  department  of  agricul- 
ture, of  the  director  of  the  state  agricultural  experiment 
station,  of  a  state  chemist,  or  under  the  control  of  any 
two  of  these.  In  any  case,  a  corps  of  inspectors  is  pro- 
vided, the  members  of  which  take  samples  of  the  fertilizers 
on  the  market  throughout  the  state.  These  samples  are 
analyzed  in  laboratories  provided  for  the  purpose,  in 
order  to  ascertain  whether  the  mixture  is  up  to  its  guar- 
antee. If  the  fertilizer  falls  below  the  guarantee,  —  allow- 
ing, of  course,  for  the  variation  permitted  by  law,  —  the 
manufacturer  is  subject  to  prosecution. 

A  more  effective  check  on  fraudulent  guarantees,  how- 
ever, is  found  in  publicity.  The  state  law  usually  pro- 
vides for  the  publication  each  year  of  the  guaranteed  and 
found  analyses  of  all  brands  inspected.     Not  only  has 


FERTILIZER   PRACTICE  559 

this  proved  effective  in  preventing  fraud,  but  it  is  really 
a  great  advantage  to  the  honest  manufacturer. 

The  expenses  for  the  inspection  and  control  of  fertilizers 
are  usually  defrayed  by  the  license  fees,  which  average 
for  the  different  states  from  ten  to  twenty  dollars  a  year 
for  each  brand  selling  for  $5  or  more  a  ton.  In  the 
eastern  states  this  fee  produces  a  net  return  greatly  in 
excess  of  the  expenses  incurred  by  the  fertilizer  inspection 
and  control,  and  consequently  has  become  the  source  of 
a  handsome  income  for  these  states. 

472.  Trade  values  of  fertilizers.  — It  has  become  cus- 
tomary for  the  authorities  charged  with  fertilizer  inspec- 
tion and  control  in  the  various  states  to  adopt  each  year 
a  schedule  of  the  trade  values  of  the  various  elements  as 
they  appear  on  the  market  in  unmixed  lots.  These 
values  are  obtained  by  averaging  all  the  wholesale  prices 
of  a  ton  for  the  various  unmixed  supplies  for  the  six 
months  preceding  March  1,  to  which  is  added  20  per  cent 
of  the  price  to  cover  cost  of  handling.  The  trade  values 
for  1912  were  as  follows : l  — 

Trade  Values  of  Plant-food  Elements  in  Raw  Ma- 
terials and  Chemicals 

Cents  a  pound 

Nitrogen  in  ammonia  salts         18^ 

Nitrogen  in  nitrates  .     .     .     .     .     .     .     .     .     .     18^ 

Organic  nitrogen  in  dry  and  fine  fish,  meat,  and  blood     20 
Organic  nitrogen  in  fine  bone,  tankage,  and  mixed 

fertilizer        19 

Organic  nitrogen  in  coarse  bone  and  tankage         .     .     15 
Organic  nitrogen  in  castor  pomace  and  cottonseed 
meal 20 

1  New  York  (Geneva),  Agr.  Exp.  Sta.,  Bui.  371,  p.  434.     1913. 


560      SOILS:    PROPERTIES  AND  MANAGEMENT 

Cents  a  pound 

Phosphoric  acid,  water-soluble 4^ 

Phosphoric  acid,  citrate-soluble  (reverted)        ...  4 

Phosphoric  acid,  in  fine  bone,  fish,  and  tankage     .  4 
Phosphoric   acid,    in   cottonseed    meal    and    castor 

pomace         4 

Phosphoric  acid,  in  coarse  fish,  bone,  tankage,  and 

ashes 3§ 

Phosphoric  acid  in  mixed   fertilizers,   insoluble   in 

water  or  ammonium  citrate  2 

Potash  as  high-grade  sulfate,  in  forms  free  from  chlo- 
rides, in  ashes,  etc.         o\ 

Potash  as  muriate        4j 

Potash  as  castor  pomace  and  cottonseed  meal      .     .  5 

It  must  be  remembered  that  these  prices  are  seaboard 
evaluations,  and  represent  the  cost  to  the  manufacturer 
of  the  elements  as  they  exist  in  the  unmixed  carriers. 
This  is  called  the  commercial  evaluation  of  a  fertilizer, 
and  is  the  first  of  a  number  of  items  that  enter  into  the 
total  cost,  or  the  price  the  farmer  must  pay  on  the  retail 
market.  The  items  that  make  up  this  ultimate  price 
may  be  listed  as  follows :  (1)  wholesale  cash  cost,  or  com- 
mercial evaluation;  (2)  cost  of  mixing;  (3)  profit  of 
manufacturers;  (4)  transportation;  (5)  storage,  com- 
mission to  agents,  bad  debts,  and  so  forth ;  and  (6)  profit 
of  retailer.  These  additional  charges  are  often  sufficient 
to  double  the  original  commercial  value  of  the  fertilize 
constituents. 

It  is  evident  that  by  knowing  the  composition  of  a  fer- 
tilizer, and  the  carriers  of  the  various  constituents,  the 
commercial  evaluation  of  the  mixture  may  be  easily  cal- 
culated.    However,  what  the  farmer  must  pay  depends 


FERTILIZER  PRACTICE  561 

to  a  large  extent  on  the  additional  charges  already  listed. 
Thus,  a  fertilizer  evaluated  at  $22  a  ton  on  the  New  York 
market  may  cost  the  farmer  $35,  or  even  $45,  after  having 
passed  through  the  hands  of  the  manufacturer  and  the 
retail  merchant.  This  commercial  evaluation,  however, 
must  not  be  confused  with  the  agricultural  evaluation, 
which  is  the  value  of  the  increased  crop  produced  by  the 
application  of  the  fertilizer.  It  is  evident  that  the  agri- 
cultural value  will  vary  with  the  soil,  the  crop,  or  the 
season,  and  may  be  above  or  below  the  total  cost  accord- 
ing to  circumstances.  In  good  fertilizer  practice,  the 
excess  of  the  agricultural  value  over  the  total  cost  of  the 
fertilizer,  all  costs  incidental  with  the  growing,  harvest- 
ing, and  marketing  of  the  increase  being  first  deducted, 
should  be  sufficient  to  give  a  handsome  profit  on  the 
investment. 

473.  The  buying  of  mixed  goods.  —  The  successful 
buying  of  mixed  fertilizers  on  the  retail  market  depends 
on  two  things :  (1)  the  selection  of  a  suitable  composi- 
tion, with  carriers  of  known  value ;  and  (2)  the  purchase 
of  high-grade  goods.  The  farmer  who  observes  these 
two  points  will  have  at  least  purchased  successfully. 
Whether  he  obtains  a  profit  from  the  use  of  the  fertilizer 
depends  on  the  balancing  of  a  number  of  factors  more  or 
less  variable  from  season  to  season. 

The  selection  of  a  suitable  fertilizer,  as  to  carriers  and 
composition  for  any  particular  crop  or  soil,  entails  first 
of  all  a  study  of  the  guarantee.  Should  the  guarantee 
be  such  as  that  already  cited,  a  large  amount  of  informa- 
tion is  at  hand  concerning  the  forms  of  the  carriers  and 
the  availability  of  the  important  constituents.  This 
knowledge,  properly  correlated  with  the  probable  needs 
of  the  crop  and  the  soil,  will  determine  whether  that 
2o 


562      SOILS:    PROPERTIES  AND  MANAGEMENT 

particular  brand  should  be  purchased  or  not.  The  real 
question  here  is  not  the  actual  quantities  of  the  elements 
in  a  ton  of  the  fertilizer,  but  their  balance  among  them- 
selves. The  actual  pounds  of  nitrogen,  phosphoric  acid, 
or  potash  applied  per  acre  can  be  governed  by  the  rate 
at  which  the  mixture  is  applied. 

The  purchase  of  high-grade  goods  is  the  second  impor- 
tant point  to  be  considered.  Data  collected  from  practi- 
cally every  state  show  that  the  higher  the  grade  of  the 
fertilizer,  both  as  to  availability  and  as  to  the  percentage 
of  the  constituents  carried,  the  greater  is  the  amount  of 
plant-food  obtained  for  every  dollar  expended.  The 
following  data,  taken  from  Vermont l  for  1909,  are  the 
average  of  one  hundred  and  thirty  brands  and  are  typical 
data  in  this  regard  :  — 


g 

«  « 

«  X 

£  w  w 

■A  « 

kt 

M  B  7. 
O  J  « 

m 

■ 
■  2 

o& 

gp 

Cost  (in  Cents) 

g"g 

Bfi 

*i 

P  w  &, 

of  One  Pound  ok 

«n£ 

Fertilizer 

5  z 

Bfcg 

B> 

SfeS 

fe££ 

§5 

S  < 
O  > 

*  JW 

ow 

^s* 

-  -  - 

a  <  S 

iJ 

fc°g 

°  w  -. 
a  «  < 

N 

P»Oft 

KjO 

ts  U  J 

«  ^2 

ow 

CQOH 

ws 

o£5 

>tfQ 

Low  grade 

$13.52 

$27.10 

$13.58 

$1.00 

.38    7.6 

8.5 

(cents) 

50.0 

Medium 

grade     . 

18.22 

30.00 

11.78 

0.65 

.31 

6.3 

7.0 

60.6 

High  grade 

26.30 

38.93 

12.63 

0.48 

.28 

5.7 

6.3 

67.6 

It  is  noticeable  at  once  that  the  lower  the  grade  of  the 
fertilizer,  the  higher  is  the  proportional  cost  of  placing 
the  goods  on  the  market.     In  other  words,  it  costs  just 

1  Hills,  J.  L.,  Jones,  C.  H.,  and  Miner,  H.  L.  Commercial 
Fertilizers.     Vermont  Agr.  Col.,  Bui.  143,  pp.  147-149.     1909. 


FERTILIZER  PRACTICE  563 

as  much  per  ton  to  market  a  low-grade  material  as  a 
high-grade  one.  This  accounts  for  the  fact  that  the  ele- 
ments are  cheaper  per  pound  in  a  high-grade  mixture, 
and  that  the  value  of  plant-food  received  for  every  dollar 
expended  is  greater. 

474.  Home-mixing  fertilizers.  —  In  comparing  the  above 
commercial  evaluations  with  the  prices  actually  paid 
by  the  farmer  on  the  retail  market,  it  is  found  that  the 
latter  shows  an  increase  ranging  from  48  to  100  per  cent. 
This  is  due  to  the  charges  for  mixing,  transportation,  han- 
dling, storage,  commission,  interest  on  capital,  profit, 
and  other  items,  made  during  the  passage  of  the  material 
from  the  wholesale  dealer  to  the  user.  In  order  to  escape 
these  costs,  many  farmers  have  begun  the  practice  of 
buying  the  separate  carriers,  thus  avoiding  these  charges 
—  except,  of  course,  that  of  transportation.  In  many 
cases  the  mixing  on  the  farm  costs  nothing,  as  it  can  be 
done  in  winter  when  the  farm  work  is  not  pressing.  Even 
if  the  farmer  must  charge  himself  with  this  mixing,  it 
seldom  amounts  to  more  than  fifty  cents  a  ton. 

As  might  be  expected,  this  practice  has  met  with  much 
opposition  from  manufacturers.  In  general  it  is  claimed 
that  the  factory  goods  are  more  finely  ground  than  those 
mixed  by  the  farmer,  and  consequently  the  ready-mixed 
goods  are  not  only  more  uniform  but  also  in  better  physi- 
cal condition.  Also,  the  manufacturer  is  able  to  treat 
certain  materials  with  acids,  and  thus  increase  their 
availability.  While  these  reasons  are  more  or  less  valid, 
good  results  may  be  expected  from  a  fertilizer  even  though 
it  may  not  be  quite  uniform,  as  the  soil  tends  to  equalize 
this  deficiency.  Moreover,  by  screening  and  by  using 
a  proper  filler,  a  farmer  can  obtain  a  physical  condition 
which  will  in  no  way  interfere  with  the  drilling  of  the  ma- 


564      80ILS:    PROPERTIES  AND  MANAGEMENT 

terial.  While,  obviously,  one  farmer  alone  cannot  ail'ord 
to  buy  direct  from  the  wholesale  dealer  because  of  the 
high  freight  charges  on  small  lots,  this  objection  is  being 
met  by  clubs  and  various  organizations  whereby  the 
single  carriers  may  be  bought  in  carload  lots. 

It  is  evident  that  when  a  farmer  mixes  his  own  fertilizer 
he  is  able  to  obtain  not  only  pure  goods,  but  high-grade 
goods  as  well,  thus  reducing  freight.  Moreover,  as  a  gen- 
eral thing  home  mixing  is  cheaper  than  buying  the  ready- 
mixed  goods.  A  quotation  from  Connecticut1  for  1 906 
illustrates  about  what  this  saving  may  be :  — 

Plant-Pood  Purchased  for  $30 


Nitrogenous  superphosphates 

Best  quality       .... 

Least  valuable  .  .  . 
Special  manures 

Best  quality       .... 

Lowest  quality  .  .  . 
Home  mixtures 

Average  of  all    .     .     .     . 


Pounds 

N 


73 
23 

69 
32 

77 


Pounds 
Pj04 


188 
279 

170 
174 

200 


Pounds 
KiO 


111 

53 

143 
66 

168 


Total 


372 

382 
272 

445 


A  third  point,  and  by  some  considered  to  be  more  im- 
portant than  those  already  discussed,  is  the  educational 
value  of  home  mixing.  No  farmer  can  mix  his  own  fertil- 
izer without  becoming  familiar  with  the  carriers,  their 
availability,  and  their  effects.  He  is  forced  to  study  their 
influence  on  the  crops  more  closely,  and  thus  is  placed 

Jenkins,  E.  H.,  and  Winton,  A.  L.  Fertilizer  Report. 
Conn.  (New  Haven)  Agr.  Expt.  Sta.,  Rept.  1906,  Part  I,  pp. 
1-106. 


FERTILIZER  PRACTICE  565 

in  a  position  to  make  changes  that  will  tend  to  a  higher 
efficiency  of  the  constituents.  The  chances  are  that  he 
will  alter  his  fertilizer  mixture  as  his  rotation  progresses 
and  his  soil  changes  in  fertility. 

Such  arguments  do  not  always  mean,  however,  that 
it  pays  to  mix  at  home.  As  a  matter  of  fact,  in  many 
cases  it  does  not  pay,  especially  where  only  a  small  amount 
of  fertilizer  is  needed  and  it  is  impossible  to  cooperate 
with  other  farmers.  As  a  general  rule,  fertilizers  should 
be  bought  by  the  method  that  will  give  the  greatest  value 
for  every  dollar  expended.  Farmers  often  can  avail 
themselves  of  the  advantage  of  both  systems  by  asking 
for  bids  from  various  manufacturers  on  carload  lots  of 
mixed  goods  having  a  certain  designated  composition. 
The  farmers  in  this  case  designate  the  carriers  as  well. 
All  the  advantages  of  machinery  mixing  may  thus  be 
gained,  with  the  lower  cost  which  has  made  home  mixing 
so  popular. 

475.  Fertilizers  not  to  be  mixed.  —  Every  farmer  who 
practices  home  mixing  should  keep  in  mind  that  there 
are  certain  fertilizers  which  should  not  be  mixed.  This 
is  due  to  the  fact  that  a  number  of  materials  carry  lime 
in  the  oxide,  the  hydrate,  or  the  carbonate  form.  This 
lime,  particularly  the  caustic  forms,  may  react  in  three 
directions,  depending  on  the  fertilizer  with  which  it  is  in 
contact :  (1)  in  setting  free  ammonia,  (2)  in  causing  re- 
version of  acid  phosphate,  and  (3)  in  producing  a  bad 
physical  condition,  especially  when  in  contact  with  ma- 
terials more  or  less  deliquescent.  Van  Slyke l  may  be 
quoted  in  this  regard  as  follows :  — 


1Van    Slyke,    L.    L.     Fertilizers    and    Crops,    pp.    485-486. 
New  York,  1912. 


W6      SOILS:    PROPERTIES  AND  MANAGEMENT 


3. 


Calcium  oxide 
Calcium  hydrate 
Wood  ashes 
Basic  slag 
Calcium  cyanamid 
Basic  calcium 
nitrate 

Calcium  oxide 
Calcium  hydrate 
Calcium  carbonate 
Wood  ashes 
Basic  calcium 
nitrates 

Calcium  oxide 
Calcium  hydrate 
Basic  calcium 
nitrate 


should   not   be 
mixed     with 


ammonium  sul- 
fate 

animal  manures, 
as  tankage, 
blood,  and  the 
like 

nitrogenous 
guanos 


.      , ,       x  ,     f  soluble  phos- 
should  not  be  , 

,       .  ,       {      phates 
mixed    with  £  ,  .    , 

[ol   any  kind 


should  not  be 
mixed    with 
(unless  applied 
immediately) 


'  sodium    nitrate 
potassium     chlo- 
ride 
kainit,    and    the 
like 


Neither  is  it  wise  to  allow  moist  acid  phosphate  to  lie 
in  contact  with  large  quantities  of  sodium  nitrate,  as 
nitric  acid  may  be  slowly  liberated  by  free  sulfuric  or 
phosphoric  acid.  Also,  large  quantities  of  calcium  cyana- 
mid should  not  be  mixed  with  acid  phosphate  because 
of  the  lime  contained  in  the  former.  If,  however,  the 
ratio  is  not  greater  than  one  to  ten,  the  results  are  bene- 
ficial, since  the  reaction,  without  causing  serious  rever- 
sion of  the  phosphate,  generates  enough  heat  to  quickly 
season  the  mixture.  The  fine  and  dry  condition  of  the 
cyanamid  is  also  conducive  to  a  good  mechanical  condi- 
tion, and  accounts  for  the  fact  that  this  material  is  in 
such  favor  with  manufacturers  of  mixed  goods. 


FERTILIZER  PRACTICE  567 

476.  How  to  mix  fertilizers.  —  As  the  various  carriers 
are  bought  under  guarantee,  the  percentages  of  nitrogen, 
phosphoric  acid,  and  potash  in  the  ingredients  to  be  mixed 
are  accurately  known.  The  calculation  of  the  amounts 
of  each  carrier  and  of  the  filler  necessary  to  make  up  a 
ton  of  a  fertilizer  having  a  certain  formula,  then  becomes 
a  matter  of  simple  arithmetic.  The  mixing  is  an  equally 
simple  operation.  The  implements  needed  in  home  mixing 
are  as  follows :    (1)   a   tight   floor,   (2)    platform    scales, 

(3)  a  sand  screen  with  from  three  to  six  meshes  to  an  inch, 

(4)  a  tamper  or  a  grinder,  (5)  shovels,  a  rake,  and  like 
tools. 

First,  the  various  ingredients,  after  being  crushed  and 
screened  if  lumpy,  are  weighed  out  in  amounts  sufficient 
for  the  unit  of  fertilizer  to  be  mixed  at  any  one  time. 
The  bulkiest  material  is  spread  on  the  floor  first  and  leveled 
uniformly  by  raking.  The  remaining  ingredients  are 
then  spread  in  thin  layers  above  the  first,  in  the  order 
of  their  bulk.  Beginning  at  one  side,  the  material  is 
next  shoveled  over,  care  being  taken  that  the  shovel 
reaches  the  bottom  of  the  pile  each  time.  The  pile  is 
then  again  leveled,  and  the  process  is  repeated  a  sufficient 
number  of  times  to  insure  thorough  mixing.  Sometimes 
a  mixing  machine  may  be  used  for  this  operation.  For 
storage  and  general  convenience,  the  fertilizer  may  be 
weighed  into  sacks  of  from  100  to  150  pounds  capacity 
and  put  in  a  dry  place  until  needed  for  use. 

A  word  of  caution  should  be  inserted  here  regarding  the 
concentration  of  the  mixture.  Some  farmers,  in  order 
to  lessen  the  work  of  mixing  and  application  in  the  field, 
raise  the  percentage  of  the  elements  exceedingly  high  — 
a  condition  very  likely  to  occur  when  high-grade  materials 
are  used.     This  is  bad  practice,  in  that  it  may  interfere 


568       SOILS:    PBOPEBTIES  AND  MANAGEMENT 

with  germination  and  may  also  injure  the  young  plants. 
Also,  it  is  likely  to  result  not  only  in  a  poor  physical  condi- 
tion but  also  in  uneven  distribution,  which  will  bring  about 
a  lowered  efficiency  of  the  fertilizer.  The  use  of  sufficient 
dry,  finely  divided  filler  will  obviate  such  dangers. 

477.  Factors  affecting  the  efficiency  of  fertilizers.  — 
The  agricultural  value  of  a  fertilizer  is  necessarily  a  vari- 
able quantity,  since,  in  applying  fertilizers,  a  material 
subject  to  change  is  placed  in  contact  with  two  wide 
variables,  the  soil  and  the  crop.  The  general  factors 
that  govern  the  effect  of  fertilizers  may  be  listed  as 
follows :  — 

1.  Seed,  crop,  and  adaptation  of  crop  to  soil.  —  It  is  quite 

evident  that  different  crops  will  respond  differ- 
ently to  the  same  fertilizer  elements.  Also,  the 
strength  of  the  seed,  the  management  of  the  crop, 
and  the  adaptation  of  crop  to  soil,  will  be  potent 
factors  in  variation. 

2.  Temperature,  sunshine,  and  rainfall.  —  These  factors 

are  meteorological  and,  of  course,  are  dominant 
in  the  growth  of  the  plant.  Rainfall  especially 
is  important,  as  an  optimum  moisture  content 
is  conducive  to  good  plant  development.  In 
general,  as  shown  by  experiments  in  Ohio  and 
Pennsylvania,  the  higher  the  rainfall,  the  greater 
is  the  efficiency  of  the  fertilizer  used. 

3.  Drainage.  —  This  is  of  great  importance  in  ferti- 

lizer practice,  since  it  places  the  soil  in  a  better 
condition  from  all  standpoints  for  plant  growth. 
In  other  words,  the  better  the  normal  soil  condi- 
tions, the  better  should  be  the  reaction  from  ferti- 
lizer application. 


FERTILIZEB  PRACTICE  569 

4.  Physical  condition  of  the  soil.  —  The  addition  of  lime 

and  organic  matter,  the  utilization  of  drainage, 
tillage,  and  the  like,  all  are  conducive  to  higher 
crop  returns  through  the  indirect  effect  on  fertilizer 
efficiency. 

5.  Lime.  —  Lime,    by   improving   physical   conditions, 

by  setting  plant-food  free,  by  correcting  acidity, 
by  stimulating  bacterial  action,  and  by  tending 
to  eliminate  toxic  materials  either  directly  or 
indirectly,  is  of  great  importance  in  fertilizer 
practice.  In  fact,  certain  fertilizers,  such  as 
ammonium  sulfate  and  acid  phosphate,  do  not 
reach  their  full  efficiency  unless  plenty  of  lime  is 
present. 

6.  Organic  matter.  —  Besides  the  effect  of  organic  matter 

on  physical  conditions  and  chemical  reactions 
which  indirectly  influence  fertilizer  action,  an  im- 
portant action  is  set  up  by  organic  matter  in  the 
encouragement  of  bacterial  functions.  As  the 
favorable  changes  of  fertilizers,  especially  those 
carrying  nitrogen,  is  due  to  biological  activity, 
the  presence  of  organic  materials  becomes  doubly 
important. 

7.  Chemical  composition  of  the  soil.  —  Since   the   full 

return  from  a  fertilizer  is  derived  when  the  ele- 
ments are  well  balanced,  the  actual  constitution 
of  the  soil  becomes  a  factor,  especially  when  ready 
availability  is  obtainable.  Therefore,  in  choosing 
a  fertilizer  and  deciding  on  the  amounts  to  apply, 
the  chemical  condition  of  the  soil  is  no  mean  factor. 

While  the  conditions  affecting  fertilizer  efficiency  have 
thus  been  so  briefly  disposed  of,  it  is  evident  that  a  more 


570       SOILS:    PROPERTIES  AND  MANAGEMENT 

detailed  consideration  of  the  question  would  be  not  only 
interesting  but  also  profitable,  would  space  permit.  One 
point  of  broader  scope,  however,  than  the  addition  of  a 
well-balanced  food  stimulation,  stands  out  clearly  in  this 
consideration.  The  necessity  of  putting  a  soil  in  any 
given  climate  into  the  best  possible  condition  for  plant 
growth  is  paramount.  This  means  that  drainage,  lime, 
humus,  and  tillage,  in  the  order  named,  must  be  raised 
to  their  highest  perfection.  Under  such  improvements 
the  further  use  of  commercial  fertilizers  may  or  may  not 
be  a  paying  investment. 

478.  Method  and  time  of  applying  fertilizers.  —  The 
distribution  of  the  fertilizer  by  means  of  machinery  is 
much  more  satisfactory  than  is  broadcasting  by  hand, 
as  the  former  method  gives  a  more  uniform  distribution. 
Cereals  and  other  crops  are  now  usually  planted  with  a 
drill  or  a  planter  provided  with  an  attachment  for  dropping 
the  fertilizer  at  the  same  time  that  the  seed  is  sown,  the 
fertilizer  being  by  this  method  placed  under  the  surface 
of  the  soil.  Broadcasting  machines  are  also  used,  which 
leave  the  fertilizer  uniformly  distributed  on  the  surface 
of  the  ground,  thus  permitting  it  to  be  harrowed  in  suffi- 
ciently before  the  seed  is  planted,  and  preventing  injury 
to  the  seed  by  the  chemical  activity  of  the  fertilizing 
material. 

Corn  planters  with  fertilizer  attachments  deposit 
the  fertilizer  beneath  the  seed,  thus  avoiding  a  possible 
detrimental  contact.  Grain  drills  do  not  do  this,  and, 
where  the  amount  of  fertilizer  used  exceeds  300  or  400 
pounds  an  acre,  it  is  better  to  apply  it  before  seeding. 
Grass  and  other  small  seeds  should  be  planted  only  after 
the  fertilizer  has  been  mixed  with  the  soil  for  several 
days.     For  crops  to  which  large  quantities  of  fertilizers 


FERTILIZER   PRACTIGE  571 

are  to  be  added,  especially  potatoes  and  garden  crops, 
it  is  desirable  to  drop  only  a  portion  of  the  fertilizer  with 
the  seed,  the  remainder  having  been  broadcasted  by  ma- 
chinery and  harrowed  in  earlier. 

479.  Fertilizing  crops.  —  Three  primary  considerations 
must  be  observed  in  the  actual  utilization  of  fertilizers : 
(1)  the  percentage  of  nitrogen,  phosphorus,  and  potash 
suited  to  the  crop  and  the  soil ;  (2)  the  availability  of  the 
carriers;  and  (3)  the  amounts  to  be  applied.  It  is  evi- 
dent, due  to  so  many  factors  that  are  difficult  to  control, 
that  fertilizer  formulas  for  different  crops  on  particular 
soils  are  difficult  to  determine.  In  fact,  such  data  can 
never  be  more  than  merely  suggestive.  Further,  the 
best  quantity  of  a  mixture  to  apply,  even  though  it  is 
perfectly  balanced,  is  a  figure  that  can  only  be  approxi- 
mated. Probably  the  largest  percentage  of  the  fertilizer 
waste  that  occurs  annually  can  be  charged  to  this  factor. 
Many  farmers  make  the  mistake  of  applying  too  much 
fertilizer.  As  a  consequence,  any  information  along  such 
lines  can  only  be  merely  suggestive,  rather  than  literal, 
it  being  understood  that  the  general  formulas  suitable 
to  various  crops,  and  the  quantities  ordinarily  applied, 
are  subject  to  wide  variations. 

The  fact  that  there  are  so  many  mixtures  on  the  market 
in  this  country  for  the  same  crops  would  be  rather  amus- 
ing, did  it  not  so  strikingly  expose  the  ignorance  of  the 
manufacturer  as  well  as  the  gullibility  of  the  public. 
Recognizing  the  need  of  standard  formulas  subject  to 
change  according  to  local  conditions,  Van  Slyke ]  has 
offered  the  following  for  general  use :  — 


1  Van   Slyke,    L.    L.     Fertilizers   and    Crops,    p.    528.     New 
York.     1912. 


572       SOILS:    PROPERTIES  AND   MANAGEMENT 


Fertilizer  Formulas  for  General  Application 


Crops 


Leguminous 
Cereal  .  . 
Garden  .  . 
Grass  .  . 
Orchard 
Root       .     . 


Percentage  Percentage  Percentage 
of  N      op  PjO»     of  KiO 


10 
5 

10 
9 

10 

7 


While  it  is  recognized  that  these  formulas  are  probably 
far  from  correct  in  their  application  to  such  groups  as 
the  garden  crops,  where  so  many  entirely  different  plants 
are  concerned,  it  is  felt  that  they  furnish  the  basis,  as 
far  as  our  knowledge  now  extends,  for  a  more  economic 
fertilization.  The  variation  of  such  mixtures  to  suit 
specific  needs  is  a  part  of  fertilizer  practice. 

The  carriers  largely  used  for  such  readily  available 
mixtures  are  sodium  nitrate,  acid  phosphate,  and  potassium 
chloride  or  sulfate.  Tankage  or  blood  is  often  substituted 
for  sodium  nitrate  where  humus  is  desirable,  while  am- 
monium sulfate  and  calcium  cyanamid  are  growing  in 
popularity.  Raw  rock  phosphate  and  basic  slag  are  used 
rather  largely  in  separate  applications,  the  amounts 
being  usually  larger  than  with  the  ordinary  fertilizer 
materials. 

The  other  phase  of  fertilizer  practice  is  in  the  amount 
to  be  applied.  With  all  the  groups  considered  above 
except  garden  and  root  crops,  the  applications  are  rela- 
tively light,  ranging  from  150  to  300  pounds  to  an  acre. 
Where  excessive  vegetative  growth  is  required,  as  in  silage, 
the  rate  may  be  increased  to  500  pounds.  In  the  top- 
dressings  of  meadows  or  grains,  the  rate  varies  from  75 


FERTILIZER  PRACTICE  573 

to  150  pounds  an  acre.  Very  often  this  dressing  is  sodium 
nitrate  alone.  With  garden  and  root  crops  the  amount 
of  fertilizer  applied  is  very  large,  ranging  from  800  to 
sometimes  as  high  as  2000  pounds.  The  cropping  here 
is  intensive,  and  the  expenditure  for  fertilization  may  be 
large  and  yet  yield  handsome  profits. 

It  must  always  be  remembered  that  in  fertilizer  prac- 
tice the  very  high  yields  obtained  under  fertilizer  stimu- 
lation are  not  always  the  ones  that  give  the  best  returns 
on  the  money  invested.  In  other  words,  the  law  of 
diminishing  returns  is  a  factor  in  the  influence  of  ferti- 
lization on  crop  yield.  This  relationship  is  clearly  shown 
by  the  curve  illustrating  the  law  of  the  minimum  (par. 
469),  in  which  the  return  for  each  increment  of  fertilizer 
becomes  less  and  less  as  the  total  quantity  added  becomes 
greater.  It  is  evident,  therefore,  that  with  an  excessive 
application  of  any  mixture,  the  returns  to  an  increment 
will  at  last  become  so  small  that  the  increased  crop  fails 
entirely  to  pay  for  even  the  fertilizer,  not  to  mention  such 
charges  as  cost  of  application,  harvesting  of  increased 
crop,  storage,  and  the  like.  The  application  of  moderate 
amounts  of  fertilizer  is  to  be  urged  for  all  soils  until  the 
maximum  paying  dose  that  may  be  applied  to  any  given 
crop  is  ascertained  by  careful  experimentation.  Over- 
fertilization  probably  accounts  for  the  fact  that  such  a 
large  proportion  of  the  fertilizers  sold  to  the  farmers  each 
year  not  only  is  entirely  wasted,  but  probably  in  some 
cases  even  becomes  detrimental  to  crop  yield. 

480.  Systems  of  fertilization.  —  During  the  evolution 
of  fertilizer  practice,  particularly  since  the  early  part 
of  the  nineteenth  century,  a  number  of  systems  of  apply- 
ing fertilizer  have  been  advocated  or  have  been  in  actual 
use.     These  may  be  listed  as  follows :  — 


574       SOILS:    PROPERTIES  AND  MANAGEMENT 

1.  Single-element  system. —  This  was  one  of    the   first 

to  be  suggested,  and  was  advocated  because1  each 
particular  crop  was  supposed  at  that  time  to 
respond  largely  to  one  element.  Thus,  nitrogen 
was  supposed  to  dominate  wheat,  rye,  and  oats; 
phosphoric  acid,  to  dominate  corn,  turnips,  and 
sorghum;  and  potash  to  dominate  potatoes, 
clover,  and  beans.  Present  knowledge  of  the 
balancing  effects  of  fertilizers  shows  this  idea  to 
be  fallacious. 

2.  Abundant  supply  of   minerals.  —  This   system   had 

its  origin  from  the  fact  that  potash  and  phosphoric 
acid  are  relatively  cheap  and  are  slowly  leached 
from  the  soil,  while  nitrogen  is  expensive  and  easily 
lost.  Such  a  plan,  therefore,  provides  always  plenty 
of  potash  and  phosphorus,  which  is  to  be  balanced 
each  season  with  sufficient  nitrogen  to  give  paying 
yields. 

3.  A  system  based  on  the  plant-food  taken  out  by  the 

crop.  —  According  to  this  plan,  as  much  plant- 
food  is  added  each  year  as  will  probably  be  taken 
out  by  the  plant,  this  being  determined  by  chemi- 
cal analyses.  This  system  overlooks  the  fact 
not  only  that  different  plants  feed  differently  on 
the  same  soil,  but  that  the  same  crop  exhibits 
marked  variability  with  change  of  season  and 
change  of  soil.  Moreover,  no  allowance  is  made 
for  losses  by  leaching,  which  are  known  to  equal 
at  times  the  losses  due  to  plant  growth. 

4.  Irrational  system.  —  This  is  the  plan  followed  by 

many  farmers  where  fertilizers  are  an  important  fac- 
tor in  soil  management.  The  formula  is  changed 
from  year  to  year,  in  a  vain  attempt  to  strike  a 


FERTILIZER   PRACTICE  575 

high  point  in  production.  The  same  continual  shift 
is  found  in  the  quantities  applied.  Too  often 
the  specific  brand  used  is  determined  by  the  trade 
name  that  it  carries  or  by  the  recommendation  of 
the  retail  merchant,  rather  than  from  a  careful 
consideration  of  the  guarantee  or  of  the  carriers 
for  each  important  element.  The  educational 
phase  of  home  mixing  should  do  much  to  eliminate 
this  system. 
5.  Fertilization  of  the  money  crop.  —  In  trucking  or  in 
general  farming  operations  one  crop  is  usually  a 
money  crop.  Naturally  its  stimulation  by  heavy 
fertilization  will  pay  better  than  applications  to 
crops  that  bring  less  on  the  market.  The  general 
plan  in  this  system  is  to  allow  the  crops  following 
the  money  crop  to  utilize  the  residuum.  When 
this  residual  influence  works  out,  the  system  is 
likely  to  be  a  profitable  one ;  but  when  the  follow- 
ing crops  fail  to  respond,  the  method  becomes 
wasteful  in  the  extreme. 

In  the  selection  of  a  system  that  will  result  in  an  ef- 
fective utilization  of  fertilizers,  only  two  of  the  plans  de- 
scribed above  need  be  considered.  In  any  fertilizer, 
phosphoric  acid  and  potash  should  always  be  present  in 
amounts  sufficient  to  more  than  balance  the  nitrogen, 
since  the  activity  of  nitrogen  is  so  pronounced.  There- 
fore a  scheme  that  calls  for  an  abundance  of  minerals  is 
a  sound  one.  This,  coupled  with  the  heavy  fertilization 
of  the  money  crop,  does  not,  however,  constitute  what 
might  be  considered  a  rational  system,  since  the  crops 
that  follow  may  or  may  not  be  adequately  supplied  with 
plant-food.     Unwise   fertilization   often   leaves   the   soil, 


576      SOILS:    PROPERTIES  AND  MANAGEMENT 

as  far  as  its  balance  is  concerned,  less  able  to  yield  a 
paying  crop  than  before.  The  careful  fertilization  of  the 
rotation,  then,  with  special  attention  to  the  money  crop, 
is  the  only  rational  system  that  can  ordinarily  be  employed, 
since  it  not  only  cares  for  the  crop  on  the  land  but  also 
looks  to  those  that  are  to  succeed.  The  attention  that 
must  necessarily  be  paid  to  the  fertility  of  the  soil  in  such 
a  system  insures  the  establishment  of  a  soil  management 
which  will  ultimately  result  in  a  great  conservation  of 
fertility,  while  at  the  same  time  raising  the  yields  and 
increasing  the  prosperity  of  the  farming  class. 


CHAPTER  XXVI 
FARM  MANURES 

Of  all  the  by-products  of  the  farm,  barnyard  manure 
is  probably  the  most  important,  since  it  affords  a  means 
whereby  the  unused  portion  of  the  crop,  the  residue  of 
the  finished  farm  product,  may  again  be  returned  to  the 
soil.  This  country  is  now  entering  on  an  era  in  which 
the  prevention  of  all  waste  is  becoming  more  and  more 
necessary  and  a  nearer  approach  to  a  self-sustaining  sys- 
tem of  agriculture  far  more  essential.  A  clear  under- 
standing of  the  composition  of  farm  manure,  the  changes 
it  undergoes,  and  its  avenues  of  loss,  and  also  of  methods 
for  its  practical  handling,  and  a  realization  of  its  effects 
both  on  soil  and  on  crop,  are  of  vital  importance.  This 
need  appeals  not  only  to  the  practical  man  but  to  the 
theoretical  and  technical  man  as  well,  for  here  is  a  field 
in  which  theory  and  practice  not  only  meet  but  widely 
overlap. 

481.  General  character  and  function  of  farm  manures. 
—  The  term  farm  manure  may  be  employed  in  reference  to 
the  refuse  from  all  animals  of  the  farm,  although,  as  a 
general  rule,  the  bulk  of  the  ordinary  manure  which  ul- 
timately finds  its  way  back  to  the  land  is  produced  by 
cattle  and  horses.  This  arises  not  only  because  these 
animals  consume  the  greater  part  of  the  grain  and  rough- 
age on  the  average  farm,  but  also  because  the  methods 
of  handling  them  make  it  easier  and  more  practicable  to 
2p  577 


578      SOILS:    PROPERTIES  AND  MANAGEMENT 

conserve  their  excreta.  Yard  manure  generally  refers 
to  mixed  manures.  The  mixing  usually  occurs  during 
storage,  either  for  convenience  in  handling  or  for  the  pur- 
pose of  checking  losses  and  facilitating  fermentation. 
Thus,  horse  and  cow  manures  are  commonly  mixed,  since 
the  too  rapid  fermentation  and  probable  loss  of  ammonia 
in  the  former  is  checked,  while  at  the  same  time  a  more 
rapid  and  much  more  complete  decay  is  encouraged  in 
the  latter. 

The  ordinary  manure  consists  of  two  original  compo- 
nents, the  solid  and  the  liquid  portion.  As  these  con- 
stituents differ  greatly,  not  only  in  composition  but  also 
in  physical  properties,  their  proportions  must  appreciably 
affect  the  quality  of  the  excreta  and  its  agricultural  value. 
Litter  added  for  bedding  or  for  adsorptive  purposes  is 
almost  always  an  important  factor,  for  while  it  prevents 
losses  of  the  soluble  constituents  it  may  at  the  same  time 
lower  the  value  of  the  product  for  a  unit  amount. 

Farm  manure  ordinarily  fulfills  two  functions  which 
are  usually  not  so  simultaneously  yet  clearly  developed 
in  any  other  material  —  that  of  a  direct  and  that  of  an 
indirect  fertilizer.  Consisting  of  73  per  cent  of  water 
and  only  27  per  cent  of  dry  matter,  the  percentages  of 
plant-food  are  necessarily  low.  As  mixed  farm  manure 
contains  on  the  average  1  0.50  per  cent  of  nitrogen,  0.25 
per  cent  of  phosphoric  acid,  and  0.60  per  cent  of  potash, 
considerable  quantities  of  plant-food  elements  are  added 
in  an  ordinary  application.  Ten  tons  of  average  manure, 
even  if  only  one-half  of  the  nitrogen,  one-sixth  of  the 
phosphorus,  and  one-half  of  the  potash  are  readily  avail- 
able, is  equivalent  to  300  pounds  of  sodium  nitrate,  60 

^ee  Analyses,  Storer  F.  H.  Agriculture,  pp.  237-248. 
New  York.     1910. 


FARM  MANURES  579 

pounds  of  acid  phosphate,  and  125  pounds  of  potassium 
chloride.  This  is  equivalent  to  the  addition  of  485 
pounds  of  an  approximately  10-2-12  ready-mixed  ferti- 
lizer. Moreover,  from  the  fact  that  so  large  an  amount 
of  the  plant-food  carried  is  not  readily  available,  it  acts 
as  a  residuum,  which  is  slowly  given  up  to  the  succeeding 
crops.  It  has  been  shown  in  England  x  that  paying  in- 
creased returns  may  lie  obtained  from  manure  four  years 
after  its  application.  At  Rothamsted,  England,2  a 
residual  impetus  was  noticeable  on  crops  forty  years  after 
the  soil  was  manured.  This,  however,  is  an  exceptional 
case. 

Farm  manure  may  act  as  an  indirect  fertilizer  in  its 
tendency  toward  improved  physical  relations.  The  addi- 
tion of  organic  matter  is  the  vital  factor  here.  Better 
tilth,  better  aeration,  improved  drainage,  and  increased 
water  capacity  are  the  necessary  accessories  to  a  rise  in 
humus  content.  The  influence  of  manure  on  the  avail- 
ability of  the  mineral  constituents  of  the  soil  is  not  the 
least  of  its  indirect  effects.  Even  the  increased  adsorp- 
tive  power  of  the  soil  should  be  mentioned,  in  its  tendency 
toward  the  counteraction  of  toxic  principles. 

Another  general  characteristic  of  average  farm  manure 
is  that,  while  it  is  a  fertilizer,  it  is  an  unbalanced  one. 
Proportional  very  roughly  to  a  10-2-12  commercial  mix- 
ture, any  one  acquainted  with  general  fertilizer  practice 
can  see  that  it  is  too  high  in  nitrogen  and  too  low  in  avail- 
able phosphoric  acid.     The  elimination  of  such  a  condi- 


1  Voelcker,  J.  A.,  and  Hall,  A.  D.  The  Valuation  of  Unex- 
hausted Manure  Obtained  by  the  Consumption  of  Foods  by 
Stock.     London.     1903. 

2  Hall,  A.  D.  Fertilizers  and  Manures,  p.  213.  New  York, 
1910. 


580      SOILS:    PROPERTIES  AND  MANAGEMENT 

tion  and  a  balancing  thereby  of  the  plant  ration  is  one 
of  the  many  problems  that  present  themselves  in  the 
economic  handling  and  utilization  of  animal  residues. 

482.  Variable  composition  of  manures.  —  The  manure 
produced  on  an  average  farm  is  rather  likely  to  vary 
markedly  in  composition  and  character  from  time  to 
time.  It  may  even  change  radically  from  one  day  to 
another.  There  are  five  general  factors  that  are  usually 
listed  as  being  responsible  for  this  variation:  (1)  litter; 
(2)  class  of  animal ;  (3)  individuality,  condition,  and  age  of 
animal ;    (4)  food  of  animal ;  and  (5)  handling  of  manure. 

483.  Litter.  —  Perhaps  under  ordinary  circumstances 
the  amount  and  character  of  the  litter  has  as  much  to 
do  with  the  variation  in  manurial  composition  as  has 
any  other  one  factor,  if  not  more.  By  an  increase  in 
the  amount  of  bedding,  the  product  becomes  bulky, 
light  in  weight,  and  difficult  to  handle.  This  is  likely 
to  be  the  case  with  manure  from  livery  stables,  where 
the  litter  is  used  to  keep  the  horses  clean  and  not  for 
purposes  of  plant-food  conservation.  That  bedding  must 
also  exert  a  marked  effect  on  chemical  composition  is 
evident  from  the  following  analyses  :  — 

Composition  of  Litter 


N 

PjOs 

K*0 

Sawdust  shavings      .... 

Oat  straw 

Peat 

0.10 
0.62 
2.63 
0.65 

0.20 
0.20 
0.20 
0.15 

0.40 
1.04 
0  17 

Leaves    

0.30 

Sawdust  and  shavings  add  little  of  value  to  the  manure 
and  really  act  as  a  diluent.     While  they  are  good  absorb- 


FARM  MANURES 


581 


ents  they  decompose  so  slowly  as  to  make  them  somewhat 
objectionable  on  light  soils.  Leaves  decompose  readily, 
but  add  little  fertility.  Oat  straw  carries  no  more  nitro- 
gen than  does  average  manure,  and  this  nitrogen,  like 
that  of  peat  or  muck,  is  not  readily  available  as  plant- 
food.  Litter,  however,  is  of  such  extreme  importance  as 
an  adsorbent  that  the  resistant  qualities  of  even  such 
materials  as  shavings  can  be  to  a  degree  ignored.  Be- 
cause of  the  influence  of  the  bedding  on  composition, 
manure  should  never  be  bought  unless  this  phase  has 
been  carefully  looked  into. 

484.  Class  of  animal.  —  The  second  factor  causing 
radical  variation  in  the  composition  of  farm  manure  is 
the  class  of  animal  by  which  it  is  produced.  The  following 
figures,1  compiled  from  Ohio,  Connecticut,  and  New  York 
(at  Cornell  University),  illustrate  this  point  clearly:  — 


Percentage  of 

H»0 

N 

P 

K 

Horse  manure  with  straw 
Cow  manure  with  straw 

62.80 
78.00 

0.57 
0.46 

0.12 
0.13 

0.54 
0.36 

A  working  horse  on  maintenance  ration  will  return  in 
the  manure  almost  all  the  nitrogen  and  minerals  taken 
as  food.  In  other  words,  the  building-up  and  the  break- 
ing-down, or  elimination,  processes  are  about  equal. 
A  young  fattening  pig,  on  the  other  hand,  will  return  only 
about  85  per  cent  of  the  nitrogen  received  as  food  and  96 
per  cent  of  the  mineral  material,  and  a  milking  cow  75 
per  cent  and  89  per  cent,  respectively. 

1  Thorne,  C.  E.     Farm  Manures,  p.  89.     New  York.     1914. 


582       SOILS:    PROPERTIES  AND  MANAGEMENT 


485.  Individuality,   condition,   and   age   of  animal.  — 

Various  animals  differ  in  capacity,  sonic  retaining  much 
more  of  the  elements  contained  in  the  food  than  do  others* 
and  consequently  producing  a  poorer  manure.  The 
service  to  which  the  animal  is  subjected  is  also  a  factor. 
A  milch  cow  will  certainly  utilize  more  nutriments  than 
an  animal  not  in  that  condition.  Age  is  perhaps  more 
accountable  for  variation  in  farm  manure  than  either  of 
the  other  two  causes.  A  young  animal  gaining  in  muscle 
and  bone  is  storing  away  large  quantities  of  nitrogen, 
phosphorus,  and  potash,  and  producing  a  manure  corre- 
spondingly poorer  in  these  ingredients. 

486.  Food  of  animal.  —  Since  the  animal  will  retain 
only  a  certain  quantity  of  the  food  elements,  it  is  reason- 
able to  suppose  that  the  richer  the  food,  the  richer  will 
be  the  corresponding  excrement,  both  liquid  and  solid. 
Such  has  proved  to  be  the  case.  Wheeler,1  in  studying 
the  rations  of  chickens,  found  the  following  difference 
in  the  manure  produced  :  — • 


Ration 

Percentage  op 

H2O 

N 

P 

K 

Fresh  hen  manure  (nitrog- 
enous ration)        .     .     . 

Fresh  hen  manure  (car- 
bonaceous ration)     .     . 

59.7 

55.3 

0.80 
0.66 

0.41 
0.32 

0.27 
0.21 

From  Ohio,2  where  the  production  of  manure  has  been 
most  thoroughly  investigated,  the  following  data  may  be 
quoted :  — 

1  Wheeler,  W.  P.  Poultry  Feeding  Experiments.  Kept. 
New  York  (Geneva)  Agr.  Exp.  Sta.,  No.  8,  p.  02.     1889. 

2  Thorne,  C.  E.,  and  others.  The  Maintenance  of  Fertility. 
Ohio  Agr.  Exp.  Sta.,  Bui.  183.     1907. 


FARM  MANURES  583 

Effect  of  Ration  on  Manurial  Composition 


Percentage  op 

N 

p 

K 

Corn  and  mixed  hay      .     .     . 
Corn,  oil  meal,  and  hay     .     . 
Corn,  oil  meal,  and  clover 

1.49 
1.55 

1.68 

0.23 
0.24 
0.26 

1.11 

1.02 
1.04 

487.  Handling  manure.  —  In  dealing  with  a  product 
of  which  almost  one-half  is  liquid,  there  is  great  danger 
that  a  considerable  amount  of  valuable  plant-food  will  be 
lost  by  leaching.  The  modification  and  consequent 
lowering  of  the  plant-food  value  of  farm  manure  is  a 
vital  question  in  the  economic  handling  of  this  product. 
Next  to  the  litter,  lack  of  care  is  perhaps  the  most  im- 
portant single  factor  concerned  in  altering  the  chemical 
composition  of  manures  in  general.  The  influence  of 
handling  is  so  clearly  brought  out  by  the  following  figures 
from  Schutt,1  on  mixed  horse  and  cow  manure,  that  further 
discussion  seems  unnecessary.  The  protected  manure  in 
this  case  was  in  a  bin  under  a  shed.  The  exposed  sample 
was  in  a  similar  bin  but  unprotected  from  the  weather :  — 


Loss  at  End  of 
Six  Months 
(Percentage) 

Loss  at  End  op 

Twelve  Months 

(Percentage) 

Protected 

Exposed 

Protected 

Exposed 

Loss  of  organic  matter 
Loss  of  nitrogen    . 
Loss  of  phosphoric  acid 
Loss  of  potash 

58 

19 

0 

3 

65 
30 
12 
29 

60 

23 

4 

3 

69 
40 
16 
36 

1  Schutt,  M.  A.     Barnyard  Manure. 
Centr.  Exp.  Farm,  Bui.  31.     1898. 


Canadian  Dept.  Agr. 


584       SOILS:    PROPERTIES  AND  MANAGEMENT 

488.  Composition  and  character  of  farm  manures.  — 
Although  the  probable  composition  of  farm  manures  is  so 
difficult  to  state  in  exact  figures,  compilations  of  the 
available  data  have  yielded  percentages  which,  while 
they  demand  a  most  liberal  interpretation,  afford  con- 
siderable light  regarding  the  differences  that  normally 
exist  between  the  excrement  of  various  animals.  Of 
these  compilations,  Van  Slyke's  is  perhaps  the  best. 

The  Composition  of  Fresh  Manure  l 


Percentage  op 

Excrement 

H»0 

N 

PtO, 

KiO 

[  Solid  80  per  cent 
Horse  |  Liquid  20  per  cent 
[Whole  manure 

75 
90 

78 

0.55 
1.35 
0.70 

0.30 

Trace 

0.25 

0.40 
1.25 
0.55 

[  Solid  70  per  cent 
Cow    {  Liquid  30  per  cent 
{ Whole  manure 

85 
92 
86 

0.40 
1.00 
0.60 

0.20 

Trace 

0.15 

0.10 
1.35 
0.45 

f  Solid  67  per  cent 
Sheep  |  Liquid  33  per  cent 
[  Whole  manure 

60 

85 
68 

0.75 
1.35 
0.95 

0.50 
0.05 
0.35 

0.45 
2.10 
1.00 

( Solid  60  per  cent 
Swine  j  Liquid  40  per  cent 
[Whole  manure 

80 
97 

87 

0.55 
0.40 
0.50 

0.50 
0.10 
0.35 

0.40 
0.45 
0.40 

Since  the  horse  does  not  ruminate  its  food,  the  manure 
is  likely  to  be  of  an  open  character.  It  is  also  a  fairly 
dry  manure,  as  is  that  from  sheep,  the  liquid  in  these  two 
manures  making  up  20  and  33  per  cent,  respectively, 
of  the  whole  product.  The  complete  manure  from  these 
two  animals  contains  78  and  68  per  cent,  respectively, 


1  Van  Slvke,  L.  L. 
York.      1912, 


Fertilizers    and    Crops,    p.    291.     New 


FARM  MANURES  585 

of  water  —  a  considerable  contrast  to  the  86  and  87  per 
cent  presented  by  the  cattle  and  swine  excrements. 
Cattle  and  swine  manures,  being  very  wet,  are  rather 
solid  and  compact.  The  air,  therefore,  is  likely  to  be 
excluded  to  a  large  degree  and  decomposition  is  relatively 
slow.  They  are  usually  spoken  of  as  cold,  inert  manures 
as  compared  with  the  dry,  open,  rapidly  heating  excre- 
ments obtained  from  the  horse  and  the  sheep. 

In  every  case  except  that  of  swine  the  liquid  portion 
of  the  various  excrements  is  much  the  richer  in  nitrogen, 
containing  on  the  average  more  than  twice  as  much  when 
compared  on  the  percentage  basis.  The  liquid  is  also 
richer  in  potash  than  the  solid,  averaging  for  the  four 
classes  of  animals  1.36  per  cent  as  compared  to  0.34  per 
cent  contained  in  the  solid  manure.  Most  of  the  phos- 
phoric acid,  however,  is  contained  in  the  solid  excrement, 
only  traces  being  found  in  the  urine  except  in  the  case 
of  the  swine.  It  is  therefore  evident  that  the  liquid 
manure,  pound  for  pound,  is  more  valuable  in  so  far  as 
the  plant-food  elements  are  concerned.  The  advantage 
leans  heavily  toward  the  urine  also  in  that  the  constit- 
uents therein  contained  are  immediately  available;  this 
cannot  be  said  of  the  solid  manure. 

489.  Actual  plant-food  in  liquid  and  solid  excrement.  — 
While  the  liquid  manure  carries  more  nutriments  to  an 
equal  weight  than  the  solid,  it  yet  remains  to  be  seen 
which  actually  carries  more  of  the  primary  food  elements. 
In  general,  more  solid  manure  is  excreted  than  liquid, 
tending  to  throw  the  advantage  toward  the  former  in  so 
far  as  total  food  elements  are  concerned.  The  following 
table,  adopted  from  Van  Slyke,1  bears  on  this  point :  — 

1Van  Slyke,  L.  L.  Fertilizers  and  Crops,  p.  295.  New 
York.     1912. 


586       -SOILS:    PROPERTIES  AND  MANAGEMENT 


Distribution   of   Plant-Food   Constituents   between   the 
Liquid  and  the  Solid  of  Whole  Manure 


Excrement 


Horse 

Cow 

Sheep      

Swine 

Average 

Average  for  horse  and 
cow 


Percentage 
of  Total 
Nitrogen 


Solid      Liquid 


62 
49 
52 
67 


57 
55 


38 
51 

48 
33 


43 
45 


Percentage 

op  Total 

Phosphoric 

Acid 


Solid      Liquid 


100 

100 

95 

88 


95 
100 


0 

0 

5 

12 


Percent  \<,i 

op  Total 

Potash 


Solid      Liquid 


56 
15 
30 
57 


40 
35 


44 
85 
70 
43 


60 
65 


It  is  seen  here  that  a  little  more  than  one-half  the 
nitrogen,  almost  all  the  phosphoric  acid,  and  about 
two-fifths  of  the  potash,  are  found  in  the  solid  manure. 
Nevertheless  this  apparent  advantage  of  the  solid  manure 
is  balanced  by  the  ready  availability  of  the  constituents 
carried  by  the  urine,  giving  it  in  total  about  an  equal 
commercial  and  agricultural  value  with  the  solid  excre- 
ment. Such  figures  are  suggestive  of  the  care  that  should 
be  taken  of  the  liquid  manure.  Its  ready  loss  of  nitrogen 
by  fermentation,  and  the  ease  with  which  all  its  valuable 
constituents  may  escape  by  leaching,  should  make  it 
an  object  of  especial  regard  in  handling. 

490.  Production  of  manure.  —  A  well-fed,  moderately 
worked  horse  will  produce  daily  from  45  to  55  pounds 
of  manure,  of  which  from  10  to  11  pounds  is  urine.  A 
cow,  on  the  other  hand,  having  a  greater  food  capacity, 
will  excrete  from  70  to  90  pounds  during  the  same  period, 
of  which  from  20  to  30  pounds  is  liquid.     Swine  and 


FARM  MANURES 


587 


sheep,  varying  so  greatly  in  weight,  will  excrete  such 
widely  different  quantities  that  it  is  difficult  and  mis- 
leading to  express  the  amount  based  on  the  individual. 
A  clearer  method  of  comparison  is  that  employed  below, 
in  which  a  thousand  pounds  in  weight  of  animal  is  made 
the  basis  of  the  calculation  :  — 

Manure  Excreted  by  Various  Farm  Animals  to  the  1000 
Pounds  Live  Weight 


Animal 

Pounds 
a  Day 

Tons  a 
Year 

Horse 1                 

50 
70 
40 
85 
34 

9.1 

Cow2 

12.7 

Steer  3   .          

7.3 

Swine  4 

15.5 

Sheep  5 

6.2 

It  is  quite  evident  that,  for  the  weight  of  animal,  the 
swine  and  the  cow  give  the  heaviest  production  of  manure 
on  the  farm,  but  it  should  be  remembered  also  that  they 
consume  the  greatest  amount  of  food.  Whether  these 
animals  are  the  most  economical  in  production  of  manure 
must  depend  largely  on  age  and  individuality. 

1  Roberts,  I.  P.,  and  Wing,  H.  H.  On  the  Deterioration  of 
Farmyard  Manure  by  Leaching  and  Fermentation.  Cornell 
Univ.  Agr.  Exp.  Sta.,  Bui.  13.  1889.  Also,  Roberts,  I.  P. 
The  Production  and  Care  of  Farm  Manure.  Cornell  Univ. 
Agr.  Exp.  Sta.,  Bui.  27.  1891.  Also,  Watson,  G.  C.  The 
Production  of  Manure.  Cornell  Univ.  Agr.  Exp.  Sta.,  Bui.  56. 
1893. 

2  Thorne,  C.  E.     Farm  Manures,  p.  97.     New  York.     1914. 

3  Thorne,  C.  E.,  and  others.  The  Maintenance  of  Fertility. 
Ohio  Agr.  Exp.  Sta.,  Bui.  183.     1907. 

4  Watson,  G.  C.  The  Production  of  Manure.  Cornell  Univ. 
Agr.  Exp.  Sta.,  Bui.  56.     1893. 

5  Van  Slvke,  L.  L.  Fertilizers  and  Crops,  p.  294.  New 
York.     1912. 


588      SOILS:    PROPERTIES  AND   MANAGEMENT 

491.  Heiden's  formulas.  —  Perhaps  a  better  and  more 
nearly  accurate  means  of  calculating  the  probable  pro- 
duction of  manure  is  from  the  food  consumed,  rather 
than  from  the  combined  weight  of  animals  kept.  Formu- 
las have  been  devised  from  experimental  data  in  Ger- 
many and  are  designated  as  Heiden's  formulas.1  IVoni 
the  amount  of  absolute  dry  matter  fed  and  the  excrement 
produced,  Heiden  was  able  to  determine  certain  definite 
relationships  of  the  latter  to  the  former.  These,  of  course, 
varied  for  different  animals,  being  2.10  for  the  horse,  3.80 
for  the  cow,  and  1.80  for  sheep.  For  example,  if  a  horse 
received  20  pounds  of  dry  matter  daily,  the  manurial 
production  would  be  42  pounds.  Such  formulas  are  of 
particular  value  on  English  farms,  where  the  incoming 
renter  must  pay  the  preceding  tenant  for  the  manure 
produced  on  the  farm  during  previous  years. 

492.  Poultry  manure.  —  The  excrement  from  poultry 
is  extremely  variable,  due  to  causes  that  have  already 
been  discussed.  In  general,  this  manure  is  much  richer 
than  that  from  other  farm  animals.  Storer2  cites  the 
following  analysis :  — 

Composition  of  Poultry  Manure 

Per  cent 

Water 0.56 

Nitrogen 1.60 

Phosphoric  acid 1.75 

Potash     . 0.90 

Lime 2.25 

1  Henry,  W.  A.  Feeds  and  Feeding,  p.  265.  Madison, 
Wisconsin.     1904. 

2  Storer,  F.  H.  Agriculture,  Vol.  1,  p.  613.  New  York. 
1910.  Also,  Vorhees,  E.  B.  Ground  Bone  and  Miscellaneous 
Samples.     New  Jersey  Agr.  Exp.  Sta.,  Bui.  84.     1891.     Also, 


FARM  MANURES  589 

It  is  evident  that  poultry  excrement  is  the  most  valu- 
able manure  produced  on  the  farm.  It  dries  readily 
and  the  loss  of  nitrogen  by  fermentation  is  not  great. 
Because  of  its  great  strength  farmers  are  very  careful 
regarding  its  application,  as  injurious  effects  on  the  crop 
may  result.  Notwithstanding  its  great  value  it  probably 
receives  less  care  than  any  other  manure  produced  on  the 
farm. 

493.  Commercial  and  agricultural  evaluation  of 
manures.  —  For  purposes  of  comparison,  experimenta- 
tion, and  sale,  farm  manures  are  often  evaluated  in  a  way 
similar  to  that  used  with  commercial  fertilizers.  The 
great  difficulty  here  lies  in  arriving  at  prices  for  the  im- 
portant constituents  which  are  at  all  comparable  with  the 
value  of  the  manure  in  the  field.  The  following  figures 
are  calculated  from  the  preceding  tables,  and  show  not 
only  the  comparative  value  of  the  fresh  excrement  from 
different  sources  but  also  what  might  be  considered  as 
fair  prices  a  ton  for  the  manures.  The  value  of  the  nitro- 
gen is  here  placed  at  ten  cents  a  pound,  -the  phosphoric 
acid  at  two  and  one-half  cents,  and  the  potash  at  four 
cents :  — ■ 

Value  of 

manure 

a  ton 

Swine  manure $1.50 

Cow  manure 1.64 

Horse  manure 1.97 

Sheep  manure       2.87 

Poultry  manure 4.80 

Average  of  cow  manure  and  horse  manure  mixed  .  1.80 

Goessman,    C.    A.     Massachusetts    State   Exp.  Sta.,    Bui.    37, 
1890,  and  Bui.  63,  1S96. 


590       SOILS:    PROPERTIES  AND  MANAGEMENT 

This  commercial  evaluation,  of  course,  must  be  applied 
with  care  because  of  the  many  factors  tending  to  vary 
the  composition  of  the  excrement.  Litter,  particularly, 
will  exert  a  great  influence  in  this  direction.  Perhaps  a 
safe  figure  as  regards  the  commercial  value  of  manure 
as  it  is  likely  to  be  handled  on  the  average  farm  is  about 
SI. 50  a  ton.  This  approaches  more  nearly  the  price  that  a 
market  gardener,  for  example,  may  pay  for  such  a  product. 

This  commercial  evaluation  must  never  become  con- 
fused with  what  is  known  as  the  agricultural  value  of  a 
manure.  The  former  is  based  on  composition,  while 
the  latter  arises  from  the  effects  as  measured  in  crop 
growth.  A  manure  of  high  commercial  value  may,  when 
placed  on  the  soil,  yield  only  a  low  to  medium  agricultural 
return.  This  latter  evaluation  is,  of  course,  the  one  of 
greatest  significance  in  agricultural  practice.  A  very 
good  example  of  this  might  be  cited  from  the  Ohio  experi- 
ments *  with  manure.  In  this  case  both  treated  and 
untreated  manures  were  evaluated  commercially  and 
were  then  applied  to  the  land.  The  value  of  the  increased 
crops  in  a  three-years'  rotation  was  then  calculated  in 
terms  of  return  to  a  ton  of  manure  applied :  — 

Commercial  and  Agricultural  Evaluation  of  Manures 


Manure 


Yard  manure  untreated  .  .  .  . 
Yard  manure  plus  floats  .  .  .  , 
Yard  manure  plus  acid  phosphate 
Yard  manure  plus  kainit .  .  .  . 
Yard  manure  plus  gypsum    .     .     , 


Commer- 
cial Value 

Agricul- 
tural 
Value 

$1.41 
2.04 
1.65 
1.45 
1.48 

$2.15 
3.31 
3.67 
2.79 
2.76 

1  Thorne,  C.  E.,  and  others.     The  Maintenance  of  Fertility. 
Ohio  Agr.  Exp.  Sta.,  Bui.  183,  pp.  206-209.     1907. 


FARM  MANURES  591 

In  practice,  then,  it  is  this  agricultural  evaluation  which 
must  be  especially  watched.  Its  expression  should  be 
not  only  in  net  yield  to  the  acre,  but  also  in  net  return 
to  a  ton  of  manure  applied. 

494.  The  fermentation  of  manure.1  —  During  the 
processes  of  digestion  the  food  of  animals  becomes  more 
or  less  decomposed  and  decayed.  This  condition  comes 
about  partly  because  of  the  digestive  processes  and 
partly  from  the  bacterial  action  that  takes  place,  largely 
in  the  lower  intestines.  Of  these  two  influences  within 
the  animal,  bacterial  activities  are  probably  of  the  greater 
importance  as  far  as  the  breaking-up  of  the  complicated 
foodstuffs  is  concerned.  The  fresh  excrement,  then,  as 
it  comes  from  the  stable,  consists  of  decayed  or  partially 
decayed  plant  materials,  with  a  certain  amount  of  broken- 
down  animal  tissue  and  mucus.  This  is  more  or  less 
intimately  mixed  with  litter  and  the  whole  mass  is  wetted, 
or  moistened,  with  the  liquid  excrement,  carrying,  as  it 
does,  considerable  quantities  of  soluble  nitrogen  and 
potash.  This  mass  of  material,  ranging  from  the  most 
complex  compounds  to  the  most  simple,  is  teeming  with 
bacteria,  especially  those  that  function  in  decay  and  putre- 
faction. The  number  very  often  runs  into  billions  to  a 
gram  of  excrement.  In  such  an  environment  it  is  of 
little  wonder  that  biological  changes  go  on  so  rapidly. 

Although  so  many  different  groups  of  organisms  live 
and  function  in  manure,  and  although  so  many  products, 
both  simple  and  complex,  are  continually  being  split 
off,  for  convenience  and  simplicity  the  bacteria  may  be 

1  Good  discussions  may  be  found  as  follows :  Lipman,  J.  G. 
Bacteria  in  Relation  to  Country  Life,  pp.  303-356.  New  York. 
1911.  Hall,  A.  D.  Manures  and  Fertilizers,  pp.  184-210. 
New  York.     1910. 


592       SOILS:    PROPERTIES  AND  MANAGEMENT 

grouped  under  two  heads,  aerobic  and  anaerobic.  The 
former  work  in  the  presence  of  oxygen,  the  latter  when 
air  is  either  lacking  or  only  very  slightly  present.  This 
grouping  is  not  a  distinct  one  by  any  means,  as  many 
organisms  may  function  not  only  in  air  but  also  when 
oxygen  is  lacking.  The  products,  however,  are  as  dif- 
ferent under  these  two  conditions  as  if  they  arose  from 
distinct  organisms. 

495.  Aerobic  action.  —  When  manure  is  first  produced 
it  is  likely  to  be  rather  loose,  and  if  allowed  to  dry  at 
once  it  becomes  well  aerated.  The  first  bacterial  action 
is  therefore  likely  to  be  rather  largely  aerobic  in  nature. 
Transformations  are  very  rapid  and  are  accompanied  by 
considerable  heat,  ranging  from  100°  to  150°  F.  and  some- 
times higher.  This  action  falls  largely  on  the  simple 
nitrogenous  compounds.  Urea  is  principally  affected, 
and  will  very  quickly  disappear  from  well-aerated  manure. 
The  reaction  is  as  follows  :  — 

CON2H4  +  2  H20  =  (NH4)2  C08 

The  ammonium  carbonate  is  a  volatile  compound,  and 
on  the  least  exposure  and  evaporation  of  the  manurial 
liquids  it  changes  into  ammonia  and  carbon  dioxide. 
Thus  nitrogen  may  be  rapidly  lost  from  manure  by  the 
unwise  allowing  of  excessive  aerobic  decay  and  decom- 
position to  proceed. 

This  complex  group  of  aerobic  putrefactive  organisms 
also  attack  to  a  certain  extent  the  more  complicated  ni- 
trogenous compounds,  as  well  as  some  of  the  simpler  car- 
bohydrates contained  in  the  solid  and  the  liquid  portions 
of  the  manure.  More  carbon  dioxide  therefore  results, 
as  well  as  certain  simplified  products  which  ultimately 
may  be  reduced  to  such  a  form  as  to  be  available  as  plant- 


FARM  MANURES  593 

food.  In  other  words,  the  whole  mass  of  the  manure 
tends  to  simpler  forms.  The  mass  becomes  decayed, 
humus  is  produced,  and  available  plant-food  is  evolved. 

496.  Anaerobic  action.  —  As  the  manure  becomes 
compacted,  especially  if  it  is  left  moist,  oxygen  is  grad- 
ually excluded  within  the  heap  and  its  place  is  taken  by 
carbon  dioxide,  which  is  given  off  during  the  process  of 
any  form  of  bacterial  activity.  The  fermentation  now 
changes  from  aerobic  to  anaerobic,  it  becomes  slower,  and 
the  temperature  falls  to  as  low  as  80°  or  90°  F.  New 
organisms  may  now  function,  and  even  some  of  the  same 
ones  that  were  active  under  aerobic  conditions  may  con- 
tinue to  be  effective.  The  process  is  now  a  deep-seated 
one  and  the  products  become  changed  to  a  considerable 
degree.  Carbon  dioxide,  of  course,  continues  to  be  evolved, 
but  instead  of  ammonia  being  formed  the  nitrogenous 
matter  is  converted  into  the  usual  putrefactive  products, 
such  as  indol,  skatol,  and  the  like.  The  carbonaceous 
matter  is  resolved  into  numerous  hydrocarbons,  of  which 
methane  (CH4)  is  prominent;  and  as  a  by-product  of 
the  breaking-down  of  the  proteins,  hydrogen  sulfide 
(H2S)  and  sulfur  dioxide  (S02)  are  evolved.  The  com- 
plex nitrogenous  and  carbohydrate  bodies  are  attacked 
with  the  splitting-off,  not  only  of  simpler  materials,  but 
often  of  those  more  complex.  Such  compounds  may  be 
listed  in  general  as  organic  acids  and  humic  bodies. 
They  of  course  ultimately  succumb  to  simplification. 

497.  Fermentation  in  general.  —  In  any  process  of 
fermentation,  acids  tend  to  form  which  if  not  neutralized 
will  render  the  mass  acid  and  impede  bacterial  activity. 
This  occurs  when  the  solid  excrement  decomposes  alone. 
The  liquid  manure,  however,  is  alkaline  and  will  tend 
to  correct  any  acidity  due  to  fermentation.     The  advan- 

2q 


594       SOILS:    PROPERTIES  AND   MANAGEMENT 


tage  of  either  handling  the  liquid  and  the  solid  together, 
or  pumping  the  liquid  over  the  solid  at  intervals,  is  there- 
fore apparent. 

The  general  changes  in  any  manure  pile  can  readily 
be  recapitulated.  First  is  the  aerobic  action,  with  escape 
of  ammonia  and  carbon  dioxide.  Next  the  manure  is 
wetted,  it  compacts,  and  the  slow,  deep-seated  decay 
sets  in  with  a  simplification  of  some  compounds,  with 
the  production  of  acids,  and  with  a  gradual  formation 
of  humic  materials.  As  the  manure  becomes  alternately 
wet  and  dry,  the  two  general  processes  may  follow  each 
other  in  rapid  succession,  the  anaerobic  bacteria  attack- 
ing the  complex  materials,  the  aerobic  affecting  both  the 
complex  and  the  simpler  compounds.  Carbon  dioxide 
is  given  off  continuously  during  the  process. 

498.  Gases  from  manure.  —  The  changes  in  the 
composition  of  the  gases  drawn  from  wet  and  compact 
manure,  as  compared  with  those  from  the  same  pile  dry 
and  open,  are  well  shown  from  results  by  Deherain.1 
The  pile  in  this  experiment  was  about  eight  feet  high :  — 

Composition  of  Gases  from  Dry  and  Moist  Manure 


Percentage  op 

C02 

o2 

CH« 

N 

[Top 

Dry  manure  <  Middle 

[  Bottom 

Wet        and  f  Top 

compact     I  Middle 

manure      [  Bottom 

7.2 
14.5 

50.8 
42.7 

49.8 

47.8 

7.0 
4.7 
0.0 
1.1 
0.0 
0.0 

0.0 
1.3 
49.2 
52.4 
48.3 
51.2 

85.8 
79.5 
0.0 
9.8 
2.2 
1.0 

1  Hall,  A.  D.    Fertilizers  and  Manures,  p.  188.    New  York.    1910. 


FARM  MANURES 


595 


It  is  noticeable  that  nitrogen  ceases  to  be  lost  under 
anaerobic  conditions,  but  the  production  of  methane  is 
much  increased.     Carbon  dioxide  is  present  at  all  times. 

499.  Change  of  bulk  and  composition  of  rotting  manure. 
—  Because  of  the  great  loss  of  carbon  dioxide  during  the 
fermentation  processes,  there  is  a  considerable  change  in 
bulk  of  the  manure.  Fresh  excrement  loses  20  per  cent 
in  bulk  by  partial  rotting,  40  per  cent  by  more  thorough 
rotting,  and  60  per  cent  by  becoming  completely  decom- 
posed. This  means  that  1000  pounds  of  fresh  manure 
may  be  reduced  to  800,  600,  or  400  pounds,  according 
to  the  degree  of  change  it  has  undergone. 

Although  considerable  loss  of  nitrogen  may  have  oc- 
curred through  aerobic  bacterial  action,  and  although 
both  nitrogen  and  the  minerals  may  have  been  consider- 
ably leached  away,  the  loss  of  carbon  dioxide  is  so  much 
greater  that  generally  there  is  an  actual  percentage  in- 
crease of  the  former  constituents  in  the  well-rotted  prod- 
uct. This  relationship  is  well  shown  by.  figures  from 
Wolff,1  in  which  the  samples  were  compared  on  the  basis 
of  equal  amounts  of  dry  matter :  — 


Composition 

of  Fresh 

and  Decomposed  Manure 

Fresh 
(Per  cent) 

Rotted 
(Per  cent) 

Ash        

3.81 
0.39 
0.45 
0.49 
0.12 
0.18 
0.10 

4.76 

Nitrogen 

0.49 

Potash       

0.56 

Lime 

0.61 

Magnesia 

0.15 

Phosphoric  acid 
Sulfuric  acid 

0.23 

0.13 

1  Aikman,   C.  M. 
burgh  and  London. 


Manures  and  Manuring,  p.  288. 
1910. 


Edin- 


596       SOILS:    PROPERTIES  AND  MANAGEMEST 

It  must  be  remembered,  however,  that  this  is  only 
general  case  and  holds  good  only  when  the  manure  hi 
had  fairly  careful  attention.  When  the  manure  has  Uh 
improperly  handled,  the  soluble  constituents  may  be 
as  soon  as  formed  and  a  rotted  product  may  result  whi( 
is  very  low  in  nitrogen,  potassium,  and  phosphorus.  It 
therefore  evident  that  the  handling  of  the  fresh  manure 
a  controlling  factor  in  the  ultimate  value  of  the  product. 

A  further  insight  into  the  condition  of  rotted  manure 
is  given  by  Voelcker,1  the  data  being  calculated  to  a  dry- 
weight  basis :  — 


ost 


is 


Soluble  organic  matter     . 
Soluble  inorganic  matter 
Insoluble  organic  matter 
Insoluble  inorganic  matter 


Fresh 
Manure 
(Per  cent) 

Rotted 
Manure 
(Per  cent) 

7.33 

15.09 

4.55 

5.98 

76.14 

51.34 

11.98 

27.59 

These  figures  show  the  increased  soluble  matter  in 
well  decomposed  manure  and  emphasize  the  value  of 
rotting.  The  great  loss  of  organic  matter  through  the 
giving-off  of  carbon  dioxide  is  also  evident. 

500.  Fire-fanging  of  manure.  —  A  change  of  a  fermenta- 
tive nature  which  sometimes  takes  place  in  loose  and  well- 
dried  manure  is  fire-fanging.  Many  farmers  consider 
this  to  be  due  to  actual  combustion,  as  the  manure  is  very 
light  in  weight  and  has  every  appearance  of  being  burned. 
This  condition,  however,  is  produced  by  fungi  instead 
of    bacteria,  and  the  dry  and  dusty  appearance  of  the 


1  Halligan,  J.  E. 
ton,  Pennsylvania. 


Soil  Fertility  and  Fertilizers,  p.  67. 
1912. 


Eas- 


FARM  MANURES 


597 


manure  is  due  to  the  mycelium,  which  penetrates  in  all 
directions  and  uses  up  the  valuable  constituents.  Manure 
thus  affected  is  of  little  value  either  as  plant-food  or  as  a 
soil  amendment. 

501.  Waste  of  farm  manures.  —  Any  system  of  agri- 
culture, in  order  to  be  permanent,  must  arrange  for  the 
addition  of  as  much  plant-food  as  is  removed' in  the  crop 
and  the  drainage  water  combined.  Even  if  all  of  the 
crop  were  returned  to  the  soil,  a  permanent  system  of 
agriculture  would  fall  far  short  of  being  established,  since 
at  least  as  much  plant-food  is  removed  by  leaching  as  by 
cropping.  As  a  matter  of  fact,  it  is  not  even  possible  to 
return  to  the  land  as  farm  manure  all  the  constituents 
taken  off  in  the  crop,  due  to  the  ease  with  which  loss  occurs. 
These  losses  may  be  grouped  under  two  general  heads: 
(1)  those  that  occur  as  the  food  passes  through  the  animal ; 
and  (2)  those  that  are  due  to  leaching  and  fermentation. 

502.  Losses  due  to  digestion.  —  A  certain  quantity  of 
material  is  necessarily  taken  from  the  original  food  as  it 
passes  through  the  animal.  This  loss  falls  most  heavily 
on  the  organic  matter  and  only  slightly  on  the  mineral 
constituents.  Wolff  *  presents  the  following  figures  aver- 
aged from  all  classes  of  animals  :  — 


Percentage    of   Original   Food    Constituents    Recovered 
in  Fresh  Manure 


Solid 
Manure 

Liquid 
Manure 

Total 

Organic  matter    .     .     . 

Nitrogen 

Minerals 

42.5 
40.1 
59.7 

3.4 
47.2 
39.0 

45.9 

87.3 
98.7 

1  Aikman,  C.  M.     Manures  and  Manuring,  pp.  228  and  232. 
Edinburgh  and  London.    1910. 


598     SOILS:   properties  and  management 

It  is  to  be  noted  that  the  organic  matter  of  the  food 
has  sustained  an  average  loss  of  about  55  per  cent,  while 
the  loss  of  nitrogen  and  of  minerals  has  been  13  per  cent 
and  2  per  cent,  respectively.  The  loss  of  the  organic 
matter  is  especially  serious,  although  it  can  be  replaced  by 
using  green  manures  and  the  practicing  of  a  proper  rotation. 
The  loss  of  nitrogen  can  be  replaced  only  by  the  growing 
of  legumes  or  by  the  addition  of  a  nitrogenous  fertilizer. 

503.  Losses  due  to  leaching  and  fermentation.  —  As 
about  one-half  of  the  nitrogen  and  two-thirds  of  the  potash 
in  farm  manures  is  in  a  soluble  condition,  the  possibility 
of  loss  by  leaching  is  very  great,  especially  where  the 
manure  is  exposed  to  heavy  rainfall.  The  loss  of  phos- 
phorus is  also  of  some  consequence.  In  addition,  the 
fermentation,  especially  that  of  an  aerobic  nature,  will 
cause  the  formation  of  ammonia,  which  may  be  lost  in 
large  quantities  if  steps  are  not  taken  to  control  such 
action.  It  is  evident  that  losses  by  leaching  may  be 
checked  considerably  by  protecting  the  manures  from 
excessive  rainfall  and  by  providing  tight  floors  in  the 
stable  or  an  impervious  bottom  in  the  manure  pit  or 
under  the  manure  pile.  Packing  and  moistening  the 
manure  will  change  the  aerobic  fermentation  to  anaerobic, 
thus  reducing  very  markedly  the  production  of  ammonia 
wyhile  allowing  a  simplification  of  the  manurial  compounds 
to  proceed  steadily.  All  wise  methods  of  handling  and 
storing  manures  provide  against  these  losses  through 
leaching  and  fermentation  by  protecting  the  manure  from 
rain  and  by  controlling  fermentation  through  moisten- 
ing and  compacting. 

It  is  very  difficult,  in  quoting  figures  for  w^aste  of 
manure,  to  separate  the  losses  due  to  leaching  from  those 
due  to  fermentation.     The  two  processes  go  on  simul- 


FARM  MANURES 


599 


taneously,  and  the  loss  from  one  source  is  dependent,  to 
a  certain  extent,  on  the  other.  It  is  only  the  nitrogen, 
however,  that  may  be  lost  by  both  fermentation  and 
leaching,  the  minerals  being  wasted  only  through  the 
latter  avenue.  A  few  figures  regarding  the  losses  to 
manures  when  exposed  to  atmospheric  conditions  may 
not  be  amiss  at  this  point :  — 

Losses  from  Manure  through  Leaching  and 
Fermentation 


New 

Jersey3 

New 
York1 

New 
York1 

Canada2 

New 
York1 

(Aver- 
age for 
eight 
years) 

Ohio* 

Kind  of  Manure 

Horse 

Horse 

Horse 

Cow 

Cow 

Steer 

Time  exposed  (days) 

183 

183 

274 

183 

77 

91 

Loss   of   nitrogen    (per- 

centage)       .... 

36 

60 

40 

41 

31 

30 

Loss  of  phosphoric  acid 

(percentage) 

50 

47 

16 

19 

19 

23 

Loss  of  potash  (percent- 

age)      

60 

76 

34 

8 

43 

58 

It  seems  evident  that  when  manure  is  exposed  to  at- 
mospheric agencies,  even  under  the  best  conditions,  the 
losses  of  nitrogen,  phosphoric  acid,  and  potash  will  be 
on  the  average  45,   30,   and  50  per  cent,   respectively. 

1  Roberts,  I.  P.,  and  Wing,  H.  H.  On  the  Deterioration 
of  Farmyard  Manure  by  Leaching  and  Fermentation.  Cornell 
Univ.  Agr.  Exp.  Sta.,  Bui.  13.     1889. 

2  Schutt,  M.  A.  Barnyard  Manure.  Canadian  Dept.  Agr., 
Centr.  Exp.  Farms,  Bui.  31.     1898. 

3  Thorne,  C.  E.     Farm  Manures,  p.  146.     New  York.      1914. 

4  Thorne,  C.  E.,  and  others.  The  Maintenance  of  Fertility. 
Ohio  Agr.  Exp.  Sta.,  Bui.  183.     1907. 


600       SOILS:    PBOPERTIES  AND  MANAGEMENT 


Under  conditions  on  the  average  farm  such  losses  may 
easily  rise  to  50  per  cent  of  all  the  constituents,  and  prob- 
ably very  much  higher  as  regards  nitrogen  and  potash. 
From  one-half  to  three-fourths  of  the  important  elements 
contained  in  the  original  food  fails  to  again  reach  the 
land.  Hall,1  quoting  from  Woods'  experiments  at  Cam- 
bridge, shows  that  about  10  per  cent  of  the  nitrogen  in 
the  food  consumed  is  retained  by  the  animal.  He  also 
shows  that  15  per  cent  of  nitrogen  is  lost  during  the  making, 
and  from  10  to  25  per  cent  during  the  storage,  of  the  ma- 
nure, even  under  the  best  conditions.  This  gives  a  total 
loss  of  nitrogen  amounting  to  from  35  to  50  per  cent.  If 
this  is  the  loss  under  the  best  conditions,  it  can  readily 
be  seen  that  the  loss  on  an  average  farm  must  approach 
65  or  75  per  cent. 

Some  idea  as  to  separate  losses  from  fermentation  and 
leaching  may  be  gained  from  data  drawn  from  Canada.2 
In  this  experiment  a  mixture  of  horse  dung  and  cow  dung 
was  divided.  One-half  was  placed  in  a  bin  under  a  shed ; 
the  other  half  was  exposed  to  the  weather,  outside  in  a 
similar  bin.  After  a  year  the  two  portions  were  analyzed 
and  the  losses  computed  :  — 


Losses  from  Manure  after 

Twelve  Months 

Protected 
(Per  cent) 

Unprotected 
(Per  cent) 

Loss  of  organic  matter 

Loss  of  nitrogen 

Loss  of  phosphoric  acid 

Loss  of  potash 

60 

23 

4 

3 

69 
40 
16 
36 

1  Hall>  A-  D-     Fertilizers  and  Manures,  p.  198.     New  York. 

2Schutt,  M.  A.     Barnyard  Manure.     Canadian  Dept.  Agr., 
Centr,  Exp.  Farm,  Bui.  31.     1898. 


FARM  MANURES  601 

Evidently  the  losses  by  fermentation  are  very  consider- 
ably augmented  by  exposure,  especially  if  the  rainfall 
is  high.  This  waste  not  only  is  very  considerable  as 
regards  the  nitrogen,  but  is  especially  high  as  far  as  the 
organic  matter  is  concerned.  Such  figures  serve  also 
to  emphasize  again  the  importance  of  shielding  manure 
in  storage  from  excessive  rainfall.  Some  water  is,  of 
course,  necessary,  but  too  much  serves  only  to  carry 
away  the  materials  already  soluble  or  rendered  soluble 
by  fermentation. 

504.  Increased  value  of  protected  manure.  —  From 
the  previous  discussion  it  is  evident  that  a  well-protected 
and  carefully  preserved  manure  will  be  higher  in  plant- 
food  constituents  than  one  not  so  handled.  Moreover, 
the  agricultural  value  of  such  manure  will  be  higher. 
This  is  shown  by  actual  tests  from  Ohio.1  Over  a  period 
of  fourteen  years,  in  a  three-years'  rotation  of  corn,  wheat, 
and  hay,  a  stall  manure  gave  a  yield  30  per  cent  higher 
than  that  with  a  yard  manure,  the  quantities  applied  in 
each  case  being  equal.  In  New  Jersey,  in  comparing 
fresh  manure  with  leached  manure  the  former  showed  a 
gain  in  crop  yield  53  per  cent  higher  than  the  latter  over 
a  period  of  three  years  immediately  following  the  appli- 
cation. Such  figures  are  worthy  of  careful  considera- 
tion by  the  average  farmer. 

505.  The  money  waste  of  manure.  —  To  make  the 
seriousness  of  the  question  of  waste  in  manures  more 
striking,  the  probable  losses  may  be  calculated  in  money 
value  for  the  United  States.  The  entire  live  stock  of  all 
kinds  in  this  country  may  be  roughly  calculated  as  equiv- 

1  Thorne,  C.  E.,  and  others.  Plans  and  Summary  Tables 
of  the  Experiments  of  the  Central  Farm.  Ohio  Agr.  Exp.  Sta., 
Circ.  120.     1912. 


602       SOILS:    PROPERTIES  AND   MANAGEMENT 

alent  in  manure  producing  capacity  to  about  100,000,000 
cattle,  each  weighing  1000  pounds.  Assuming  that 
each  animal  will  produce  manure  to  the  value  of  $21 
a  year  and  that  the  cattle  are  yarded  for  four  months, 
the  total  value  of  excrement  produced  during  the  yarding 
period  would  be,  in  round  numbers,  8700,000,000.  If 
only  one-third  of  the  value  of  the  manure  is  lost  by  mis- 
handling, an  annual  waste  of  §233,000,000  would  occur. 
This  is  a  very  conservative  estimate  regarding  the  losses 
of  farm  manure  throughout  the  United  States.  The  an- 
nual sale  of  commercial  fertilizers  in  this  country,  prob- 
ably amounting  to  over  $100,000,000,  is  entirely  inade- 
quate to  replace  this  loss. 

506.  Handling  of  manures. *  —  The  ultimate  considera- 
tion in  a  study  of  farm  manures  comprises  the  best 
methods  of  economic  handling,  both  as  to  labor  and  as  to 
the  saving  of  the  constituents  carried  by  the  product.  The 
greater  the  amount  of  plant-food  that  can  be  saved  in 
the  manure  and  returned  to  the  land,  the  less  will  be  the 
necessity  of  commercial  sources  of  these  elements.  Many 
methods  present  themselves  as  being  more  or  less  effica- 
cious, but  none  are  absolutely  perfect,  as  losses  by  fer- 
mentation are  bound  to  occur  even  though  leaching  is 
entirely  prevented.  Methods  of  handling  are  usually 
chosen  because  of  their  adaptability  to  particular  cir- 
cumstances, rather  than  because  of  the  exact  amount 
of  valuable  constituents  that  they  will  conserve. 


1  Good  discussions  of  handling  farm  manure  are  as  follows : 
Hart,  E.  B.  Getting  the  Most  Profit  from  Farm  Manure. 
Wisconsin  Agr.  Exp.  Sta.,  Bui.  221.  1912.  Beal,  W.  H.  Barn- 
yard Manure.  U.  S.  D.  A.,  Farmers'  Bui.  192.  1904.  Roberts, 
I.  P.  The  Fertility  of  the  Land,  Chapter  IX,  pp.  188-213. 
New  York.     1904. 


FARM  MANURES  603 

507.  Care  of  manure  in  the  stalls.  —  Considerable  loss 
to  manure  occurs  in  the  stable,  due  to  fermentation  and 
leaching.  Before  the  litter  can  absorb  the  liquid,  it 
is  likely  to  ferment  and  to  leach  away  in  exceptional 
amounts.  Therefore  the  first  care  is  as  to  bedding,  which 
should  be  chosen  for  its  absorptive  properties,  its  cost, 
and  its  cleanliness.  The  following  table  1  expresses  the 
absorptive  capacity  of  some  common  litters  :  — 

Absorptive  Power  of  Bedding  for  Water 

Per  cent 

Wheat  straw .  220 

Oak  leaves 162 

Peat 600 

Sawdust 435 

Spent  tan       450 

Air-dry  humous  soil 50 

Dry  peat  moss 1300 

Muck 200 

The  amount  of  litter  to  be  used  is  determined  by  the 
character  of  the  food.  If  the  food  is  watery,  the  bedding 
should  be  increased.  In  general,  the  litter  may  amount 
to  about  one-third  of  the  dry  matter  of  the  food  consumed. 
Sheep  require  about  a  pound  of  bedding  a  head,  cattle 
from  eight  to  ten  pounds,  and  horses  from  six  to  seven 
pounds.  No  more  litter  than  is  necessary  to  keep  the 
animal  clean  and  to  absorb  the  liquid  manure  should  be 
used,  as  the  excrement  is  thus  diluted  unnecessarily  with 
material  which  often  does  not  carry  large  quantities  of 
fertilizing  ingredients. 

1  Beal,  W.  H.  Barnyard  Manure.  U.  S.  D.  A.,  Farmers' 
Bui.  192.     1904. 


604       SOILS:    PROPERTIES  AND  MANAGEMENT 

The  next  care  is  that  floors  shall  be  tight,  so  that  free 
liquids  cannot  drain  away  but  will  be  held  in  contact 
with  the  absorbing  materials.  The  preserving  of  manures 
in  stalls  with  tight  floors  has  been  for  years  a  common 
method  of  handling  dung  in  England.  The  trampling 
of  the  animals,  and  the  continued  addition  of  litter  with 
the  liquid  and  solid  excrement,  explain  the  reason  for 
the  success  of  the  method.  The  following  data,  from 
Ohio,1  show  the  relative  recovery  of  food  elements  in 
manure  produced  on  a  cement  floor  and  on  an  earth  floor, 
respectively.  The  experiment  was  conducted  with  steers 
over  a  period  of  six  months. 

Recovery    of   Food    Elements   in   Manure    Produced 
on  Cement  Floor;  on  Earth  Floor 


Per  Cent 


Nitrogen  . 
Phosphorus 
Potash  .     . 


62.4 

78.9 

78.4 


508.  Hauling  directly  to  the  field.  —  Where  it  is  pos- 
sible to  haul  directly  to  the  field,  this  practice  is  to  be 
advised,  since  opportunities  for  excessive  losses  by  leach- 
ing and  fermentation  are  thereby  prevented.  Manure 
may  even  be  spread  on  frozen  ground  or  on  the  top  of 
snow,  provided  the  land  is  fairly  level  and  the  snow  is 
not  too  deep.  This  system  saves  time  and  labor,  and 
when  leaching  does  occur  the  soluble  portions  of  the  ma- 
nure are  carried  directly  into  the  soil. 

509.  Cement  pit.  —  Very  often  it  is  not  convenient 

1  Thome,  C.  E.  The  Maintenance  of  Fertility.  Ohio 
Agr.  Exp.  Sta.,  Bui.  183,  p.  199.     1907. 


FARM  MANURES  605 

nor  possible,  especially  in  certain  parts  of  the  year,  to 
haul  manure  directly  to  the  field.  Means  of  storage 
must  therefore  be  provided.  Some  farmers,  if  the  amount 
of  manure  produced  on  their  lands  is  large,  find  it  prof- 
itable to  construct  manure  pits  of  concrete.  These 
storage  pits  are  usually  rectangular  in  shape,  with  a  shed 
covering,  and  with  open  ends  so  that  a  team  may  drive 
in  at  one  end  and  out  at  the  other.  In  such  a  pit  leaching 
is  prevented  by  the  covering  and  by  the  solid  bottom. 
By  keeping  the  manure  carefully  spread  and  well  mois- 
tened, fermentation  may  proceed  with  a  minimum  loss 
of  nitrogen.  Some  dairymen  even  go  so  far  as  to  utilize 
a  cistern,  into  which  is  shoveled  both  the  liquid  and  the 
solid  manure.  Later,  when  fermentation  has  proceeded 
sufficiently,  the  material  is  pumped  out  and  applied  to 
the  land.  This  method  is  not  to  be  advocated  in  this 
country  except  under  particular  conditions. 

510.  Covered  barnyard.  —  Another  method  of  storage 
is  by  means  of  a  covered  barnyard.  Such  a  yard  must 
have  an  impervious  bottom.  The  manure  is  spread  out 
in  the  yard,  and  if  animals  are  allowed  to  exercise  here 
the  manure  is  kept  thoroughly  packed  as  well  as  damp. 
The  storage  of  manure  in  deep  stalls,  a  favorite  method 
in  England,  is  similar  to  this  system  and  has  been  shown 
to  be  very  economical.  It  also  affords  an  opportunity 
for  the  mixing  of  the  manure  from  different  classes  of 
animals.  The  desirability  of  this  has  already  been  shown 
regarding  horse  and  cow  excrements.  The  advantages 
of  trampling,  as  far  as  the  keeping  qualities  of  manure 
are  concerned,  are  clearly  shown  by  the  following  figures 
taken  from  the  work  of  Frear : *  — 

1  Frear,   W.     Losses   of   Manure.     Pennsylvania  Agr.    Exp. 
Sta.,  Bui.  63.     1903, 


606       SOILS:    PROPERTIES  AND   MANAGEMENT 
Loss  of  Manure  in  Covered  Sheds 


Percentage  op 

N 

K2O 

P*Os 

Covered  and  trampled  .     .     . 
Covered  and  untrampled  .     . 

5.7 
34.1 

5.5 
19.8 

8.5 
14.2 

Throwing  manure  in  heaps  under  a  shed  and  allowing 
hogs  to  work  the  mass  over,  is  an  economical  practice  so 
far  as  food  utilization  is  concerned.  It  interferes,  how- 
ever, with  proper  and  economical  packing  of  the  manure. 
The  question  to  be  decided  is  whether  the  added  food 
value  of  the  manure  overbalances  the  extra  losses  by 
fermentation  incurred  by  the  rooting  of  the  swine. 

511.  Piles  outside.  —  Very  often  it  is  necessary  to 
store  manure  outside,  fully  exposed  to  the  weather. 
When  this  is  the  case,  certain  precautions  must  be  ob- 
served. In  the  first  place,  the  pile  should  be  located  on 
level  ground  far  enough  from  any  building  so  that  it 
receives  no  extra  water  therefrom  in  times  of  storm.  The 
earth  under  the  pile  should  be  slightly  dished  in  order 
to  prevent  loss  of  excess  water.  If  possible,  the  soil 
of  the  depression  should  be  puddled,  or,  better,  lined  with 
cement. 

The  sides  of  the  heap  should  be  perpendicular,  so  as 
to  shed  water  readily.  The  manure  must  be  kept  moist 
in  dry  weather  in  order  to  decrease  aerobic  action.  Each 
addition  of  manure  should  be  packed  in  place,  the  fresh 
on  and  above  the  older.  This  allows  the  carbon  dioxide 
from  the  well-rotted  dung  to  pervade  the  fresher  and 
looser   portions,    thus    quickly    establishing   the    aerobic 


FARM  MANURES  607 

conditions  so  essential  to  economic  and  favorable  fer- 
mentation. 

Placing  fresh  manure  in  small  heaps  in  the  field  to  be 
spread  later,  is,  in  the  first  place,  poor  economy  of  labor. 
Moreover,  it  encourages  loss  by  fermentation,  while  at 
the  same  time  the  soluble  portions  of  the  pile  escape  into 
the  soil  immediately  underneath.  There  is  thus  a  poor 
distribution  of  the  essential  elements  of  the  dung,  and 
when  the  manure  is  finally  spread,  an  overfeeding  of 
plants  at  one  point  and  an  underfeeding  at  another  results. 
A  low  efficiency  of  the  manure  is  thus  realized.  This 
method  of  handling  manure  is  not  to  be  recommended. 

512.  Distribution  of  manure  in  the  field.  —  In  the 
actual  application  of  manure  to  the  land,  certain  general 
principles  should  always  be  kept  in  mind.  In  the  first 
place,  evenness  of  distribution  is  to  be  desired,  since  it 
tends  to  raise  the  efficiency  of  the  manure  by  encouraging 
a  more  uniform  plant  growth.  This  evenness  of  spread- 
ing is  much  aided  by  fineness  of  division.  Moreover,  it 
is  generally  better,  especially  in  diversified  farming  on 
medium  to  heavy  soils,  to  decrease  the  amounts  at  each 
spreading  and  apply  oftener.  Thus,  instead  of  adding 
20  tons  to  the  acre,  10  tons  would  be  applied  and  twice 
as  much  area  covered.  The  applications  would  then 
be  made  oftener.  A  larger  and  quicker  return  in  net 
crop  yield  per  ton  of  manure  applied  would  be  realized. 
This  has  been  strikingly  shown  by  the  Ohio  experiments  1 
over  a  test  for  eighteen  years  in  a  three-years  rotation  of 
wheat,  clover,  and  potatoes,  the  manure  being  placed  on 
the  wheat  and  affecting  the  clover  and  the  potatoes  as  a 

1  Thorne,  C.  E.,  and  others.  Plans  and  Summary  Tables 
of  the  Experiments  at  the  Central  Farm.  Ohio  Agr.  Hxp. 
Sta.,  Circ.  120,  p.  108.     1912. 


608     SOILS:  properties  and  management 

residuum.     The  results  are  expressed  in  yield  per  ton  of 
manure  applied :  — 

Yield  to  the  Ton  of  Manure  when  Applied  in 
Different  Amounts 


Wheat 
(Bushels) 

Clover 
(Pounds) 

Potatoes 
(Bushels) 

4  tons  to  the  acre    .... 

8  tons  to  the  acre    .... 

16  tons  to  the  acre    .... 

8.0 
4.1 
2.4 

177 

150 

99 

37.3 
19.4 
11.6 

Not  only  is  the  increased  efficiency  from  lower  appli- 
cations apparent,  but  a  great  recovery  of  the  manurial 
fertility  in  the  crops  also  results.  The  Ohio  experiments 
have  shown  that  in  the  first  rotation  after  the  manure 
is  applied,  a  recovery  may  be  expected  from  a  treatment 
of  8  tons  25  to  30  per  cent  higher  than  from  one  of 
16  tons. 

Evenness  of  application  and  fineness  of  division  are 
greatly  facilitated  by  the  use  of  a  manure  spreader.  This 
also  makes  possible  the  uniform  application  of  small 
amounts  of  manure,  even  as  low  as  five  or  six  tons  to  the 
acre.  It  is  impossible  to  spread  so  small  an  amount  by 
hand  and  obtain  an  even  distribution.  Moreover,  a 
spreader  lessens  the  labor  and  more  than  doubles  the 
amount  of  manure  one  man  can  apply  a  day.  When 
any  quantity  of  manure  is  to  be  handled,  a  manure  spreader 
will  pay  for  itself  in  a  season  or  two  at  the  most. 

Whether  manure  should  be  plowed  under  or  not  depends 
largely  on  the  crop  on  which  it  is  used.  On  timothy  it 
is  spread  as  a  top-dressing.  Ordinarily,  however,  it  is 
plowed    under.    This    is    particularly    necessary    if    the 


FARM  MANURES  609 

manure  is  long,  coarse,  and  not  well  rotted.  It  should 
not  be  turned  under  so  deep,  however,  as  to  prevent  ready 
decay.  If  manure  is  fine  and  well  decomposed,  it  may 
be  harrowed  into  the  surface  soil.  The  method  employed 
depends  on  the  crop,  the  soil,  and  the  condition  of  the 
manure.  The  amount  to  be  applied  varies  consider- 
ably. Eight  tons  to  the  acre  would  be  a  light  dressing, 
15  tons  a  medium  dressing,  and  25  tons  heavy  for  an 
ordinary  soil.  On  trucking  lands,  however,  as  high  as 
50  or  100  tons  is  often  used. 

513.  Reinforcement  of  manure.  —  The  reinforcement 
of  farm  manures  is  designed  to  accomplish  two  things  in 
the  handling  of  this  product :  (1)  checking  loss  by  leaching 
and  fermentation,  and  (2)  balancing  the  manure  and 
rendering  its  agricultural  value  higher.  Four  chemicals 
may  be  used  in  this  reinforcement :  gypsum  (CaSO^, 
kainit  (KC1,  mostly),  acid  phosphate  (CaH^PO^  + 
CaS04),  and  floats  (raw  rock  phosphate,  Ca3(P04)2). 

Gypsum  is  supposed  to  act  on  the  ammonia,  changing 
it  to  ammonium  sulfate,  a  stable  compound.  It  is  rather 
insoluble,  however,  so  that  its  action  is  slow.  It  may  be 
applied  in  the  stable  or  on  the  manure  pile.  The  rate 
is  about  100  pounds  to  the  ton  of  manure.  It  has  no 
balancing  effect. 

Kainit  is  added  to  react  with  any  ammonia  that  may 
be  produced  and  also  to  increase  the  potash  in  the  manure. 
It  is  soluble,  and  because  of  its  caustic  tendencies  it  must 
not  come  into  contact  with  the  feet  of  the  animals.  It 
must  not  be  spread  on  the  manure,  therefore,  until  the 
stock  has  been  removed.  Since  manure  is  unbalanced 
as  to  phosphorus,  the  agricultural  value  of  this  reinforce- 
ment is  likely  to  be  slight.  Kainit  is  usually  added  at 
the  rate  of  50  pounds  to  the  ton  of  manure. 
2s 


610       SOILS:    PROPERTIES  AND  MANAGEMENT 

Acid  phosphate,  when  used  as  a  reinforcing  agent,  is 
applied  at  the  rate  of  50  pounds  to  the  ton  of  manure. 
It  is  soluble,  and  therefore  becomes  intimately  mixed 
with  the  excrement.  It  adds  phosphorus,  in  which 
manure  is  especially  lacking.  Its  gypsum  may  react 
with  the  ammonia.  Theoretically  it  should  prevent  loss 
by  fermentation,  as  well  as  function  as  a  balancing  agent. 
It  must  not  come  into  contact  with  the  feet  of  farm  ani- 
mals. 

Raw  rock  phosphate,  or  floats,  is  a  very  insoluble  com- 
pound, and  consequently  reacts  but  slowly  with  the 
soluble  constituents  of  manure.  Carrying  such  a  large 
percentage  of  phosphorus,  it  tends  to  balance  the  product 
and  to  raise  its  agricultural  value.  It  is  supposed  that 
the  intimate  relationship  between  the  phosphate  and  the 
decaying  manure  increases  the  availability  of  the  former 
to  plants  when  the  mixture  is  added  to  the  soil.  Xo 
increased  solubility,  however,  as  determined  by  chemical 
means,  has  ever  been  as  definitely  shown  to  occur  (see 
par.  439).  The  reinforcement  is  usually  at  the  rate  of 
100  pounds  to  the  ton  of  manure. 

514.  Benefits  from  reinforcing.  —  Experimental  data 
have  shown  that  these  various  reinforcements  have  no 
effect  on  the  nature,  function,  and. number  of  the  bacterial 
flora.  Their  conserving  influence,  if  any,  when  the  ma- 
nure is  exposed,  must  be  in  checking  leaching  and  in 
preventing  loss  of  ammonia.  The  following  figures 
from  Ohio  experiments  !  show  how  slight  this  conserving 
effect  is.  The  reinforcement  was  at  the  rate  of  40  pounds 
to  the  ton  :  — 


1  Thome,    C.     E.     Maintenance    of    Fertility.     Ohio    Agr. 
Exp.  Sta.,  Bui.  183,  p.  206.     1907. 


FARM  MANURES 


611 


Conserving   Effect   of   Reinforcing   Agents    on   Manure 
Exposed  for  Three  Months 


Treatments 


No  treatment  . 
Gypsum       .     . 
Kainit     . 
Floats     .     .     . 
Acid  phosphate 


Value  op 

a  Ton  of 

Manure 

Percentage 
op  Loss 

In  January 

In  April 

$2.19 

$1.41 

36 

2.05 

1.48 

38 

2.24 

1.45 

35 

2.81 

2.04 

24 

2.34 

1.65 

29 

It  is  immediately  evident  that  kainit  and  gypsum  do  not 
conserve  the  manure,  and,  although  acid  phosphate  and 
floats  show  some  influence,  it  is  slight.  The  principal 
benefit  from  reinforcing  manure,  if  any,  must  therefore 
be  as  a  balancing  agent.  The  figures  from  Ohio  1  over  a 
period  of  fourteen  years  in  a  rotation  of  corn,  wheat,  and 
hay  may  be  taken  as  evidence  regarding  this  point. 
The  manure  was  added  to  the  corn  at  the  rate  of  8  tons 
to  the  acre.  The  reinforcing  was  40  pounds  to  the  ton 
of  manure  in  every  case  :  — 


The  Reinforcing  of  Fresh  Manure 


Treatment 

Total  Net  In- 
creased Value 
op  Crop  to  the 
Rotation 

Net  Increased 

Yield  to  the  Ton 

of  Manure 

Manure  plus  floats       .... 
Manure  plus  acid  phosphate 
Manure  plus  kainit      .... 
Manure  plus  gypsum  .... 
Manure  alone 

$35.94 
38.55 
29.67 

28.48 
26.48 

$4.49 
4.82 
3.71 
3.56 
3.31 

1  Thome,  C.  E.,  and  others.  Plans  and  Summary  Tables 
of  the  Experiments  at  the  Central  Farm.  Ohio  Agr.  Exp.  Sta., 
Cir.  120,  p.  112.     1912. 


612      SOILS:     PROPERTIES  AND  MANAGEMENT 

This  balancing  effect  may  be  shown  in  another  way. 
Let  it  be  supposed  that  to  10  pounds  of  poultry  manure 
having  a  composition  of  1.6  per  cent  nitrogen,  1.5  per 
cent  phosphoric  acid,  and  0.9  per  cent  potash,  there  are 
added  4  pounds  of  sawdust,  4  pounds  of  acid  phosphate, 
and  2  pounds  of  kainit.  The  manure  is  rendered  drier, 
and  its  composition  becomes  0.8  per  cent  nitrogen,  3.7 
per  cent  phosphoric  acid,  and  1.5  per  cent  potash.  It  is 
evident,  from  this  and  the  data  previously  given,  that 
the  principal  benefit  of  reinforcing  manure  lies  in  the 
balancing  influence,  and  that  acid  phosphate  and  floats 
are  the  most  desirable  to  use. 

515.  Lime  and  manure.  —  Very  often  it  would  be  a 
saving  of  labor  to  apply  lime  and  manure  to  the  soil  at 
the  same  time.  This  can  readily  be  done  with  the  car- 
bonated forms.  Such  lime  may  be  mixed  with  the 
manure,  either  in  the  stable  or  in  the  pile,  without  any 
danger  of  detrimental  results.  The  close  union  of  the 
lime  and  the  organic  matter  may  even  increase  the  solu- 
bility of  the  former.  Caustic  compounds  of  lime,  how- 
ever (CaO  and  Ca(OH)2),  must  be  kept  from  manure. 
These  active  forms  react  with  the  ammonium  carbonate 
coming  from  the  urea,  and  cause  the  liberation  of  the 
ammonia,  which  may  be  readily  lost  in  the  air :  — 

CON2H4  +  2  H20  =  (NH4)2C03 
(NH4)2C03  +  Ca(OH)2  =  CaC03  +  2  NH4OH 

A  stable  or  a  shed  containing  manure  may  be  at  once 
deodorized  by  the  use  of  quicklime,  but  only  by  the  loss 
of  much  nitrogen,  which  costs  on  the  market  eighteen  or 
twenty  cents  a  pound.  Caustic  lime  and  manure  may  be 
applied  to  the  same  soil  by  applying  the  lime  ten  days 
or  two  weeks  before  the  manure.     The  lime  will  then 


FARM  MANURES  613 

have  had  time  to  leach  into  the  soil  or  to  largely  change 
to  a  carbonate  form. 

516.  Composting.  —  A  compost  is  usually  made  up  of 
alternate  layers  of  manure  and  some  vegetable  matter 
that  is  to  be  decayed.  Layers  of  sod  or  of  humous  soil 
are  often  introduced.  The  manure  is  used  to  supply 
the  decay  organisms  and  to  start  the  action.  The  foun- 
dation of  such  a  humus  manufactory- is  usually  soil,  and 
the  pile  is  preferably  capped  with  earth.  The  compost 
should  be  kept  moist  in  order  to  prevent  loss  of  ammonia 
and  to  encourage  vigorous  bacterial  action.  Acid  phos- 
phate or  raw  rock  phosphate  and  a  potassium  fertilizer 
are  often  added,  to  balance  up  the  mixture  and  make  it  a 
more  effective  fertilizer.  Lime  is  also  introduced,  to  react 
with  such  organic  acids  as  may  tend  to  form  and  to  inter- 
fere with  proper  decay.  Undecayed  plant  tissue,  such  as 
sod,  leaves,  weeds,  grass,  sticks,  or  organic  refuse  of  any 
kind,  may  thus  be  changed  slowly  to  a  humus  which  will  be 
valuable  in  building  up  the  soil  and  in  nourishing  plants. 
Even  garbage  may  be  disposed  of  in  such  a  manner. 

517.  Manure  and  muck.  —  Muck  soil  recently  re- 
claimed from  a  swamp  condition  is  usually  treated,  if 
possible,  with  a  dressing  of  manure.  This  is  not  so  much 
for  the  purpose  of  adding  plant-food  as  to  supply  decay 
and  decomposition  organisms  that  will  break  down  the 
complicated  humic  compounds  into  such  forms  as  may 
be  utilized  by  the  crop.  Plenty  of  lime  is  therefore  essen- 
tial in  muck,  in  order  to  render  the  effects  of  this  inocu- 
lation effective  and  lasting. 

518.  Effects  of  manure  on  the  soil.  —  The  direct  fer- 
tilizing effect  of  manure  is  by  no  means  its  greatest 
influence.  In  the  first  place,  manure  as  it  rots  down 
produces  humus.     This  humus  increases  the  absorptive 


614     soils:  properties  and  management 

capacity  of  the  soil.  In  clays  it  promotes  granulation, 
while  in  sands  it  acts  as  a  binding  agent.  Under  all  con- 
ditions it  promotes  granulation  and  tilth.  The  capacity 
of  a  soil  to  resist  drought  is  raised;  its  aeration  is  in- 
creased and  drainage  is  promoted.  All  these  changes 
tend  to  benefit  plant  growth  and  to  produce  those  indirect 
fertilizing  effects  that  are  characteristic  of  farm  manure. 

From  the  chemical  standpoint,  the  presence  of  manure 
in  the  soil  tends  to  increase  organic  acids,  notably  car- 
bonic acid.  The  soil  minerals  are  thus  rendered  more 
easily  soluble.  The  case  of  the  influence  of  manure  on 
the  action  of  raw  rock  in  the  soil  has  already  been  cited. 
The  humus,  also,  may  combine  with  certain  of  the  mineral 
elements  and  hold  them  in  a  form  more  easily  available 
to  crops.  Nor  is  the  chemical  influence  of  farm  manure 
the  final  effect.  The  modification  of  the  soil  flora  can 
by  no  means  be  passed  by.  Not  only  are  millions  of 
organisms  added  by  an  application  of  manure,  but  those 
already  present  in  the  soil  are  vastly  stimulated  by  this 
fresh  acquisition  of  humic  materials.  Nitrification,  am- 
monification,  and  nitrogen  fixation  are  all  increased  to  a 
remarkable  degree. 

519.  Residual  effect  of  manure.  —  No  other  fertilizing 
material  exerts  such  a  marked  residual  effect  as  does 
manure.  This  is  partly  because  of  its  indirect  physical 
and  biological  influences,  and  partly  because  of  the  stimu- 
lated root  development  of  the  crops  grown.  The  greatest 
residual  influence,  however,  is  brought  about  by  the  slowly 
decomposable  nature  of  the  manure,  only  a  small  per- 
centage being  recovered  in  the  first  crop  grown  after  the 
manure  is  applied.     Hall1  presents  the  following  data 

1  Hall,  A.  D.  Fertilizers  and  Manure,  p.  210.  New  York, 
1910. 


FARM  MANURES 


615 


from  Rothamsted.  The  crop  was  mangolds,  and  the  re- 
covery of  the  constituents  carried  by  the  manure  was 
very  low :  — 

Recovery  of  Nitrogen  in  a  Crop  of  Mangolds 


Treatment 

Per  Acre 

Yield  in 
Tons 

Percentage 
Recovery 

Nitrate  of  soda    .     . 
Ammonium  salts 
Rape  cake  .... 
Manure 

550  pounds 
400  pounds 
2000  pounds 
14  tons 

17.95 
15.12 
20.95 
17.44 

78.1 
57.3 
70.9 
31.6 

The  length  of  time  through  which  the  effects  of  an 
application  of  farm  manure  may  be  detected  in  crop 
growth  is  very  great.  Hall 1  cites  data  from  the  Roth- 
amsted experiments  in  which  the  effects  of  eight  yearly 
applications  of  14  tons  each  were  apparent  forty  years 
after  the  last  treatment.  This  is  an  extreme  case;  ordi- 
narily, profitable  increases  may  be  obtained  from  manure 
only  from  two  to  five  years  after  the  treatment.  The  fact 
remains,  nevertheless,  that  of  all  fertilizers  farm  manure 
is  the  most  lasting,  lends  the  most  stability  to  the  soil, 
and  is  really  a  soil  builder  par  excellence. 

520.  Place  of  manure  in  the  rotation.  —  With  a  num- 
ber of  trucking  crops,  the  application  of  manure  directly 
to  the  crop  year  after  year  has  proved  to  be  advisable. 
In  an  ordinary  rotation,  however,  where  less  intensive 
methods  are  employed,  it  is  evident  that  manure  may 
vary  in  its  effect  according  to  the  place  in  the  rotation  at 


1  Hall,  A.  D. 
1910. 


Fertilizers  and  Manure,  p.  213.     New  York. 


616       SOILS:    PROPERTIES  AND  MANAGEMENT 

which  it  is  applied.  This  has  proved  to  be  the  case  with 
commercial  fertilizers,  and  the  fact  is  also  becoming 
recognized  in  the  economic  use  of  farm  manures. 

In  general,  hay  has  derived  more  benefit  from  the  re- 
sidual food  than  almost  any  other  crop  in  the  rotation. 
At  the  Pennsylvania  Experiment  Station,1  in  a  rotation  of 
corn,  wheat,  and  hay  over  a  test  for  twenty-five  years,  in 
which  manure  was  applied  in  equal  amounts  to  the  corn 
and  wheat,  the  results  were  as  follows :  — 


Percentage  Increase  from  Use  of  Manure,  and  Value  of 
that  Increase 


Treatment 

Corn 

Oats 

Wheat 

Hat 

6  tons  manure 
Cost  $9       .     . 

37  per  cent 
$10.85 

28  per  cent 
$3.66 

73  per  cent 
$9.70 

39  per  cent 
$6.55 

The  same  fact  has  been  clearly  shown  in  the  Ohio 
experiments 2  covering  a  term  of  eighteen  years.  The 
query  immediately  arising  here  is :  If  hay  responds  so 
well  to  residual  feeding,  why  not  apply  the  manure 
directly  to  it?  On  this  point  the  following  figures  from 
the  Illinois  Experiment  Station  3  may  be  presented,  com- 
paring the  response  of  corn  and  oats  when  manured  to 
the  yield  of  clover  with  the  same  treatment :  — 


1  Hunt,  T.  F.  General  Fertilizer  Experiments.  Ann.  Rept. 
Pennsylvania  Agr.  Exp.  Sta.,  1907-1908,  pp.  68-93. 

2Thorne,  C.  E.,  and  others.  Plans  and  Summary  Tables 
of  the  Experiments  at  the  Central  Farm.  Ohio  Agr.  Exp.  Sta., 
Cir.   120,  pp.   101-105.     1912. 

3  Hopkins,  C.  G.  Thirty  Years  of  Crop  Rotation  in  Illinois. 
111.  Agr.  Exp.  Sta.,  Bui.  125,  p.  337.     1908. 


FARM  MANURES 


617 


Treatment 

Average  Percentage 
Increase 

Total  Value  of 
Increase 

Corn  and 
Oats 

Clover 

Corn  and 
Oats 

Clover 

Manure 

Manure,       lime,       and 
phosphate     .... 

11 

30 

92 
141 

$   7.53 

12.21 

$10.08 
15.48 

When  hay  is  included  in  any  rotation  it  is  evident  that 
the  best  results  from  manure  may  be  obtained  by  placing 
it  on  this  crop.  This,  however,  is  often  not  advisable, 
especially  where  the  amount  of  manure  is  limited.  A 
commercial  fertilizer  may  take  its  place  on  the  hay,  al- 
lowing the  farm  manure  to  be  utilized  on  special  crops. 
When  applied  to  hay  it  should  be  spread  as  a  light  top- 
dressing.  When  manure  is  used  for  such  a  crop  as  corn, 
however,  it  is  best  plowed  under,  as  the  amounts  added 
per  acre  are  often  large.  Farm  manure  in  judicious 
amounts  may  be  harrowed  in  or  plowed  under  in  orchards. 

521.  Resume.  —  From  the  general  discussion  already 
presented,  it  is  evident  that  barnyard  manure,  from  the 
standpoint  of  soil  fertility,  is  the  most  valuable  by-product 
of  the  farm.  A  careful  farmer  will  therefore  attempt  to 
utilize  it  in  the  most  economical  way.  The  handling  of 
manure  in  such  a  manner  that  only  a  small  waste  will 
occur  from  the  time  when  the  manure  is  voided  until  it 
has  reached  the  land  again,  is  not  an  easy  problem. 
Manure  is  so  susceptible  to  the  loss  of  valuable  ingredients, 
both  by  leaching  and  by  fermentation,  that  careful  methods 
must  be  employed.  The  utilization  of  tight  floors  in  the 
stable  and  of  covered  sheds  or  manure  pits  is  to  be  ad- 


618      SOILS:    PROPERTIES  AND  MANAGEMENT 

vised.  Hauling  immediately  to  the  field  is  a  wise  pro- 
cedure. Yet  even  with  the  best  of  care  a  loss  of  from  30 
to  50  per  cent  is  often  incurred.  A  permanent  system  of 
agriculture  evidently  cannot  be  established  by  simply 
returning  all  the  manure  possible  to  the  land.  Neverthe- 
less, it  is  certainly  worth  the  while  of  any  farmer  to  use 
at  least  some  care  in  the  handling  of  this  product.  Even 
reasonable  attention  would  save  for  the  soils  of  this  coun- 
try thousands  of  dollars'  worth  of  manurial  fertility 
which  is  now  carried  awav  in  the  streams  and  rivers. 


CHAPTER  XXVII 
GREEN  MANURES1 

From  time  immemorial  the  turning-under  of  a  green 
crop  to  supply  organic  matter  to  the  soil  has  been  a  com- 
mon agricultural  practice.  Records  show  that  the  use  of 
beans,  vetches,  and  lupines  for  such  a  purpose  was  well 
understood  by  the  Romans,  who  probably  borrowed  the 
practice  from  nations  of  still  greater  antiquity.  The  art 
was  lost  to  a  great  extent  during  the  Dark  Ages,  but  was 
revived  again  as  the  modern  era  was  approached.  At 
the  present  time  green-manuring  is  considered  a  part  of 
a  well-established  system  of  soil  management,  and  is 
given  a  place,  where  possible,  in  every  rational  plan  for 
permanent  soil  improvement. 

522.  Effects  of  green-manuring.  —  The  effects  of  turn- 
ing under  green  plants  are  both  direct  and  indirect  — 
direct  as  to  the  influence  on  the  succeeding  crop,  and  in- 
direct as  to  the  action  on  the  physical  condition  of  the 
soil  so  treated.  In  the  first  place,  certain  ingredients  are 
actually  added  to  the  soil  by  such  a  procedure.  The  car- 
bon, oxygen,  and  hydrogen  of  a  plant  come  largely  from 

1  Penny,  C.  L.  Cover  Crops  as  Green  Manures.  Delaware 
Agr.  Exp.  Sta.,  Bui.  60.     1903. 

Storer,  F.  H.     Agriculture,  pp.  137-175.     New  York.      1910. 

Lipman,  J.  G.  Bacteria  in  Relation  to  Country  Life,  Chapter 
XXIV,  pp.  237-263.     New  York.     1911. 

Piper,  C.  V.  Leguminous  Crops  for  Green  Manuring. 
U.  S.  D.  A.,  Farmers'  Bui.  No.  278.     1907. 

Spillman,  W.  J.  Renovation  of  Worn-out  Soils.  U.  S.  D.  A., 
Farmers'  Bui.  No.  245.     1906. 

619 


620      SOILS:    PROPERTIES  AND  MANAGEMENT 

the  air,  and  the  plowing-under  of  a  crop  therefore  in- 
creases the  store  of  such  constituents  in  the  soil.  It  the 
plant  is  a  legume  and  the  nodule  organisms  are  active, 
the  nitrogen  content  of  the  soil  is  also  augmented.  The 
mineral  parts  of  the  turned-under  crop,  of  course,  come 
from  the  soil  originally  and  they  are  merely  turned  back 
to  it  again.  As  they  return,  however,  they  are  in  intimate 
union  with  organic  materials,  and  are  thus  readily  avail- 
able as  plant-food  as  the  decay  process  goes  on.  Indeed 
they  are  much  more  readily  available  than  they  previously 
were,  when  the  green-manuring  crop  acquired  them. 
Actual  additions  are  thus  made  to  the  soil,  together  with 
a  promotion  of  an  increased  availability  of  the  constit- 
uents dealt  with. 

Green  manures  may  function  also  as  cover  crops,  in 
so  far  as  they  take  up  the  extremely  soluble  plant-food 
and  prevent  it  from  being  lost  in  the  drainage  water.  The 
nitrates  of  the  soil  are  of  particular  importance  in  this 
regard,  as  they  are  very  soluble  and  are  adsorbed  only 
slightly  by  the  soil  particles.  Besides  this,  green  manures, 
especially  those  with  long  roots,  tend  to  carry  food  up 
from  the  subsoil,  and  when  the  crop  is  turned  under 
this  material  is  deposited  within  the  root  zone.  Again, 
the  added  organic  material  acts  as  a  food  for  bacteria, 
and  tends  to  stimulate  biological  changes  to  a  marked 
degree.  This  bacterial  action  is  especially  prone  to  in- 
crease the  production  of  carbon  dioxide,  ammonia,  ni- 
trates, and  organic  acids  of  various  kinds,  which  are  very 
important  in  plant  nutrition.  The  humus  that  results 
from  this  decay  increases  the  adsorptive  power  of  the  soil, 
and  promotes  aeration,  drainage,  and  granulation  —  con- 
ditions that  are  extremely  important  in  successful  crop 
growth. 


GREEN  MANURES  621 

523.  Quantities  of  plant  constituents  added  by  green- 
manuring.  —  In  an  average  crop  of  green  manure,  from 
five  to  ten  tons  of  material  is  turned  under.  Of  this, 
from  one  to  two  tons  is  dry  matter,  and  from  four  to  eight 
tons  water.  Of  this  dry  matter  a  great  proportion  is  car- 
bon, hydrogen,  and  oxygen  —  a  clear  gain  to  the  soil  in 
so  far  as  these  constituents  are  concerned.  The  amount 
of  nitrogen  added  to  a  soil  if  the  green  manure  is  a  legume  * 
is  a  difficult  question  to  decide.  Much  depends  on  the 
virulence  of  the  organisms  occupying  the  nodules.  These 
bacteria  are  in  turn  much  influenced  by  plant  and  soil 
conditions.  Hopkins  2  estimates  that  about  one-third  of 
the  nitrogen  in  a  normal  inoculated  legume  comes  from 
the  soil  and  two-thirds  from  the  air.  He  also  considers 
that  one-third  of  the  nitrogen  exists  in  the  roots.  It  is 
evident,  therefore,  that  in  general  the  nitrogen  found  in 
the  tops  will  be  a  rough  measure  of  the  nitrogen  fixed  by 
the  soil  organisms.  If  this  is  returned  to  the  soil,  there 
is  a  clear  gain  of  just  that  amount. 

If  the  preceding  assumption  is  correct,  clover3  would 
actually  add  to  every  acre  about  40  pounds  of  nitrogen 


1  Smith,  CD.,  and  Robinson,  F.  W.  Influence  of  Nodules 
on  the  Roots  upon  the  Composition  of  Soybean  and  Cowpea. 
Mich.  Agri.  Exp.  Sta.,  Bui.  224.     1905. 

Hopkins,  C.  G.  Alfalfa  on  Illinois  Soil.  Illinois  Agr.  Exp. 
Sta.,  Bui.  76.     1902. 

Hopkins,  C.  G.  Nitrogen  Bacteria  and  Legumes.  Illinois 
Agr.  Exp.  Sta.,  Bui.  94.     1904. 

Shutt,  F.  T.  The  Nitrogen  Enrichment  of  Soils  through  the 
Growth  of  Legumes.  Canadian  Dept.  Agr.,  Rept.  Centr.  Exp. 
Farms,   1905,  pp.    127-132. 

2  Hopkins,  C.  G.  Soil  Fertility  and  Permanent  Agriculture, 
p.  223.     Boston,  1910. 

3  Penny,  C.  L.  The  Growth  of  Crimson  Clover.  Delaware 
Agr.  Exp.  Sta.,  Bui.  67.     1905. 


622      SOILS:    PROPERTIES  AND  MANAGEMENT 

per  ton,  alfalfa  about  50,  cowpeas  43,  and  soy  beans 
53  pounds.  These  figures,  even  though  they  may 
be  far  from  correct,  at  least  give  some  idea  as  to  the 
possible  addition  of  nitrogen  by  green-manuring  prac- 
tices, and  show  how  the  soil  may  be  enriched  by  such 
management.  As  in  the  case  of  farm  manures,  the  in- 
direct effects  of  such  a  procedure  may  override  the 
direct  influences,  making  the  use  of  legumes  as  green- 
manuring  crop  less  necessary  than  at  first  thought  might 
be  supposed. 

524.  Decay  of  green  manure.  —  As  a  green  crop  enters 
the  soil,  the  process  of  its  decay  is  the  same  as  that  of 
any  plant  tissue  that  becomes  a  part  of  the  soil  body. 
The  organisms  that  are  active  are  those  common  to  the 
soil,  together  with  such  bacteria  as  are  carried  into  the 
soil  on  the  turned-under  crop.  The  decay-  should  be 
accomplished  under  aerobic  conditions  so  that  only 
beneficial  products  may  result.  Plenty  of  water  is  a 
necessity,  as  otherwise  the  soil  would  be  robbed  of  a 
part  of  its  available  moisture  in  facilitating  the  process  of 
decay.  When  proper  decay  has  occurred,  end  products 
should  result  which  can  be  utilized  as  plant-food.  The 
intermediate  compounds  that  are  formed  should  yield  a 
black  humus,  should  readily  split  up  into  simple  com- 
pounds, and  should  be  in  general  beneficial,  both  directly 
and  indirectly,  to  crop  growth.  The  decay  of  green 
manure  under  conditions  of  poor  drainage  and  improper 
aeration  is  likely  to  cause  the  generation  of  materials 
detrimental  to  the  proper  development  of  plants. 

525.  Crops  suitable  for  green  manures.  —  The  crops 
that  may  be  utilized  as  green  manures  are  usually  grouped 
under  two  heads,  legumes  and  non-legumes.  Some  of  the 
common  green  manures  are  as  follows:  — 


GREEN  MANURES 


623 


Legu 

raes 

Non-legumes 

Annual 

Biennial 

Cowpea 

Red  clover 

Rye 

Soy  bean 

White  clover 

Oats 

Peanut 

Alsike  clover 

Mustard 

Vetch 

Alfalfa 

•     Mangels 

Canada  field  pea 

Sweet  clover 

Rape 

Velvet  bean 

Buckwheat 

Crimson  clover 

Hairy  vetch 

When  other  conditions  are  equal,  it  is  of  course  always 
better  to  choose  a  leguminous  green  manure  in  preference 
to  a  non-leguminous  one,  because  of  the  nitrogen  that  may 
be  added  to  the  soil.  However,  it  is  so  often  difficult  to 
obtain  a  catch  of  some  of  the  legumes  that  it  is  poor 
management  to  turn  the  stand  under  until  after  a  number 
of  years.  Again,  the  seed  of  many  legumes  is  very  expen- 
sive, almost  prohibiting  their  use  as  green  manures. 
Among  the  legumes  most  commonly  grown  as  green  ma- 
nures, cowpeas,  soy  beans,  and  peanuts  may  be  named. 
Many  of  the  other  legumes  do  not  so  fit  into  the  common 
rotations  as  to  be  handily  turned  under  as  a  green  manure. 

For  the  reasons  already  cited,  the  non-legumes  have  in 
many  cases  proved  the  more  popular  and  economic  as 
green  manures.  Rye  and  oats  are  much  used  because 
of  their  rapid,  abundant,  and  succulent  growth  and  be- 
cause they  may  be  accommodated  to  almost  any  rotation. 
They  are  hardy  and  will  start  on  almost  any  kind  of  a 
seed  bed.  They  are  thus  extremely  valuable  on  poor  soils. 
Often  the  value  of  such  a  green  manure  as  oats  is  greatly 
increased  by  sowing  peas  with  it.  The  advantages  of  a 
legume  and  a  non-legume  are  thus  combined. 


624       SOILS:    PBOPEBTIES  AND  MANAGEMENT 

526.  When  to  use  green  manures. — The  indiscrimi- 
nate use  of  green  manures  is  of  course  never  to  be  ad- 
vised, as  the  soil  may  be  injured  thereby  and  the  normal 
rotation  much  interfered  with.  When  soils  are  poor  in 
nitrogen  and  humus,  they  are  very  often  in  poor  tilth. 
This  is  true  whether  the  texture  of  the  soil  be  fine  or 
coarse.  The  turning-under  of  green  crops  must  be  judi- 
cious, however,  in  order  that  the  soil  may  not  be  clogged 
with  undecayed  matter.  Once  or  twice  in  a  rotation  is 
usually  often  enough  for  such  treatments.  Proper  drain- 
age must  always  be  provided.  In  regions  where  the  rain- 
fall is  scanty,  very  great  caution  must  be  observed  in  the 
handling  of  green  manures.  The  available  moisture  that 
should  go  to  the  succeeding  crop  may  be  used  .in  the 
process  of  decay,  and  the  soil  left  light  and  open,  due  to  an 
excess  of  undecomposed  plant  tissue. 

527.  When  to  turn  under  green  crops.  —  It  is  generally 
best  to  turn  under  green  crops  when  their  succulence  is 
near  the  maximum.  In  this  case  a  large  quantity  of  water 
is  carried  into  the  soil,  and  the  draft  on  the  original  soil 
moisture  is  less.  Again,  the  succulence  encourages  a 
rapid  and  more  or  less  complete  decay,  with  the  maximum 
production  of  humus  and  end  products.  The  plowing 
should  be  done,  if  possible,  at  a  season  when  a  plentiful 
supply  of  rain,  occurs.  The  effectiveness  of  the  manuring 
is  thereby  much  enhanced. 

528.  How  to  turn  under  green  material.  —  In  general, 
in  turning  under  green  manures  the  furrow  slice  should 
not  be  thrown  over  flat,  since  the  green  crop  is  then  de- 
posited as  a  continuous  layer  between  the  surface  soil 
and  the  subsoil.  Capillary  movement  is  thus  impeded 
until  a  more  or  less  complete  decay  has  occurred,  and  the 
succeeding  crop  may  suffer  from  lack  of  moisture. 


GREEN  MANURES  625 

The  furrow  ordinarily  should  be  turned  only  partly 
over,  and  thrown  against  and  on  its  neighbor.  The  green 
manure  is  then  distributed  evenly  from  the  surface  down- 
ward to  the  bottom  of  the  furrow.  When  decomposition 
occurs  the  humic  materials  are  evenly  mixed  with  the 
whole  furrow  slice.  Moreover,  this  method  of  plowing 
does  not  interfere  with  the  capillary  movements  of  water, 
and  in  actual  practice  is  a  great  aid  in  drainage  and 
aeration. 

529.  Green  manures  and  lime.  —  The  decay  of  organic 
matter  in  the  soil  is  always  accompanied  by  the  produc- 
tion of  organic  acids.  Such  acids  tend  to  form  in  large 
amount,  especially  if  the  fermenting  matter  is  of  a  suc- 
culent nature.  The  need  of  plenty  of  lime  under  such 
conditions  is  clearly  apparent,  as  a  soil  of  a  neutral  or  an 
acid  character  may  assume  a  bad  condition  during  the 
process  of  humic  decay.  Lime  may  be  added  to  the  green- 
manure  seeding  and  be  turned  under  with  that  crop. 
The  amendment  would  thus  be  in  very  close  contact  with 
the  decaying  vegetable  tissue.  Ordinarily,  however,  the 
application  of  lime  at  some  point  in  the  rotation  is  suffi- 
cient. 

530.  Green  manure  and  the  rotation.  —  Very  often  it 
is  somewhat  of  a  problem  as  to  when,  in  an  ordinary  rota- 
tion, a  green  manure  may  be  introduced  so  that  it  may 
fit  in  well  with  the  crops  grown.  In  a  rotation  of  corn, 
oats,  wheat,  and  two  years  of  hay,  a  green  manure  might 
be  introduced  after  the  corn.  This  would  not  be  a  very 
good  practice,  however,  as  a  cultivated  crop  should 
usually  follow  a  green  manure  so  as  to  facilitate  decom- 
position and  decay.  In  such  a  rotation  the  plowing- 
under  of  the  hay  stubble  is  really  a  form  of  green-manur- 
ing,  there   being  a   considerable   accumulation  of  roots, 

2s 


626       SOILS:    PROPERTIES  AND  MANAGEMENT 

stubble,  and  aftermath  on  the  soil.  When  a  rotation  of 
this  kind  is  used  it  is  better  either  to  supply  organic 
matter  in  other  ways,  or  to  alter  or  break  the  rotation  in 
such  a  manner  as  to  admit  of  a  more  advantageous  use 
of  green  crops. 

Where  trucking  crops  are  grown  and  no  very  definite 
rotation  is  adhered  to,  green-manuring  is  easier.  It  is 
especially  facilitated  when  cover  crops  are  grown,  as  in 
orchards.  Soiling  operations  also  favor  the  easy  and 
profitable  use  of  green  manures.  In  general  it  may  be 
said  that  the  organic  matter  obtained  from  such  a  source 
should  be  supplemented  by  farmyard  manures  where 
possible.  A  better  balanced  and  richer  soil  humus  is 
more  likely  to  result. 


CHAPTER  XXVIII 
LAND   DRAINAGE 

Land  drainage  l  is  the  process  of  withdrawing  from  the 
soil  the  superfluous  or  gravitational  water  occurring  in 
the  larger  spaces  within  the  normal  root  zone.  Excess 
moisture  in  the  soil  interferes  with  ventilation,  keeps 
down  the  temperature,  and  seriously  disturbs  the  physical 
nature  of  the  soil.  Any  means  that  permits  the  free  flow 
from  the  soil  of  the  gravitational  water  affords  drainage. 
Many  methods  are  used,  according  to  circumstances. 
Indications  of  the  need  of  drainage  are  the  presence  of 
free  water  in  the  surface  soil  and  in  excavations  into  the 

1  Elliott,    C.    G.     Engineering    for    Land    Drainage.     New. 
York.     1912. 

Faure,  L.  Drainage  et  Assainissement  Agricole  des  Terres. 
Paris.     1903. 

King,  F.  H.  Irrigation  and  Drainage,  Part  II.  New  York. 
(Revised  edition.     1909.) 

Klippart,  J.  H.  Principles  and  Practice  of  Land  Drainage. 
Cincinnati.     1894. 

Woodward,  S.  M.  Land  Drainage  by  Means  of  Pumps. 
U.  S.  D.  A.,  Office  Exp.  Sta.,  Bui.  No.  243.     1911. 

Warren,  G.  M.  Tidal  Marshes  and  their  Reclamation. 
U.  S.  D.  A.,  Office  Exp.  Sta.,  Bui.  No.  240.     1911. 

Elliott,  C.  G.  Drainage  of  Farm  Lands.  U.  S.  D.  A., 
Farmers'  Bui.  No.   187.     1904. 

Miles,  M.     Land  Drainage.     New  York.     1897. 

See  also  the  following  bulletins  of  state  experiment  stations : 
Michigan,  Sp.  56;  Maryland,  186;  New  York  (Cornell),  254; 
Utah,  123;  Wisconsin,  138,  199,  229;  Ontario,  Canada,  174, 
175. 

627 


628       SOILS:    PROPERTIES  AND  MANAGEMENT 

subsoil;  and  the  tendency  of  the  soil  to  puddle  and  bake 
when  dry.  When  the  wetness  is  prolonged,  the  accu- 
mulation of  organic  matter  in  the  surface  soil  imparts  a 
dark  color.  Poor  drainage  causes  a  mottled  color  in  the 
subsoil,  and  in  extreme  cases  a  pale  gray  color  resulting 
from  excessive  leaching.  When  the  land  is  in  crops  the 
wet  places  are  recognized  by  their  miry  condition  in  early 
spring  and  after  rains,  and  by  the  slow  starting  of  the 
crop.  In  meadows  the  grass  is  often  winterkilled,  leaving 
only  those  weeds  that  can  withstand  the  conditions. 
Heaving  of  soil  is  another  indication  of  wetness.  In  tilled 
crops  the  wet  spots  are  often  marked  by  the  small  growth 
of  the  plants  and  by  curled,  wilted  leaves  in  dry  periods. 
In  orchards  weakened  and  missing  trees  are  in  many 
cases  an  indication  of  defective  drainage,  especially  in  the 
subsoil,  where  the  roots  of  older  trees  seek  to  develop. 

Steeply  sloping  hill  land  may  need  drainage  quite  as 
much  as  flat  land  if  it  has  a  compact  subsoil  overlaid  by  a 
porous  topsoil.  Water  is  then  trapped  in  the  soil,  and  is 
removed  very  slowly  by  percolation  on  top  of  the  hard 
subsoil  and  by  evaporation.  It  is  wet  land  in  need  of 
drainage! 

531.  Extent  of  drainage  needed  in  humid  regions.  — 
The  amount  of  farm  land  in  need  of  some  drainage  is  very 
large.  Besides  the  land  commonly  designated  as  swamp 
and  marsh,  there  are  very  large  areas  of  land  devoted  to 
crop  production,  the  yields  from  which  are  reduced  by  the 
excess  of  water  that  they  contain  at  certain  seasons  of  the 
year.  The  extent  of  swamp  land  varies  in  different  coun- 
tries, but  is  likely  to  aggregate  about  five  per  cent  of  the 
total  area.  The  cropped  land  in  need  of  some  drainage  is 
very  much  larger,  and  roughly  aggregates  three-fourths  of 
the  total  improved  land  surface.     The  temporary  wetness 


LAND  DRAINAGE  629 

that  much  land  experiences  is  often  more  injurious  than 
the  prolonged  wetness  of  swamp  land.  On  the  latter 
there  is  no  loss  except  on  the  investment  value  of  the  land, 
which  is  likely  to  be  low.  On  the  tilled  land,  however,  a 
considerable  sum  of  money  is  expended  for  labor,  seed, 
and  perhaps  fertilizers  and  manures,  without  corresponding 
returns.  The  loss  under  these  conditions  may  be  heavy. 
For  the  ordinary  farm  and  garden  crops,  the  fluctuation 
of  the  soil  moisture  from  a  condition  of  somewhat  pro- 
longed saturation  to  the  dry  and  often  hard  condition 
that  usually  results  is  exceedingly  difficult  to  withstand. 
Drainage  is  concerned  not  only  with  the  surface  and  the 
topsoil  water,  but  also  with  the  subsoil  water  to  the  depth 
to  which  the  roots  of  crops  normally  penetrate. 
'  532.  History  of  drainage.  —  The  need  for  soil  drainage 
in  the  production  of  the  ordinary  farm  and  garden  crops 
on  many  soils  has  been  recognized  from  the  beginning  of 
historic  times.  The  old  Roman  husbandman  Cato,1  and 
his  successors  of  the  next  ten  centuries,  in  their  writings 
on  agriculture  pointed  out  the  importance  of  draining  wet 
soil,  and  Cato  explains  how  bundles  of  faggots  should  be 
buried  in  trenches  in  the  land.  In  western  Europe 2 
artificial  drainage  has  been  practiced  for  some  hundreds 
of  years.  In  England  within  the  last  two  hundred  years 
drainage  by  means  of  pipes  has  become  a  general  practice. 
The  practice  of  underdrainage  by  means  of  clay  tile 
was  begun  in  America  in  the  early  part  of  the  nineteenth 

1  Cato,   M.   P.     Roman  Farm  Management  by  a  Virginia 
Gentleman.     New  York.     1913. 

2  Elliott,    C.    G.     Engineering    for    Land    Drainage.     New 
York.     1912. 

,  Miles,  M.     Land  Drainage,  Chapter  VI.     New  York.     1892. 
French,   H.   F.     Farm   Drainage,    Chapter  II.     New  York. 
1859. 


630       SOILS:    PROPERTIES  AND    MANAGEMKM 

century.  John  Johnston,1  a  Scotchman  living  near 
Geneva,  New  York,  carried  out  the  most  extensive  Q& 
these  pioneer  enterprises,  beginning  about  1835.  A  very 
thorough  system  of  tile  drains,  aggregating  about  sixty 
miles  in  length,  was  installed  on  his  farm  of  three  hundred 
acres,  and  these  drains  are  still  in  operation  and  are  pro- 
ducing excellent  results. 

533.  Effects  of  land  drainage  on  the  soil.  — The  need 
and  value  of.  thorough  drainage  of  the  soil  can  often  be 
better  appreciated  after  a  careful  summary  of  its  effects 
on  the  properties  that  determine  crop  growth.  From  a 
study  of  these  it  may  be  seen  that  for  the  production  of 
the  ordinary  upland  crops  a  reasonable  amount  of  soil 
drainage  is  the  first  requisite.  It  may  well  be  termed 
the  foundation  of  good  soil  management.  The  more 
noticeable  effects  are  as  follows  :  — 

1.  Drainage  permits  the  development  of  the  granular 
structure  in  soils,  especially  in  those  containing  much 
clay,  and  thereby  permits  the  creation  of  a  much  better 
tilth.  This  tilth  is  brought  about  by  the  frequent  changes 
in  moisture  content  of  the  soil  made  possible  by  drainage, 
coupled  with  other  natural  and  artificial  agencies,  as  has 
already  been  explained.  As  a  result  the  soil  maintains 
the  open  and  friable  condition  favorable  for  the  absorp- 
tion of  rain  water,  and  the  circulation  of  the  water  in 
the  spaces  in  the  soil  without  interference  with  the  crop 
roots.  The  tendency  of  the  soil  to  puddle  and  form 
large,'  hard  lumps  is  reduced. 

2.  The  withdrawal  of  the  excess  water  from  the  larger 
spaces  in  the  soil  permits  the  admission  of  air  into  those 

1Mellen,  C.  R.  History  and  Results  of  Drainage  on  the 
John  Johnston  Farm.  Proc.  New  York  State  Drainage  Assoc., 
pp.  27-32.       1912-1913. 


LAND  DRAINAGE  631 

spaces.  This  results  in  better  ventilation.  The  free 
movement  downward  through  the  soil  of  the  waves  of 
saturation  accelerates  the  process  of  deep  soil  ventilation 
by  driving  the  contaminated  air  out  through  the  under- 
drains  while  fresh  air  is  drawn  in  behind  the  wave  of  soil 
moisture. 

3.  The  removal  of  the  excess  moisture  by  drainage 
permits  the  soil  to  maintain  a  higher  average  tempera- 
ture. The  high  specific  heat  of  water  as  compared  with 
the  soil  causes  the  presence  of  water  to  be  the  chief  deter- 
mining factor  in  soil  temperature.  Further,  the  process 
of  evaporation  of  the  excess  water  from  the  soil  requires 
a  tremendous  amount  of  heat.  The  use  of  solar  heat  to 
warm  useless  water  and  to  remove  it  by  evaporation  is 
avoided  by  draining  away  this  excess.  Drained  soil  not 
only  maintams  a  higher  average  temperature  in  summer, 
but  warms  up  earlier  in  spring  to  a  temperature  for 
planting  seeds.     This  gives  a  longer  growing  season. 

4.  The  improved  ventilation  resulting  from  drainage 
permits  the  roots  of  plants  to  penetrate  deeper  into  the 
soil,  where  they  come  in  contact  with  a  larger  supply  of 
moisture  and  food.  One  of  the  indications  of  the  need  of 
drainage  is  the  shallow  root  development  of  crops.  Stag- 
nant water  in  a  saturated  soil  is  as  resistant  to  the  pene- 
tration of  upland  crops  as  is  the  hardest  rock  (see  Fig. 
63). 

5.  The  improved  physical  condition  of  the  soil  that 
results  from  drainage  permits  the  retention  of  a  larger 
amount  of  film  water,  and  this,  in  time  of  drought,  re- 
sults in  a  much  larger  available  supply  of  moisture  to 
the  crops. 

G.  The  improved  physical  condition  of  the  soil  permits 
better  internal  circulation  of  water,  by  which  the  films  are 


632     soils:  properties  and  management 

renewed  and  the  excess  water  is  permitted  to  pass  away 
quickly  in  the  drainage  channels. 


Fig.  63. — Area  of  land  nearly  level,  but  having  compact  subsoil  with 
undulating  subsurface,  thereby  causing  wet  pockets  that  force 
plants  to  form  short  roots.  Weeds  are  abundant  in  such  areas. 
Drainage  removes  the  water  and  permits  deeper  penetration  of  the 
plant  roots,  thus  enlarging  their  feeding  zone. 


7.  The  improved  ventilation  and  higher  temperature 
due  to  drainage  promote  the  activity  of  decay  organisms, 
by  which  fresh  organic  matter  is  changed  into  forms  that 
may  be  used  as  food  by  crops.  This  aids  in  the  formation 
of  humus,  with  its  beneficial  physical  effects  on  the  soil. 

8.  The  higher  temperature,  better  ventilation,  better 
distribution  of  moisture  and  of  decayed  organic  matter, 
together  with  the  deeper  penetration  of  roots,  make  avail- 
able a  larger  amount  of  mineral  elements  from  the  soil 
particles. 

9.  It  may  now  be  recognized  that  there  is  a  distinct 
sanitary  aspect  to  soil  management.  The  accumulation 
of  materials  of  a  toxic  nature  is  promoted  by  poor  drain- 
age, and  their  destruction  is  hastened,  and  perhaps  in 
part  their  formation  is  prevented,  by  the  conditions 
that  accompany  good  soil  drainage. 

10.  Drainage  reduces  heaving.  Heaving,  or  the  lifting 
of  crops  by  frost  action  in  the  soil,  indicates  the  presence 


LAND  DRAINAGE  638 

of  too  much  moisture  in  the  soil  in  proportion  to  its 
pore  space.  When  water  freezes  it  expands  one-eleventh 
of  its  volume.  If  the  soil  is  too  nearly  saturated,  this  ex- 
pansion is  expressed  at  the  surface  of  the  soil  by  a  lifting, 
or  heaving,  which  is  exceedingly  injurious  to  most  crops 
that  pass  the  winter  in  the  soil.  It  breaks  their  roots 
and  gradually  lifts  the  smaller  plants  out  of  the  ground 
if  the  process  is  many  times  repeated.  When  the  soil  is 
drained  so  that  free  air  spaces  are  distributed  through 
the  mass,  the  expansion  of  the  water  as  it  freezes  is  taken 
up  in  these  spaces  without  heaving  at  the  surface. 

11.  Drainage  reduces  erosion  of  soils  by  withdrawing 
the  water  through  the  soil  instead  of  permitting  it  to 
accumulate  to  the  point  where  it  must  move  over  the 
surface,  often  with  serious  erosive  action.  In  order  that 
the  drains  may  be  efficient,  the  soil  above  the  drains  must 
be  sufficiently  porous  to  permit  the  removal  of  the  water 
as  fast  as  it  accumulates. 

12.  Thorough  soil  drainage  greatly  increases  the  effi- 
ciency of  all  equipment  and  practices  used  in  crop  produc- 
tion on  the  farm.  There  is  a  longer  time  in  which  to  do 
the  work,  a  longer  season  in  which  the  crop  may  grow, 
and  usually  less  labor  is  required  in  order  to  fit  the  land 
and  keep  it  properly  tilled.  Further,  the  crop  matures 
more  evenly  and  is  likely  to  be  of  better  quality.  The 
need  for  a  commercial  fertilizer  is  reduced  because  of  the 
higher  efficiency  of  the  soil. 

13.  Prompt  and  thorough  drainage  of  a  wet  soil  results 
in  a  large  increase  in  yield  and  quality  of  crops.  All  the 
common  farm,  garden,  and  orchard  crops  are  injured  by 
a  saturated  condition  of  the  soil,  and  the  changes  that 
accompany  the  correction  of  that  condition  permits  a 
large  growth  of  the  plants.     The  fundamental  nature  of 


634       SOILS:    PROPERTIES  AND  MANAGEMENT 

those  changes,  and  therefore  the  basic  importance  of  good 
drainage  of  the  soil,  is  indicated  by  this  summary  of 
effects.  Even  where  ordinary  yields  of  crops  can  be 
grown,  improved  drainage  will  usually  increase  the  yield 
10  per  cent  or  more;  and  increases  of  several  hundred 
per  cent  are  in  many  cases  realized  where  the  conditions 
before  drainage  were  particularly  bad.  Land  in  need  of 
drainage  is  in  many  cases  fertile  in  all  other  respects,  and 
when  the  soil  moisture  is  properly  adjusted  it  responds 
with  large  yields.  Proper  drainage  should  be  the  starting 
point  in  any  permanent  improvement  of  the  soil. 

534.  Methods  of  drainage.  —  Two  general  methods  of 
drainage  are  employed  :  (1)  open  ditches,  and  (2)  closed 
drains,  or  underdrains. 

Open  ditches  are  most  satisfactory  where  the  volume 
of  water  to  be  moved  is  very  large.  The  general  drainage 
of  a  region  is  usually  carried  in  open  ditches.  They  are 
used  where  the  land  is  exceedingly  flat,  and  especially  if 
the  land  level  is  very  near  the  level  of  the  water  in  the 
outlet  channel  so  that  only  a  small  head  can  be  developed. 
They  are  used  also  where  a  temporary  result  is  desired. 

There  are  many  objections  to  open  ditches,  either  large 
or  small,  especially  as  applied  to  tilled  land.  They  waste 
a  considerable  area  of  land  in  the  channel  and  on  the 
banks,  and  they  interfere  with  free  tillage  operations.  In 
the  case  of  small  field  ditches  this  interference  is  serious. 
The  ditch  bank  promotes  the  growth  of  weeds.  The 
shallow  surface  trenches  commonly  used  to  remove  stand- 
ing water  from  the  land  are  of  very  low  efficiency,  since 
they  do  not  remove  the  water  from  the  subsoil  and  often 
are  so  shallow  that  the  surface  soil  remains  almost  satu- 
rated. Water  flows  slowly  in  such  rough,  irregular 
channels. 


LAND  DRAINAGE  635 

The  cost  of  maintenance  of  a  system  of  open  ditches  is 
heavy,  because  of  erosion,  the  accumulation  of  silt,  and 
the  growth  of  weeds,  all  of  which  make  frequent  repairs 
necessary. 

Underdrains  when  properly  constructed  are  more 
permanent  than  open  ditches  and  cost  less  for  mainte- 
nance. They  do  not  interfere  with  surface  operations. 
The  better  grade  gives  them  a  relatively  larger  carrying 
capacity  than  open  ditches  have,  and  their  greater  depth 
below  the  surface  permits  much  higher  efficiency  in  the 
removal  of  excess  moisture  from  the  root  zone. 

535.  Construction  of  small  open  ditches.  —  Small 
field  ditches  may  be  used  in  the  field  to  remove  small 
accumulations  of  surface  water.  They  usually  consist  of 
a  furrow  run  in  the  lowest  parts  and  made  with  a  large 
single  shovel  plow,  with  a  turning  plow,  or  with  a  two- 
way  plow  having  moldboards  to  turn  the  soil  on  either 
side.  Another  modification  in  the  construction  of  open 
ditches,  which  is  frequently  combined  with  the  foregoing, 
is  the  use  of  "  dead  furrows."  The  land  is  plowed  in 
narrow  beds  two  or  three  rods  in  width,  with  a  deep 
"  dead  "  furrow  between  each  which  drains  off  some  of 
the  surplus  water  from  the  higher  parts  of  the  intervening 
area.  A  further  modification  is  sometimes  used  in  plant- 
ing cultivated  spring  crops  on  wet  land.  Ridges  are 
thrown  up  along  each  row  and  the  seed  is  planted  on  these 
ridges.     The  intervening  trench  affords  some  drainage. 

536.  Construction  of  large  open  ditches.  —  Where 
larger  volumes  of  water  must  be  removed,  a  larger  channel 
is  necessary,  its  size  being  determined  by  the  area  to  be 
drained,  the  grade  of  the  ditch,  its  length,  its  straight ness, 
and  the  smoothness  of  the  sides  and  bottom.  The  ideal 
shape  for  the  ditch  for  the  largest  carrying  capacity  is  a 


636       SOILS:    PROPERTIES  AND  MANAGEMENT 

semicircle.  In  this  form  the  ditch  is  one-half  as  deep  as 
it  is  wide  at  the  surface.  This  brings  the  minimum  sur- 
face in  contact  with  the  moving  water.  The  tendency 
of  the  banks  to  cave  near  the  top,  as  well  as  the  diffi- 
culty of  constructing  such  a  form,  has  led  to  the  modifi- 
cation of  the  walls  to  an  inclined  slope  that  is  normally 
one  to  one,  or  an  angle  of  forty-five  degrees.  This  angle 
is  further  modified  by  the  nature  of  the  soil  through  which 
the  ditch  passes,  and  is  steeper  in  clay  soil  and  less 
steep  in  loose  sandy  soil.  Where  the  land  is  very  flat 
and  near  the  level  of  the  water  in  the  outlet  channel,  it 
may  be  desirable  to  deepen  the  ditch  considerably  below 
the  minimum  level  of  water  in  order  to  increase  the  flow 
during  freshets. 

The  shape  may  be  further  modified  where  the  volume 
of  water  to  be  carried  varies  excessively.  A  wide  channel 
may  be  provided  to  accommodate  the  flood  water,  and 
in  the  bottom  of  this  channel  a  smaller  channel  may  be 
provided  for  the  normal  flow,  of  such  a  size  that  it  is  more 
likely  to  be  kept  clean  and  free  than  would  a  ditch  of 
larger  cross  section  in  which  the  water  would  be  shallow. 

An  open  ditch  should  be  kept  as  straight  as  possible  so 
as  to  avoid  erosion  of  the  banks  where  turns  occur.  Change 
of  direction  should  begin  gradually  and  should  have  the 
maximum  curvature  at  the  middle  of  the  turn.  It  should 
then  pass  gradually  on  into  the  straight  line  of  the  new 
direction. 

The  grade  will  naturally  conform  in  a  large  measure  to 
the  surface  of  the  ground,  but  it  may  need  to  be  modified 
from  the  natural  grade  where  the  slope  is  so  steep  as  to 
cause  serious  erosion.  This  difficulty  receives  special 
attention  in  constructing  canals  to  carry  irrigation  water. 
Sandy   soils   having  low   cohesion   are   most   subject  to 


LAND   DRAINAGE  637 

erosion  on  high  grades.  Fine-textured  clays  are  least 
affected  by  erosion.  The  grades  and  rates  of  flow  that 
are  permissible  depend  largely  on  the  size  of  the  ditch. 
A  velocity  of  three  feet  a  second  is  usually  the  maximum 
that  is  permissible.  It  may  be  a  little  higher  in  clay, 
and  should  be  a  third  lower  in  silt  and  fine  sandy  loam. 
This  rate  of  flow  may  be  attained  in  ditches  where  the 
water  is  several  feet  deep  by  a  fall  of  only  six  inches  to  a 
foot  a  mile.  In  small  ditches  where  the  water  is  a  foot 
or  less  in  depth  the  grade  may  be  from  fifty  to  sixty  feet 
a  mile,  and  in  heavy  clay,  especially  if  it  is  compact  and 
stony,  a  still  higher  grade  will  not  cause  serious  washing. 

These  limits  depend  to  a  large  extent  on  the  amounts 
of  sediment  that  the  water  carries.  Material  in  suspen- 
sion greatly  increases  erosive  action  on  the  ditch  walls. 

In  constructing  open  ditches  care  should  be  taken  to 
deposit  the  earth  several  feet  back  from  the  edge  of  the 
channel.  This  is  desirable  for  two  reasons :  first,  it  re- 
moves the  weight  from  the  unsupported  bank,  where 
caving  is  very  likely  to  occur  when  the  soil  is  saturated ; 
second,  it  provides  a  larger  throat  for  the  stream  should 
it  be  inclined  to  overflow. 

Another  method  of  constructing  an  open  ditch,  es- 
pecially in  wet  grass  land,  is  to  form  a  broad,  shallow 
channel  by  the  use  of  a  road  scraper.  The  earth  is 
gradually  worked  back  a  rod  or  more,  and  the  walls  are 
so  flat,  even  with  a  ditch  three  feet  deep,  that  crops  grow 
and  may  be  collected  in  the  bottom  of  the  ditch.  This 
system  reduces  the  loss  of  land  and  the  interference  with 
farm  operations. 

537.  Construction  of  early  types  of  underdrains.  — 
Any  material  or  condition  that  affords  an  underground 
passage  for  the  flow  of  water  measurably  fulfills  the  func- 


638      SOILS:    PROPERTIES  AND  MANAGEMENT 

tion  of  an  underdrain.  Many  methods  and  materials 
have  been  employed.  One  used  in  England  in  clay  soil 
is  termed  mole  drainage.  A  plow  having  a  long,  thin 
shank,  with  a  molelike  or  cigar-shaped  point  at  the 
bottom,  is  slowly  drawn  through  the  soil  by  teams  or  a 
capstan.  The  passage  formed  persists  for  several  years 
in  the  finer  and  more  coherent  classes  of  soil,  and  may 
do  good  service.  Soil  free  from  stones  and  having  a  con- 
siderable degree  of  plasticity  is  necessary  for  this  method 
to  have  much  value. 

In  ancient  times,  and  in  pioneer  days  in  America, 
bunches  of  faggots,  brush,  poles,  rails,  straw,  and  wooden 
boxes  of  triangular  or  square  shape,  have  been  extensively 
employed  for  underdrainage  and  have  been  very  useful. 
They  may  still  have  some  use,  but  they  have  generally 
been  superseded  by  more  permanent,  if  not  more  efficient, 
materials. 

538.  Stone  drains.  —  Wherever  stones  are  abundant 
they  have  been  placed  in  trenches  in  some  manner  and 
often  have  served  for  many  years  to  facilitate  the  removal 
of  excess  water  from  the  soil.  Where  there  are  flat  stones 
they  may  be  arranged  to  form  a  continuous  throat. 
Several  systems  of  arrangement  have  been  used.  All 
throated  drains  are  more  likely  to  be  closed  by  sediment 
than  a  drain  with  no  single,  distinct  throat.  Perhaps 
the  safest  arrangement  is  to  place  flat  stones  on  edge  in 
the  trench,  with  their  faces  parallel  to  one  another  and 
to  the  walls  of  the  ditch,  depending  on  the  irregularities 
between  their  faces  for  the  flow  of  the  water.  Flat  stones 
are  placed  over  the  top  of  the  vertical  stones.  Where 
round  stones  are  available  the  safest  method  is  to  place 
them  in  the  trench  without  any  arrangement  except  to 
put  the  small  stones  on  the  surface.     The  water  will  find 


LAND  DRAINAGE 


639 


its  way  through  the  openings.  All  stone  drains  are 
likely  to  be  of  short  duration  because  of  obstructions  that 
develop  in  the  channel  by  the  accumulation  of  sediment, 
often  promoted  by  the  burrowing  of  animals.  The 
throat  of  a  ditch,  to  receive  stone  or  brush,  should  be 
relatively  large  (see  Fig.  64). 


Fig.  64. — The  most  common  types  of  drainage  tile  and  other  materials 
used  for  land  drainage.  (1),  cobblestones  with  smaller  pieces  of 
stone  on  top  ;  (2) ,  flat  stones  placed  face  to  face  and  parallel  to  line 
of  ditch ;  (3)  and  (4) ,  throated  drains  constructed  of  flat  stones 
used  in  different  ways ;  (5),  pole  drain;  (6),  triangular  box  drain; 
(7).  square  box  drain.  Note  construction  for  admission  of  water 
along  lower  edge ;  (8) ,  horseshoe  tile  laid  on  a  board ;  (9) ,  horse- 
shoe tile,  bottom  attached;  (10),  single  sole  tile  with  round  open- 
ing; (11),  double  sole  tile;  (12),  hexagonal  tile;  (13),  round  tile; 
(14),  Y-shaped  junction  piece  ;    (15),  elbow  piece. 


539.  Tile  drains.  —  Modern  underdrainage  is  usually 
accomplished  by  means  of  short  sections  of  pipe  of  burned 
clay  or  concrete,  placed  in  the  ground  sufficiently  deep  to 
lower  the  water  table  in  the  subsoil  to  the  desired  depth 
within  two  or  three  days.  They  are  given  an  accurate 
grade,  and  this,  coupled  with  the  smooth,  hard  channel 
which  is  not  subject  to  erosion,  makes  them  a  very  em- 


640       SOILS:    PROPERTIES  AND  MANAGEMENT 

cient  as  well  as  a  very  permanent  means  of  hind  drainage 
at  relatively  small  cost.  If  they  are  well  installed  and  of 
good  material,  they  should  continue  to  operate  for  cen- 
turies with  very  little  attention.  As  noted  above,  tile 
drains  have  been  in  continuous  operation  in  America  for 
seventy-five  years  and  are  still  firm  and  efficient. 

540.  Quality  of  tile.  —  There  may  be  a  considerable 
range  in  the  quality  of  tile  made  of  either  clay  or  concrete. 
Clay  tile  is  made  of  several  grades  of  clay  and  sand  mixed 
and  burned  at  a  high  temperature.  Material  that  is 
fused  slightly  is  thereby  vitrified,  and  forms  a  tile  having 
a  very  dense,  impervious  wall.  This  is  vitrified  tile. 
Burned  at  a  lower  temperature  the  walls  are  more  porous 
and  less  resistant.  Some  material  does  not  fuse  at  any 
temperature  to  which  it  may  be  raised,  and  produces  a 
tile  having  soft,  porous  walls.  This  makes  soft,  or  brick, 
tile.  Still  another  grade  of  tile  is  made  of  clay  —  usually 
fire  clay  —  dipped  into  a  salt  solution  before  firing.  This 
gives  a  smooth  glaze,  commonly  seen  in  sewer  tile.  This 
is  glazed  tile. 

Of  the  three  grades  mentioned,  the  vitrified  tile  is 
normally  the  best  because  of  its  strength  and  resistance 
to  the  destructive  agencies  in  the  soil.  The  most  notice- 
able of  these  agencies  is  frost.  Even  burned  clay  cannot 
resist  the  destructive  action  of  freezing  water.  Any  tile 
that  has  walls  porous  enough  to  absorb  an  appreciable 
amount  of  water  —  and  the  larger  the  amount,  the  greater 
is  the  danger  —  will,  if  frozen  and  thawed  a  few  times,  be 
shattered  into  flakes.  The  walls  of  soft  tile  will  absorb 
capillarily  from  8  to  20  per  cent  of  moisture,  and  under 
the  action  of  frost  will  go  to  pieces  rapidly.  Glazed  tile 
is  less  injured,  especially  when  the  glaze  is  intact;  but 
once  a  crack  has  formed  the  tile  is  rapidly  destroyed. 


LAND  DRAINAGE  641 

The  vitrified  tiles  have  walls  so  dense  that  they  absorb 
less  than  3  or  4  per  cent  of  moisture,  and  often  less  than 
2  per  cent,  so  that  they  are  much  less  vulnerable  to  frost 
action.  Good  tile  should  be  well  formed  and  should 
give  a  clear  ring  when  struck  with  a  hammer. 

Concrete  tile  of  good  quality  may  be  made,  but  the 
quality  is  normally  inferior  to  that  of  the  best  vitrified 
tile.  The  porosity  is  likely  to  be  5  to  10  per  cent.  To 
make  good  cement  tile  requires  a  rich  proportion  of  cement, 
good  sand,  and  as  wet  a  mixing  and  molding  as  is  prac- 
ticable. Several  machines  of  both  farm  and  factory  size 
are  in  the  market  for  molding  concrete  tile.  • 

Water  enters  tile  through  the  joints,  not  through  the 
walls.  Even  the  most  porous  tiles  having  a  high  absorp- 
tion do  not  permit  an  appreciable  amount  of  water  to 
pass  through  the  walls.  Therefore,  soft  tiles  have  no 
higher  efficiency  than  vitrified  tiles,  and,  owing  to  the 
risk  of  freezing,  the  effectiveness  of  a  line  of  porous  tiles 
is  much  jeopardized.  Since  water  enters  at  the  joints  of 
the  tile,  short  lengths  are  more  efficient  than  long  lengths. 
The  usual  length  of  sections  of  tile  under  12  inches  in 
diameter  is  12  to  13  inches.  In  larger  sizes,  where  the 
carrying  function  predominates  over  the  collecting  func- 
tion, lengths  of  2  feet  are  employed. 

541.  Shapes  of  tile.  —  Tile  should  have  a  round  open- 
ing and  a  round  or  a  hexagonal  exterior.  A  flat-bottomed 
opening  is  objectionable  because  it  reduces  the  flow  and 
promotes  the  accumulation  of  sediment.  U-shaped  tiles 
with  flat  sides  are  called  horseshoe,  or  single-sole  tile. 
This  shape  is  unsatisfactory.  Tiles  are  often  warped 
in  the  process  of  drying  and  burning,  and  the  last-named 
shape  does  not  allow  a  close  joint  to  be  formed  by  turn- 
ing the  tile.  Round  and  hexagonal  shapes  permit  turn- 
2t 


642       SOILS:    PROPERTIES  AND  MANAGEMENT 

ing  until  a  good  joint  is  formed.  An  earlier  type  was 
the  U-shaped  tile  laid  on  a  board.  These  tiles  are  easily 
broken  by  the  pressure  of  the  earth.  They  are  no  more 
efficient  than  the  ordinary  round  tile. 

542.  Protection  of  joints.  —  Soil  water  should  enter 
the  tile  at  the  lower  side  of  the  joint.  Any  unusual 
opening  in  the  joint  should  be  on  the  lower  side.  If  the 
soil  has  low  coherence,  such  as  may  be  the  case  with  fine 
sand  and  silt,  the  upper  half  of  the  joint  should  be  pro- 
tected against  the  entrance  of  sediment.  A  cap  of  paper 
or  of  burlap  cloth,  two  or  three  inches  wide  and  long  enough 
to  cover  the  upper  half  of  the  joint,  may  be  used. 

Other  methods  of  protecting  joints  are  to  cover  them 
with  clay,  thick  cement  mortar,  or  the  sod  and  granular 
soil  from  the  surface.  The  last  named  is  most  commonly 
employed.  Filters  may  be  constructed  by  placing  around 
the  tile  a  layer  of  coarse  sand  or  gravel,  cinders,  straw, 
or  leaves.  Where  the  soil  is  of  a  serious  quicksand  nature 
(clean,  fine  sand  or  silt  filled  with  water),  it  may  be  desir- 
able to  place  a  bed  of  gravel  or  cinders  under  the  tiles 
as  well  as  around  them.  The  entrance  of  water  from  the 
lower  side  of  the  joint  in  small  trickles  will  generally 
prevent  any  difficulty  from  sediment.  Water  should 
flow  from  a  drain  approximately  clear,  and  any  other 
condition  usually  indicates  a  too  rapid  entrance  of  water. 
Where  the  soil  is  a  fine  clay  with  high  cohesion,  the  ends 
of  tiles  should  not  be  so  close  together  as  in  loose  soil. 
The  tops  may  sometimes  be  separated  an  eighth  of  an 
inch  with  entire  safety.  In  such  cases  it  is  especially  im- 
portant to  return  the  soil  to  the  trench  in  a  dry  condition 
and  to  place  the  topsoil  next  to  the  tile. 

543.  Entrance  of  roots  into  tile.  —  The  entrance  of 
roots  into  the  joints  of  tile  drain  sometimes  causes  an 


LAND   DRAINAGE  643 

obstruction  by  breaking  up  into  such  a  mass  of  fine  root- 
lets that  the  tile  is  finally  closed.  Any  kind  of  tree  or 
plant  may  cause  this  difficulty  if  permitted  to  develop 
under  certain  conditions.  Trouble  from  roots  occurs  only 
where  the  tile  carries  water  from  a  spring  or  some  other 
continuous  source,  so  that  in  dry  periods  the  water  may 
leak  out  at  the  joints  into  the  adjacent  dry  soil.  This 
leads  the  roots  in  the  direction  of  the  tile.  In  the  ab- 
sence of  such  a  spring,  plant  roots  do  not  appear  to  inter- 
fere with  drains.  Where  a  drain  carrying  water  con- 
tinuously comes  near  a  tree,  especially  if  the  adjacent 
soil  is  likely  to  become  dry,  the  joints  of  tile  should  be 
closed  by  cement. 

544.  Protections  of  joints  on  curves.  —  Special  care 
may  be  needed  in  order  to  protect  the  joints  on  turns 
where  the  outer  side  may  be  too  open.  The  larger  the 
size  of  the  tile,  the  longer  will  be  the  opening  on  a  given 
curve.  Short  turns  should  not  be  made.  Stones  are 
usually  unsafe  material  to  place  around  the  joints  of  a 
tile  under  such  conditions,  especially  in  soil  that  is  likely 
to  erode  easily.  If  so  used,  special  care  should  be  em- 
ployed to  protect  the  joints  with  caps. 

545.  Foundation  for  tile.  —  Tiles  should  have  a  firm 
foundation,  and  if  the  bottom  of  the  ditch  is  soft  it  may 
be  advisable  to  bed  them  in  sand  or  cinders  or  lay  them 
on  a  board.  Soft  muck  and  quicksand  make  this  most 
necessary.  Ordinarily  the  bottom  of  the  .trench  is  finished 
on  the  undisturbed  earth,  which  affords  a  firm  setting. 

546.  Arrangement  of  drainage  systems.  —  The  ar- 
rangement of  a  system  of  underdrains  should  be  deter- 
mined by  the  slope  of  the  land  and  the  structure  of  the 
soil.  No  fixed  rule  can  be  laid  down.  The  aim  must  be 
to  place  the  drains  in  the  line  of  movement  of  water  in 


644       SOILS:    PROPERTIES  AND  MANAGEMENT 

the  soil,  and  thereby  intercept  its  flow.  The  need  of 
drainage  may  arise  from  several  conditions.  It  is  always 
indicated  by  the  occurrence  of  a  stratum  of  rather  com- 
pact soil  which  intercepts  the  natural  flow  of  water  and 
brings  it  within  the  root  zone.  Sometimes  this  obstruc- 
tion is  near  the  surface,  sometimes  it  is  several  feet  below 
the  surface.  The  water  may  be  brought  to  the  surface 
in  a  single  spring  or  in  a  series  of  springs,  in  the  latter 
case  forming  a  seepage  line.  The  retaining  layer  may 
have  an  uneven  surface  and  form  basins  and  hollows 
disguised  by  a  covering  of  porous  soil.  For  all  these 
reasons,  the  drainage  conditions  of  the  soil  and  the  lines 
of  movement  of  water  through  it  should  be  studied  as 
fully  as  possible  before  the  drainage  system  is  planned. 
The  main  lines  should  first  be  located.  Where  the  land 
is  in  need  of  drainage  in  parts,  a  few  lines  of  tile  will 
accomplish  this.  Springy  holes  should  generally  be  tapped 
by  the  most  direct  route.  Often,  short  wing  drains  may 
be  necessary  at  the  upper  end,  to  collect  the  underground 
flow.      (See  Fig.  65.) 

Where  there  is  a  line  of  seepage  at  nearly  a  uniform 
level,  a  drain  placed  across  the  slope  at  the  upper  edge 
of  the  wet  area,  and  if  possible  cutting  to  the  underlying 
hard  stratum,  will  intercept  the  flow  and  meet  the  needs  of 
the  lower  land.     This  is  an  intercepting  system  of  drains. 

Where  the  land  is  more  nearly  uniform  in  its  need  of 
drainage,  a  regular  system  is  required  and  will  usually 
result  in  a  saving  of  tile.  This  arrangement  should 
approximate  a  rectangular  system,  in  order  to  avoid 
double  drainage  where  lateral  tile  join  the  main  line. 
This  may  of  course  be  modified  according  to  conditions. 
The  line  of  tile  should  be  as  long  as  is  practicable  for 
convenience  in  construction.     To  this  end,  if  the  field 


LAND  DRAINAGE  645 

is  wide  in  proportion  to  the  length  of  the  main  drains, 
the  subdrains  may  branch  out  laterally  at  a  right  angle 
or  less.  If  the  laterals  on  either  side  of  the  main  drain 
join  at  the  same  acute  angle,  the  "  herring-bone  "  system 


Fig.  65. — Sectional  view  of  soil  and  rock  formation,  showing  the  under- 
ground movement  of  water  and  the  position  of  resulting  wet  areas 
on  the  -urface.  In  addition  to  the  springy  places,  the  soil  is  kept 
wet  by  the  seepage  of  water  along  the  top  of  the  compact  subsoil. 
This  figure  also  illustrates  the  reason  for  locating  a  cross  drain 
above  the  springy  area  in  order  to  effect  drainage.  This  method 
cuts  off  the  water  supply. 

is  formed.  If  the  main  drain  is  situated  in  the  wettest 
part  of  the  field,  this  system  has  some  advantage.  If  the 
field  is  long  and  very  narrow,  the  main  drain  may  be  along 
the  short  side  of  the  field,  with  long  laterals  leading  up  the 
slope.  If  the  land  is  of  about  equal  wetness  on  a  slope, 
the  drains  should  extend  up  and  down  rather  than  across 
the  slope. 

547.  Grade  of  tile  drains.  —  Where  the  land  is  rela- 
tively flat  or  uneven,  a  survey  should  be  made  in  order 
to  determine  the  distribution  and  extent  of  the  grades. 
This  is  necessary  in  arranging  the  system.  Where  the 
grades  are  simple,  the  arrangement  may  be  determined 
by  the  eye,  if  the  man  in  charge  is  experienced. 


646     soils:  properties  and  MANAGEMKX I 

Tile  drains  operate  best  on  a  grade  of  one  or  two  feet 
in  a  hundred.  Larger  grades  are  permissible,  but  in 
such  cases  the  earth  should  be  carefully  packed  around  the 
tile  in  filling.  Tile  will  operate  even  on  the  very  slight 
grade  of  one  or  two  inches  in  a  hundred.  In  this  case 
the  minimum  size  of  tile  should  be  larger  than  on  high 
grades,  and  the  distribution  of  the  fall  should  be  very 
uniform.  Every  part  of  the  operation  of  planning  and 
construction  should  be  guided  by  readings  of  an  accurate 
level. 

548.  Depth  of  drains.  —  The  depth  of  tile  drains  should 
ordinarily  be  from  two  feet  to  three  and  one  half  feet. 
The  former  depth  should  be  the  one  for  clay  loam 
or  other  moderately  impervious  soil,  and  is  adequate 
for  most  crops  having  a  shallow  root  penetration.  The 
greater  depth  should  be  used  on  sandy  and  gravelly  soil 
and  where  deep-rooted  perennials  are  to  be  grown.  Under 
special  conditions  the  drains  may  be  laid  deeper  or  less 
deep  than  these  figures.  On  very  dense  clay  or  where  "a 
very  impervious  hardpan  exists,  the  drains  may  be  placed 
a  little  nearer  the  surface,  since  their  function  is  primarily 
to  remove  the  water  trapped  near  the  surface.  To 
intercept  deep  underground  flow  or  to  secure  an  outlet 
for  it,  or  where  especially  deep  rooting  of  crops  is  desired, 
drains  may  be  laid  deeper  than  the  normal. 

Where  the  soil  is  sufficiently  porous  to  permit  reasonably 
free  percolation  of  water,  as  in  gravelly  and  sandy  sub- 
soils, the  deeper  drains  operate  earlier  after  a  rain  and  are 
the  more  efficient.  The  number  of  drains  necessary  is 
also  reduced  by  laying  them  deeper.  Where  the  subsoil 
is  relatively  impervious,  shallow  drains  should  be  in- 
stalled and  placed  as  near  the  top  of  the  impervious  layer 
as  is  practicable.     A  shallow  trench  should  be  formed  in 


LAND  DRAINAGE 


647 


the  compact  layer  to  receive  the  tile,  and  if  its  depth 
exceeds  half  the  diameter  of  the  tile  special  care  should 
be  taken  to  place  the  topsoil  or  some  other  porous  material 
on  the  tile  and  around  the  joints  in  order  to  insure  the 
entrance  of  water. 

549.  Distance  between  drains.  —  The  interval  be- 
tween drains  must  be  determined  by  the  nature  and  the 
wetness  of  the  soil  and  the  value  of  the  crops  produced. 
In  soil  where  drains  must  be  installed  at  a  depth  of  two 
and  a  half  feet  or  less,  for  general  farming  the  interval 
between  drains  must  ordinarily  be  not  more  than  fifty 
feet.  Where  they  may  be  placed  deeper,  the  interval 
may  be  correspondingly  greater. 

The  number  of  feet  and  rods  of  tile  required  when  the 
lines  are  laid  regularly  at  a  specified  distance  apart  is 
given  in  the  following  table  :  — 


Distance  Apart  in  Feet 

Tile  to  the  Acre 

Feet 

Rods 

20 

2,178 

131.50 

25 

1,740 

105.42 

30 

1,452 

88.00 

40 

1,089 

65.75 

50 

870 

52.71 

80 

545 

32.88 

100 

435 

26.36 

150 

290 

17.57 

200 

218 

13.18 

Under  the  influence  of  the  drains  the  physical  nature 
of  the  surface  soil  and  of  the  subsoil  gradually  changes 
and  undergoes  improvement.     Lines  of  seepage  develop, 


648       SOILS:    PROPERTIES  AND  MANAGEMENT 


and  the  drain  gradually  increases  in  efficiency.  In  heavy 
soil  and  in  soils  having  hardpan  properties,  several  seasons 
may  be  required  for  this  change  in  the  soil  to  spread 
three  or  four  rods  from  the  drains.  The  problem  is  to 
remove  the  excess  water  from  the  soil  at  the  maximum 
distance  from  the  drains  in  time  to  avoid  serious  injury 
to  the  crop. 

550.  Construction  of  drainage  trenches  for  tile.  — 
Trenches  should  be  as  small  as  possible  and  yet  permit 
the  ready  introduction  of  the  tile.  Unless  the  tiles  to  be 
used  are  of  the  larger  sizes,  the  ditch  should  be  made 
from  twelve  to  fourteen  inches  wide,  with  vertical  sides. 
Where  leveling  instruments  are  employed,  the  course 
of  the  ditch  is  staked  out  and  the  grade  cord  is  stretched 
a  definite  distance  above  the  proposed  grade  line  of  the 
ditch,  to  guide  the  workmen.  In  hand  digging,  the  earth 
is  thrown  out  with  a  narrow-pointed  spade,  the  loose  earth 


Fig.  66.— Tools  for  ditching,  (l)  and  (2),  ditching  spades  for  remov- 
ing the  major  part  of  the  earth  from  the  ditch;  (3),  grading  scoop 
used  to  finish  the  bottom  of  the  ditch  and  the  grade  ;  (4) ,  skeleton 
spade  adapted  for  use  in  very  plastic  soil ;  (5) ,  shovel  for  removing 
loose  earth;  (6),  hook  used  to  place  tile  in  deep,  narrow  trenches; 
(7) ,  pick  for  loosening  stone  and  hard  earth. 


LAND  DRAINAGE  649 

is  cleaned  out  with  a  round-pointed  shovel,  and  the  bottom 
is  finished  to  a  smooth,  perfect  grade  by  means  of  the 
grading  scoop,  which  also  rounds  the  bottom  of  the  trench 
into  shape  to  receive  the  tile.  (See  Fig.  66.)  Care 
should  be  taken  not  to  excavate  the  trench  below  the 
grade  line,  so  that  the  tile  may  have  a  firm  bed. 

Horse  and  engine  power  are  now  very  generally  applied 
to  trench  digging.  Several  types  of  plows  drawn  by 
horses  are  available  to  loosen  the  soil,  and  some  types 
are  arranged  to  follow  the  grade  and  to  elevate  the  loose 
earth  out  of  the  trench.  Several  types  of  engine-driven 
machines  are  in  use  where  the  land  is  not  excessively 
stony.  They  cut  the  trench  to  the  full  depth  at  one 
operation,  and  are  constructed  so  as  to  follow  a  perfect 
grade,  so  that  tile  may  be  laid  as  fast  as  the  machine 
progresses. 

551.  Laying  tile.  —  Where  two  lines  of  tile  join  they 
should  come  together  at  an  acute  angle,  forming  a  Y 
so  that  the  two  streams  of  water  will  have  the  minimum 
interference  and  the  collection  of  sediment  will  be  pre- 
vented. If  the  lines  are  arranged  at  right  angles,  one  of 
the  strings  may  be  turned  down  grade  in  the  form  of  a 
curve  in  the  last  rod  of  its  course,  to  make  the  proper 
union.  Junction  pieces  or  Y's  may  be  bought  in  the 
smaller  size  of  tile.  They  are  rated  by  the  diameter  of 
the  lateral  and  main  branches ;  for  example,  a  3  X  6 
junction  indicates  a  three-inch  lateral  and  a  six-inch 
main.  A  lateral  tile  should  enter  the  main  drain  with  a 
slight  drop.  A  small  tile  should  enter  a  larger  main 
drain  at  the  horizontal  center  of  the  latter. 

The  tiles  are  placed  in  the  trench  by  hand,  or,  if  the 
trench  is  deep  or  the  tiles  are  very  large,  by  means  of 
some  mechanical   arrangement  such  as  a  hook.     Their 


650       SOILS:    PROPERTIES  AND  MANAGEMENT 

ends  are  put  in  line  and  as  close  together  as  conditions 
seem  to  indicate  is  necessary.  Any  covering  or  filling 
material  is  then  put  in  place.  The  tile  should  be  placed  in 
the  trench  as  soon  as  the  latter  is  finished,  and  the  trench 
should  then  be  at  least  partially  filled  with  earth  in  order 
to  avoid  danger  from  freezing  or  from  the  caving-in  of 
the  walls.  The  first  lot  of  earth  —  usually  from  the  sur- 
face —  is  carefully  placed  about  the  tiles  and  packed 
in  so  as  to  hold  them  in  position.  This  is  called  the 
blinding,  or  back-filling.  The  later  filling  may  be  accom- 
plished in  any  convenient  way. 

552.  Size  of  tile.  —  The  size  of  tile  must  be  deter- 
mined by  the  amount  and  rate  at  which  the  water  must  be 
removed,  the  grade  of  the  drain,  and  the  nature  of  the 
soil.  The  small  lateral  drains  whose  function  is  to  collect 
the  water  from  the  soil  will  usually  be  of  three  or  four 
inches  internal  diameter.  Drains  smaller  than  this 
should  not  be  used  because  of  their  inclination  to  become 
clogged.  Small  tiles  are  relatively  more  affected  than 
larger  tiles  by  the  inevitable  slight  imperfections  in  the 
grade.  The  high  friction  of  the  walls  in  small  tiles  to 
the  moving  water  reduces  the  rapidity  of  flow  and  en- 
courages the  accumulation  of  sediment.  In  soil  some- 
what of  the  nature  of  quicksand,  and  where  the  grade  is 
less  than  one  foot  in  a  hundred,  no  tile  smaller  than  four 
inches  in  diameter  should  be  used.  As  the  drainage 
water  is  collected  hj  the  different  lines,  the  size  of  the 
tiles  must  increase  correspondingly. 

553.  Amount  of  water  to  be  removed  from  land.  — 
Many  things  affect  the  amount  of  water  to  be  removed 
from  a  given  area  of  land.  The  more  important  of  these 
are  the  rainfall,  the  occurrence  of  springs,  surface  accu- 
mulation, the  storage  capacity  of  the  soil,  and  rate  of 


LAND  DRAINAGE  651 

evaporation.  Underdrains  are  designed  with  a  capacity 
to  remove  only  part  of  the  normally  largest  rainfall 
in  a  period  of  twenty-four  hours.  The  absorptive 
power  of  the  soil  and  its  hindrance  to  the  flow  of 
water  through  its  pores  permits  the  use  of  a  tile-drain 
system  capable  of  removing  from  one-quarter  to  one- 
half  inch  of  water  over  the  drainage  basin  in  twenty- 
four  hours.  This  is  termed  the  drainage  coefficient  of  the 
area.  The  drainage  coefficient  of  the  system,  especially 
if  it  is  a  large  system,  should  be  determined  after  careful 
study  of  the  amount  and  distribution  of  the  rainfall 
and  the  extent  to  which  surface  and  subterranean  water 
is  accumulated. 

554.  Carrying  capacity  of  a  tile-drain  system.  —  The 
carrying  capacity  of  a  system  of  tile  drains  depends  on 
their  respective  sizes,  their  grade,  or  fall,  their  total  length, 
their  depth  in  the  ground,  the  straightness  of  their  course, 
and  the  smoothness  of  the  interior  of  the  tile.  Some  of 
these  factors  affect  the  flow  directly  as  they  increase, 
others  inversely.  The  two  most  important  elements 
in  determining  the  capacity  of  a  drain  are  the  diameter 
and  the  grade.  The  capacity  of  a  drain  varies  as  the 
square  of  the  diameter.  Doubling  the  grade  increases 
the  capacity  by  approximately  one-third.  With  cer- 
tain additional  corrections  and  modifications,  all  the 
factors  that  affect  the  flow  have  been  put  together  in  a 
formula  to  determine  the  necessary  size  of  the  outlet 
tile  for  a  given  area.  This  formula,  known  as  Ponce- 
let's  formula,  as  modified  by  Elliott !  for  large  systems, 
is  as  follows  :  — 

1  Elliott,  C.  G.  Engineering  for  Land  Drainage,  Chapters 
VII,  VIII,  IX.     New  York.     1912. 


652       SOILS:    PROPERTIES  AND  MANAGEMENT 


(IM-8 


'-^«r 


(2)  Q  =  oF 


-f-544 

yl  =  acres  to  be  drained 

C  =  coefficient  of  drainages  selected  for  the  area  in 
cubic  feet  per  second.  It  is  determined  by 
the  depth  of  water  to  be  removed  in  twenty- 
four  hours 

Q  =  quantity  of  water  the  tile  will  discharge,  in 
cubic  feet  per  second 

a  =  area  of  tile  in  square  feet 

V  =  velocity  in  feet  per  second 

d  =  diameter  of  tile  in  feet 

/    =  length  of  tile  in  feet 

h  =  head,  or  difference  in  elevation  between  outlet 
and  upper  end,  in  feet 

b  =  sum  of  amounts  of  head  in  laterals,  in  feet 

n  =  number  of  laterals 

K  =  depth  of  tile  below  soil  surface  at  upper  end,  in 
feet 

48  and  54  are  factors  that  take  account  of  gravity, 
the  size  of  the  tile,  and  the  roughness  of  the 
walls.  The  former  figure  is  larger  for  tile 
more  than  twelve  inches  in  diameter. 

The  first  formula  determines  the  number  of  acres  that 
a  given  size  of  tile  will  drain,  by  dividing  the  quantity 
of  water  to  be  removed  by  the  coefficient  of  drainage 
selected  for  the  region. 


LAND  DRAINAGE 


653 


i 


The  second  formula  determines  the  quantity  of  water 
possible  to  remove,  by  multiplying  the  area  of  the  cross 
section  of  the  tile  by  the  velocity  of  flow. 

The  third  formula  is  used  to  determine  the  velocity  of 
flow  of  water  in  the  outlet  tile. 

In  a  small  system,  where  the  laterals  are  relatively 
unimportant  and  where  the  soil  is  fairly  close,  the  velocity 
formula  may  be  much  simplified  as  follows :  — 


7  =  48 


dh 


l+54d 


The  term  ^  K  is  used  only  where  the  soil  is  so  very 
porous  that  the  ready  movement  of  the  water  through 
the  soil  has  an  influence  on  the  flow  in  the  tile. 

Coefficients  of  drainage  and  their  equivalents  in  cubic 
feet  per  second  of  discharge  are  as  follows :  — 


Depth  op  Water  in  Inches  Removed 
in  Twenty-four  Hours 

Cubic  Feet  to  a  Second  of  Dis- 
charge 

Fraction 

Decimal 

To  an  Acre 

To  a  Square  Mile 

1 

3 
t 

1 
2 

1 
4 

1.00 
0.75 
0.50 
0.25 

0.0420 
0.0315 
0.0210 
0.0105 

26.9 

20.2 

13.4 

6.7 

From  the  above  formula  Elliott  has  calculated  the 
number  of  acres  of  land  drained  by  outlet  tiles  of  different 
sizes  and  grades  where  the  coefficient  is  one-fourth  of  an 
inch  in  twenty-four  hours  and  where  the  main  is  1000 
feet  in  length  :  — 


654     soils:  properties  and  management 


Acres  from  which  a  Main  Tile  Laid  on  Grades  Indicated 
may  Adequately  Receive  Drainage  Water 


Diameter 

Grades 

to  a  Hundred  Feet  in  Decimals  op  a  Foot  with 
Approximate  Equivalents  in  Inches 

of  Tile 
(in  Inches) 

J  inch 

1  inch 

2  inches 

3  inches 

6  inches 

9  inches 

0.04 

0.08 

0.16 

0.25 

0.50 

0.75 

Acres 

Acres 

Acres 

Acres 

Acres 

Acres 

5 

17.3 

19.1 

22.1 

25.1 

32.0 

37.7 

6 

27.3 

29.9 

34.8 

39.6 

.50.5 

59.4 

7 

39.9 

44.1 

51.1 

58.0 

74.5 

87.1 

8 

55.7 

61.4 

71.2 

80.9 

103.3 

121.4 

9 

74.7 

82.2 

95.3 

108.4 

138.1 

162.6 

10 

96.9 

106.7 

123.9 

140.6 

179.2 

211.1 

12 

152.2 

167.7 

194.6 

221.1 

281.8 

331.8 

555.  Cost  of  drainage.  —  The  cost  of  tile  drainage 
depends  on  many  things,  including  especially  the  size  of 
the  tile,  the  frequency  of  the  drains,  the  depth,  the  nature 
of  the  soil,  the  method  of  digging,  and  the  price  of  labor. 
The  cost  of  tile  varies  in  different  regions  and  increases 
rapidly  with  the  size. 

The  following  schedule  will  serve  merely  as  a  general 
guide  to  the  range  in  price  a  thousand  feet  and  a  rod  when 
purchased  in  car  lots  :  — 


Size  (Diameter  in  Inches) 

2 

3 

4 

5 

6 

8 

Cost  a  thousand 

feet    .... 

Cost'a  rod      .     . 

$  10-  $14 

$.16-$.21 

$  13-$  18 
120-M7 

$  18-$  25 
$.27-$.38 

$  25-$  32 
$.38-$.48 

$  35-$  45 
$.53-$.68 

$65-$80 
$.98-$1.12 

The  cost  for  digging  the  trench  of  course  varies  widely. 
In  medium  soil  free  from  stone,  for  a  ditch  two  and  one- 


LAND   DRAINAGE 


655 


half  feet  deep  to  receive  tile  up  to  ten  inches  in  diameter, 
the  cost  may  be  from  fifteen  cents  to  fifty  cents  a  rod, 
with  an  average  of  about  thirty-five  cents.  The  cost  can 
sometimes  be  reduced  by  the  use  of  a  power  machine. 
In  stony  and  hardpan  soil  the  cost  may  be  very  much 
higher  than  these  estimates.  The  deeper  trench  is  rela- 
tively the  more  expensive  to  construct. 

Laying  the  tile,  filling  the  trench,  and  other  miscel- 
laneous operations  for  the  smaller  sizes  of  tile  will  cost 
at  least  ten  cents  a  rod.  This  makes  a  total  cost  for  four- 
inch  tile  of  about  80  cents  a  rod,  $5  a  hundred  feet,  and 
$260  a  mile. 

Records  are  available  of  the  cost  of  drainage  on  an 
extensive  area  of  cultivated  farm  land  in  northern  Ohio,1 
where  the  soil  is  chiefly  a  medium  clay  loam,  somewhat 
stony,  and  where  the  depth  was  two  to  three  and  one-half 
feet.  Some  of  the  work  was  done  by  hand  and  some  with 
the  aid  of  a  traction  ditching  machine.  A  fairly  low  price 
prevailed  for  tile,  the  size  ranging  from  three  to  thirteen 
inches. 

The  results  are  as  follows  :  — 


Cost  op  Installing  Tile 
Drains 

Hand  work 

Machine  work 

Area  (in  acres) 

Number  of  rods  of  tile 

Cost  of  installation  per  rod  .... 
Average  cost  of  tile  per  rod  .... 
Average  number  of  rods  to  the  acre 
Average  cost  to  the  acre 

40 

2,560 

$0.4489 

$0.2445 

48 
$33.28 

188 

8,835 

$0.3746 

$0.2445 

48 

$29.72 

1Goddard,   L.   H.,   and   Tiffany,   H.    O. 
Drainage.     Ohio  Agr.  Exp.  Sta.,  Circ.  147. 


The   Cost  of  Tile 
1914. 


656      SOILS:    PROPERTIES  AND  MANAGEMENT 

556.  Storm  channels.  — -  Where  large  volumes  of  water 
must  be  carried  for  a  short  time  in  addition  to  the  normal 
flow,  a  medium-sized  tile  drain  may  be  combined  with  an 
open  surface  channel  for  carrying  away  the  flood  water. 
The  open  channel  is  located  a  little  to  one  side  of  the 
tile  drain  so  that  the  latter  may  not  be  displaced  by  pos- 
sible erosion.  The  open  surface  channel  is  made  broad 
and  shallow  in  order  to  avoid  interference  with  tillage 
operations,  and  if  erosion  is  likely  to  occur,  it  may  be 
kept  in  grass. 

557.  Silt  basins.  —  Silt  basins  are  wells  in  the  line 
of  tile  drains,  for  collecting  sediment  that  might  other- 
wise be  deposited  in  the  tile.  The  course  of  the  drain 
is  intercepted  and  a  small  well  is  sunk  two  or  more  feet 
below  the  bottom  of  the  drain.  The  well  extends  to  the 
surface  of  the  ground  and  has  a  cover.  The  inlet  drains 
come  in  at  a  slightly  higher  level  than  the  outlet.  The 
heavy  sediment  drops  to  the  bottom,  whence  it  may  be 
removed  from  time  to  time.  The  end  of  the  outlet  tile 
is  finished  with  an  elbow,  turned  down  so  as  to  prevent 
the  entrance  of  sticks  or  other  floating  material.  The 
walls  of  the  well  may  be  made  of  wood,  concrete,  or 
brick. 

558.  Surface  intakes.  —  The  admission  of  surface  water 
into  a  tile  drain  should  always  be  managed  with  great 
care  to  remove  the  heavier  sediment  or  other  material 
that  might  obstruct  the  tile.  Screen  boxes  should  be 
used.  The  screen  should  incline  to  the  intake  at  an  angle 
of  fifty  or  sixty  degrees,  so  that  floating  material,  instead 
of  obstructing  the  flow,  will  be  pushed  upward  out  of  the 
course. 

559.  Outlets.  —  As  few  outlets  as  is  practicable  should 
be  constructed  for  tile  drains,  and  these  should  have  a 


LAND  DRAINAGE 


657 


Fig.  67. — A  drainage  plan  of  an  area  of  land  exhibiting  many  differences 
as  to  soil,  slope,  and  degree  of  wetness.  Herein  are  shown  the  kinds, 
sizes,  and  arrangements  of  drains  necessary  to  provide  efficient 
drainage  under  the  various  conditions. 


2u 


658       SOILS:    PROPERTIES  AND  MANAGEMENT 

drop  and  be  well  protected  by  wing  walls.  Line-  .,i" 
drains  should  be  connected  in  systems  for  this  purp< 
Unless  the  drain  has  a  high  grade  the  outlet  should  not  be 
covered  by  water.  The  end  of  the  tile  should  be  pro- 
tected by  a  gate  or  a  series  of  rods  to  prevent  the  entrance 
of  small  animals. 

560.  Muck  and  peat  soil.  —  Muck  and  peat  soil  should 
usually  be  drained  by  open  ditches  at  first.  After  learn- 
ing the  nature  of  the  material  and  the  structure  of  the 
subformation,  it  may  be  found  permissible  to  install  tile 
in  the  smaller  ditches.  When  the  organic  material  is 
more  than  four  feet  deep,  so  that  tile  could  not  be  laid 
on  a  hard  bottom,  much  risk  is  involved  in  its  use  due 
to  the  excessive  shrinkage  of  such  soil  when  the  surplus 
water  is  removed  and  when  even  moderate  drying  occurs. 
If  the  area  is  fed  by  springs  so  that  the  water  level  will 
be  kept  permanently  at  the  base  of  the  tile,  the  shrinkage 
will  be  very  small  and  the  tile  may  usually  be  laid  with 
safety,  especially  if  placed  on  boards  to  aid  in  keeping 
the  alignment.  In  so-called  dry  peat,  where  the  subsoil 
may  dry  out  seriously  in  summer,  the  use  of  tile  is  inad- 
visable. In  muck  soil,  which  has  a  finer  texture  resulting 
from  a  more  advanced  stage  of  decay,  tile  drains  may  be 
used  with  greater  safety. 

The  distance  between  drains  in  muck  should  be  from 
one  hundred  to  five  hundred  feet,  depending  much  on 
the  nature  of  the  subsoil.  Since  the  surface  is  likely  to 
be  relatively  flat,  nothing  smaller  than  four-  or  five-inch 
tile  should  be  employed  and  the  joints  should  be  carefully 
protected  as  described  above. 

Since  the  capillary  power  of  muck  soil  is  low,  the  water 
table  should  not  be  lowered  more  than  from  two  to  three 
feet,  depending  on  the  quality  of  the  soil.     While  the 


1 


LAND  DRAINAGE  659 

bottom  of  the  open  ditch  may  go  below  this  level,  it  is 
often  advisable  to  insert  check  gates  to  hold  the  water 
level  when  it  has  been  lowered  to  the  desired  depth. 

561.  Drainage  of  irrigated  and  alkali  lands.  —  Exces- 
sive irrigation  and  the  occurrence  of  underground  seep- 
age has  resulted  in  the  water-logging  of  extensive  tracts 
of  arid  and  semiarid  land,  and  in  the  serious  accumulation 
of  alkali  salts  in  the  surface  soil.  An  effective  remedy 
for  this  condition  is  the  installation  of  a  thorough  system 
of  drains,1  preferably  underdrains,  coupled  with  heavy 
irrigation  by  means  of  which  the  excess  salt  is  leached  out 
in  the  drainage  water.  The  most  seriously  alkaline  land 
is  now  being  effectively  reclaimed  by  drainage,  for  the 
production  of  alkali-sensitive  crops. 

For  this  purpose  drains  are  installed  deeper  than  is  the 
custom  in  humid  regions,  in  order  to  reduce  the  capillary 
rise  of  moisture  to  the  surface  of  the  soil,  where  the  alkali 
salts  are  deposited  in  injurious  amounts.  The  drains 
are  often  placed  at  depths  of  from  four  to  six  feet.  Special 
care  is  also  taken  to  intercept  the  underground  seepage. 
Sometimes  the  seepage  water  from  leaky  canals  and  reser- 
voirs and  from  over-irrigation  may  pass  long  distances 
in  porous  gravel  strata  and  rise  to  the  surface  of  the  land 
on  encountering  some  impervious  obstruction.  In  such 
cases  wells  may  be  sunk  many  feet  to  the  water-bearing 
stratum,  and  the  water  thus  conducted  away  in  drains 
far  enough  below  the  surface  to  avoid  injury  to  the  soil. 

Many  special  problems  are  encountered,  such  as  the 
occurrence  of  hardpan  —  usually  a  stratum  cemented  by 
alkaline  carbonates  —  and  the  development  of  a  serious 

1  Elliott,  C.  G.  Development  of  Methods  of  Draining  Irri- 
gated Lands.  U.  S.  D.  A.,  Office  Exp.  Sta.,  Ann.  Rept., 
pp.  489-501.     1910. 


660       SOILS:    PROPERTIES   AND    MANAGEMENT 

quicksand  condition  of  soil.  The  hardpan  may  need  to 
be.  partially  broken  up  by  dynamite.  The  latter  condi- 
tion may  require  the  placing  of  the  tile  on  boards  or  the 
use  of  wooden  box  drains  to  keep  the  alignment. 

Coupled  with  deep  drainage,  sufficient  irrigation  water 
is  employed  to  produce  heavy  percolation,  by  means  of 
which  the  excess  salt  is  removed.  The  most  alkaline 
land  can  usually  be  reclaimed  in  two  or  three  years  of 
leaching. 

562.  Vertical  drainage.  —  A  gravity  outlet  for  drainage 
is  sometimes  difficult  to  provide.  In  such  a  case  it  may 
be  possible  to  remove  the  drainage  water  through  some 
porous  stratum  below  the  surface.  There  must  be  such 
a  porous  stratum  within  reach  below  the  surface,  in  order 
to  render  the  method  of  vertical  drainage  practicable. 
Basin-shaped  areas  without  an  outlet  may  be  wet  because 
of  the  accumulation  of  a  thin  layer  of  clay  or  other  im- 
pervious sediment  in  its  lowest  part,  beneath  which  at 
a  short  distance  is  a  porous  gravel  or  sand  formation. 
Anything  that  perforates  this  impervious  layer  and  keeps 
open  the  passage  will  afford  drainage.  Wells  several 
feet  in  diameter  may  be  constructed  and  filled  with  stone. 
Tile  drains  and  open  drains  have  been  emptied  into  such 
structures.  An  opening  of  temporary  efficiency  may  be 
formed  by  a  charge  of  dynamite.  The  tendency  of  such 
an  opening,  however,  is  to  become  clogged. 

A  second  condition  under  which  vertical  drainage  may 
be  advisable  exists  in  a  soil  that  is  underlaid  within  a  few 
hundred  feet  by  a  limestone  or  other  porous  rock  forma- 
tion into  which  the  surface  water  may  be  emptied.  A 
casing  may  be  installed  to  protect  the  walls  of  the  well 
and  to  reach  from  the  surface  to  the  porous  stratum. 
In  addition  a  trapped  intake,  coupled  with  a  silt  basin, 


LAND  DRAINAGE  661 

may  be  placed  at  the  top  of  the  well  to  insure  its  continu- 
ous operation.  Extensive  systems  of  underdrains  are 
reported  to  have  been  discharged  by  this  arrangement, 
where  it  might  otherwise  have  been  necessary  to  go  a 
long  distance  in  order  to  obtain  an  outlet.  It  should  be 
noted  that  in  many  cases  a  sufficiently  porous  stratum 
is  lacking  in  the  structure  of  the  surface  portion  of  the 
earth,  so  that  the  method  could  not  often  be  employed. 

563.  Drainage  by  means  of  explosives.  —  The  use  of 
explosives  for  promoting  drainage  has  been  proposed 
for  three  conditions  :  — 

1.  To  break  up  a  hard  subsoil  and  possibly  make  a 
connection  with  a  more  porous  stratum  below,  so  that  the 
soil  could  better  handle  the  normal  rainfall.  This  is 
closely  related  to  the  operations  of  subsoiling. 

2.  To  break  through  a  thin  impervious  layer  in  the 
bottom  of  a  wet  basin-shaped  area.  This  is  identified 
with  vertical  drainage  described  above. 

3.  To  open  up  channels  for  drainage  purposes.  This 
use  is  the  most  extensive.  By  proper  distribution  of  the 
charges  of  explosives,  coupled  with  favorable  soil  and 
weather  conditions,  a  very  good  channel  can  be  opened 
by  this  method.  It  is  suited  only  to  the  excavation  of 
open  ditches  of  medium  size,  three  feet  or  more  in  width, 
and  it  has  the  greatest  advantages  where  the  land  is 
much  obstructed  by  stone  or  stumps.  The  force  of  the 
explosive  largely  clears  the  ditch  of  earth  and  obstructions. 
No  very  accurate  grading  of  the  bottom  of  the  ditch  can 
be  accomplished  by  this  method. 

564.  Resume.  —  The  removal  of  the  excess  water 
from  the  soil  by  any  means  constitutes  drainage  and  is 
one  of  the  most  fundamental  operations  in  soil  manage- 
ment.    The  effects  of  adequate  drainage  are  numerous 


662      SOILS:    PROPERTIES  AND  MANAGEMENT 

and  far-reaching.  In  its  accomplishment  the  physical 
properties  of  the  soil  and  its  moisture  relations  must  be 
taken  into  account.  Whether  open  ditches  or  under 
drains  are  employed  depends  on  the  local  conditions,  but 
where  practicable  underdrains  are  always  to  be  chosen. 
While  the  cost  of  drainage  is  a  considerable  sum,  the 
improvement  when  well  made  is  of  long  duration  and  the 
cost  may  therefore  be  distributed  over  a  long  period. 
The  benefits  accrue  not  only  in  increased  crops,  which 
are  generally  large,  but  also  in  the  saving  of  expense  in 
operation.  Good  drainage  is  the  basis  of  good  soil 
management. 


CHAPTER   XXIX 
TILLAGE 

While  the  farmer  depends  somewhat  largely  on  the 
weathering  agencies  for  granulation  of  his  soil,  maximum 
tilth  can  be  obtained  only  by  certain  external  operations. 
The  advantages  to  be  derived  from  drainage  have  been 
pointed  out.  The  importance  of  the  addition  of  lime 
and  organic  matter  as  a  means  of  soil  improvement  has 
been  emphasized.  Yet,  after  all  these  have  been  pro- 
vided, a  further  fundamental  practice  remains  to  be 
followed.  This  practice  is  tillage,  or  the  manipulation 
of  the  soil  by  means  of  implements  so  that  its  struc- 
tural relationships  may  be  made  better  for  crop  growth. 
Tillage  is  so  general  in  its  application,  so  pronounced  in 
its  effects,  and  so  complex  in  its  modes  of  operation,  and 
has  to  do  with  so  many  machines  employing  different 
mechanical  principles,  that  it  requires  discussion  by  itself. 

565.  Objects  of  tillage.  —  Tillage  aims  to  accomplish 
three  primary  purposes :  (1)  modification  of  the  struc- 
ture of  the  soil;  (2)  disposal  of  rubbish  or  other  coarse 
material  on  the  surface,  and  the  incorporation  of  manures 
and  fertilizers  into  the  soil ;  (3)  deposition  of  seeds  and 
plants  in  the  soil  in  position  for  growth. 

The  most  prominent  of  these  purposes  is  the  modifi- 
cation of  the  soil  structure.  This  affects  the  retention 
and  movement  of  moisture,  aeration,  and  the  absorption 
and  retention  of  heat,  and  either  promotes  or  retards  the 

663 


66±      SOILS:    PROPERTIES  AND  MANAGEMES  1 

growth  of  organisms.  Through  all  these  factors  the  com- 
position of  the  soil  solution,  and  finally  the  penetration 
of  plant  roots,  is  influenced.  The  creation  of  a  soil 
mulch  is  merely  a  change  in  the  structure  of  the  soil  at 
such  times  and  in  such  a  manner  as  will  prevent  evapora- 
tion of  moisture.  For  this  reason  it  is  essential  to  under- 
stand the  relation  of  soil  structure  to  the  movement  of 
moisture  in  managing  the  mulch.  In  fine-textured  soils, 
in  which  the  granular  or  crumb  structure  is  most  desired, 
tillage  may  have  an  important  influence  on  the  formation 
or  destruction  of  granules.  As  has  been  pointed  out, 
any  treatment  that  increases  the  number  of  lines  of  weak- 
ness in  the  soil  structure  facilitates  the  action  of  the  mois- 
ture films  and  the  colloidal  material  in  solidifying  the 
soil  granules.  Tillage  shatters  the  soil  and  breaks  it  into 
many  small  aggregates  which  may  be  further  drawn 
together  and  loosely  cemented  as  a  result  of  the  evapo- 
ration of  moisture.  The  more  numerous  the  lines  of 
weakness  produced,  the  more  pronounced  is  the  granu- 
lation; and,  conversely,  the  fewer  the  lines  of  weakness 
produced,  the  more  coarse  and  cloddy  is  the  structure. 

566.  Implements  of  tillage.  —  The  implements  adapted 
to  the  manipulation  of  the  soil  are  very  numerous,  and 
embrace  many  types.  Many  operations  are  compre- 
hended by  the  term  tillage,  which  includes  the  use  of  all 
those  implements  that  are  used  to  move  the  soil  in  any 
way  in  the  practice  of  crop  production.  It  includes  the 
smallest  hand  implements  as  well  as  the  heaviest  trac- 
tion machinery. 

567.  Effects  on  the  soil.  —  All  these  operations  may 
be  divided  into  two  groups,  according  to  their  effect  on 
the  soil,  —  those  that  loosen  the  soil  structure,  and  those 
that    compact    the    soil    structure.     In    the    subsequent 


TILLAGE  665 

paragraphs  of  this  chapter  the  effect  of  the  commoner 
types  of  tillage  implements  on  the  soil  are  pointed  out 
as  a  guide  to  their  selection  for  the  accomplishment  of 
a  desired  modification.  Good  soil  management  consists, 
first,  in  analyzing  the  soil  conditions,  in  order  to  deter- 
mine the  change  that  should  be  effected ;  and  second,  in 
the  selection  of  the  implement  or  other  treatment  that 
will  most  readily  and  economically  accomplish  the  object. 

568.  Classes  of  tillage  implements.  —  According  to 
their  mode  of  action,  tillage  implements  may  be  divided 
into  three  groups,  —  plows,  cultivators,  packers  and 
crushers. 

569.  Plows.  —  The  primary  function  of  a  plow  is  to 
take  up  a  ribbon  of  soil,  twist  it  upon  itself,  and  lay 
it  down  again  bottom  side  up, .  or  partially  so.  In  the 
process  two  things  result :  (1)  if  the  soil  is  in  proper  condi- 
tion for  plowing,  it  will  be  shattered  and  broken  up; 
(2)  the  soil  is  partially  or  wholly  inverted,  and  any  rubbish 
is  put  beneath  the  surface. 

570.  Pulverizing  action  of  the  plow.  —  In  twisting,  the 
soil  tends  to  shear  into  thin  layers,  as  already  pointed 
out  (par.  128).  These  layers  are  moved  unequally  upon 
each  other,  as  the  leaves  of  a  book  when  they  are  bent. 
The  result  should  be  a  very  complete  breaking-up  of  the 
soil.  How  thorough  the  breaking-up  will  be  will  depend 
on  (1)  the  condition  of  the  soil,  and  (2)  the  type  of  plow. 
As  to  the  condition  of  the  soil,  there  is  a  certain  optimum 
moisture  content  at  which  the  best  results  will  be  obtained. 
That  condition  of  moisture  is  the  one  that  is  best  for 
plant  growth.  Any  departure  from  this  optimum  moisture 
content  will  result  in  less  efficient  work.  It  has  been  said 
that,  in  proportion  to  the  energy  required,  the  plow  is 
the  most  efficient   pulverizing   implement   used   by   the 


666       SOILS:    PROPERTIES  AND   MANAOEMl 

farmer.  The  optimum  moisture  content  for  plowing  b 
indicated  by  that  moist  state  in  which  a  mass  of  the  soil, 
when  pressed  in  the  hand,  will  adhere  without  puddling 
but  may  be  readily  broken  up  without  injury  to  the 
intimate  soil  structure.  This  is  a  much  more  critical 
stage  for  fine-textured  soils  than  for  coarse-textured  ones. 
Sandy  soils  are  not  greatly  altered  by  plowing  when  out 
of  optimum  moisture  condition.  On  the  other  hand,  if 
a  clay  is  plowed  when  it  is  saturated  with  water,  it  will 
be  thoroughly  puddled  and  will  dry  out  into  a  hard,  lumpy 
condition.  Such  a  structure  requires  a  considerablr  time 
to  remedy. 

571.  Types  of  plows  (Fig.  68).  —  There  are  two  gen- 
eral types  of  turning  plows,  the  common  moldboard  plow 
and  the  disk  plow.  Their  mode  of  action  is  quite  dif- 
ferent, although,  so  far  as  the  soil  is  concerned,  the  result 
is  much  the  same.  The  moldboard  plow  seems  to  have 
a  wider  application  than  the  disk  plow,  but  both  have 
a  particular  sphere  of  usefulness. 

The  disk  plow  is  essentially  a  large  revolving  disk 
set  at  such  an  angle  that  it  cuts  off  and  inverts  the  soil, 
at  the  same  time  pulverizing  it  fairly  effectively  after 
the  manner  of  the  moldboard  plow.  One  advantage 
claimed  for  the  disk  plow  is  its  lighter  draft  for  the  same 
amount  of  work  done,  due  to  its  having  rolling  friction  in 
the  soil  instead  of  sliding  friction.  In  practice  it  appears 
to  be  especially  effective  on  very  dry,  hard  soil  and  in 
turning  and  covering  rubbish. 

For  any  given  texture  of  soil  and  any  given  soil  condi- 
tion, there  is  a  type  of  plow,  a  shape  of  moldboard,  and 
a  depth  of  furrow  slice,  that  will  give  the  best  results. 
This  fact  is  to  be  kept  constantly  in  mind  in  plowing  soil. 
Sod  land  requires  a  different  shape  of  plow  from  fallow 


TILLAGE 


667 


land,  sandy  land  from  clay  land.  Rubbish  on  the  surface 
may  be  handled  by  one  plow  and  not  by  another.  On 
wet  clay  one  should  use  a  different  shape  of  plow  from 
that  which  is  preferable  for  dry  soil. 


"tomatoes      ^-«A 


Atotffoo^tftr 

0*f*,SIOE 

^\.s*0ez*CHAei£  sou 


B£AM  Cievtf 

■rrtt  »r#tiei. 


Fig.  68. — The  plow.  (1),  modern  walking  plow  with  parts  named; 
(2),  types  of  moldboard  for  (a)  fallow  ground,  light  soil,  (b)  fallow 
ground,  clay  soil,  (c)  sod  ground,  (d)  general  purpose,  fairly  well 
suited  to  a  wide  range  of  soil  conditions  ;  (3) ,  deep-tilling  disk  plow  ; 
(4) ,  subsoiler ;  (5) ,  plow  attachments  :  (a)  jointer,  (b)  knife  or 
beam  colter,  (c)  fin  colter,  (d)  rolling  colter. 


572.  Shapes  of  moldboard  plows.  —  Of  the  moldboard 
type  there  are  two  general  shapes :  (1)  The  long,  sloping 
moldboard,  which  rises  very  gradually  and  has  little  or 


668       SOILS:    PROPERTIES  AND  MANAGEMENT 

no  overhang,  found  on  what  is  called  the  sod  plow.  This 
neatly  cuts  off  the  roots  at  the  bottom  of  the  slice,  slowly 
and  gradually  twists  the  soil  over  without  breaking  the 
sod,  and  lays  it  smoothly  up  to  the  previous  furrow  slice. 
(2)  The  short,  steep  moldboard  with  a  marked  overhang. 
This  is  not  adapted  to  sod  land,  because  it  breaks  up 
the  sod  and  shoots  it  over  in  a  rough,  jagged  manner 
with  uneven  turning.  But  on  fallow  land,  to  which  it 
is  adapted,  it  very  completely  breaks  up  the  soil  and 
throws  it  over  in  a  nearly  level,  mellow  mass.  The  pul- 
verizing effect  is  obviously  much  greater  than  with  the 
sod  plow.  Since  the  steep  moldboard,  or  fallow-ground, 
plow  exerts  the  most  force  on  the  soil  in  a  given  time  at 
a  given  speed  of  movement,  it  follows  that  if  a  particular 
soil  is  over- wet  it  should  be  plowed  with  the  sod  plow; 
while,  if  it  must  be  plowed  when  too  dry,  the  fallow-ground 
plow  will  be  more  effective  —  disregarding  the  draft, 
which  will  probably  be  larger  in  the  latter  case. 

573.  Position  of  the  furrow  slice  (Fig.  69).  — Con- 
siderable care  should  be  taken  concerning  the  angle  at 
which  the  furrow  slice  is  placed.  It  is  seldom  desirable 
to  completely  invert  the  soit.  If  it  is  too  flat,  the  stubble 
and  rubbish  are  matted  at  the  bottom  of  the  furrow  and 
tend  to  interfere  with  capillary  movement  for  a  consid- 
erable period.  This  may  cause  serious  difficulty  on 
spring-plowed  soil,  where  the  capillary  connection  does 
not  have  time  to  be  renewed  before  a  crop  occupies  the 
land.  If,  on  the  other  hand,  the  furrow  is  too  steep,  the 
proper  pulverization  does  not  take  place  and  the  turning- 
under  of  stubble  and  rubbish  is  not  satisfactorily  accom- 
plished. The  stubble  and  rubbish  are  likely  to  interfere 
with  subsequent  operations.  7 

The  best  angle  at  which  to  turn  the  furrow  slice  is 


TILLAGE 


669 


about  from  30°  to  40°  with  the  horizontal.  A  furrow  thus 
set  furnishes  ready  entrance  for  rain  water  and  facilitates 
the  best  of  aeration  for  the  soil.  Such  an  angle  is  espe- 
cially to  be  recommended  for  turning  under  green  manures. 
The  capillary  connections  with  the  subsoil  are  not  broken 
and  the  green  material  is  well  distributed  from  the  top 
to  the  bottom  of  the  furrow.  Where  a  sod  is  to  be  plowed, 
a  flatter  turning  of  the  furrow  is  advocated  in  order  to 
increase  the  packing  and  avoid  the  danger  of  the  sod's 
interfering  with  subsequent  cultivation. 


Fig.  69.  —  Section  of  plowed  land  showing  the  correct  proportions  and 
position  of  the  furrow  slice  as  left  by  a  moldboard  plow.  The  effect 
of  the  jointer  in  turning  under  the  edge  of  the  furrow  slice  as  well 
as  the  position  of  turned  under  vegetation  is  apparent. 


574.  Depth  and  width  of  furrow.  —  There  is  a  general 
relation  between  the  width  of  the  furrow  slice  and  its 
depth.     In  general,  it  may  be  said  that  this  ratio  is  about 


670      SOILS:    PROPERTIES  AND  MANAGEMENT 

two  in  width  to  one  in  depth.     The  greater  the  depth,  the 
less  in  proportion  may  be  the  width  of  the  furrow  dice. 

On  clay  soil  in  particular,  there  is  also  a  relation  be- 
tween depth  and  condition.  A  wet  soil  should  be  plowed 
more  shallow,  other  things  being  equal,  than  a  dry  soil, 
because  the  puddling  action  is  less.  On  a  dry  soil  the 
depth  should  be  increased,  in  order  to  increase  the  pul- 
verization. Combining  these  principles,  then,  it  may  be 
said  that  if  a  clay  soil  must  be  plowed  when  too  wet,  it 
should  be  plowed  with  a  sod  plow  and  to  as  shallow  a 
depth  as  is  permissible.  But  on  an  over-dry  soil  the 
opposite  conditions  should  be  fulfilled  —  that  is,  the  use 
of  a  steep  moldboard  and  to  an  increased  depth.  Like- 
wise, on  sandy  soil,  where  the  aim  is  generally  to  compact 
the  structure,  this  may  be  furthered  by  deep  plowing 
with  a  steep  moldboard  when  the  land  is  over-wet. 

575.  Plow  sole.  —  In  connection  with  this  phase  of 
the  subject  it  is  important  to  consider  what  is  often  called 
the  "  plow  sole,"  —  that  is,  the  soil  at  the  bottom  of  the 
furrow,  which  bears  the  weight  of  the  plow  and  the  tram- 
pling of  the  team,  and  which  under  a  uniform  depth  of 
plowing  does  not  become  loosened.  In  clay  soil,  espe- 
cially, it  gradually  becomes  more  compact,  developing 
in  time  something  of  a  hardpan  character,  which  is  detri- 
mental to  the  circulation  of  air  and  moisture  and  inter- 
feres with  the  penetration  of  plant  roots.  Consequently, 
occasional  deep  plowing,  or  even  subsoiling,  is  recom- 
mended to  break  up  this  unfavorable  soil  structure. 
There  is  less  tendency  for  the  disk  plow  than  for  the  mold- 
board  plow  to  form  the  "  sole." 

576.  Hillside  plow.  —  The  hillside  plow  is  a  modified 
form  of  the  moldboard  plow.  It  has  a  double  curvature 
to  the  moldboard,  so  that  it  is  essentially  two  plows  in 


TILLAGE  671 

one.  The  plow  swings  on  a  swivel  in  such  a  way  that 
it  may  be  locked  on  either  the  right  or  the  left  side.  It 
removes  the  necessity  of  plowing  in  beds,  and,  by  per- 
mitting all  the  work  to  be  done  from  one  side,  enables 
the  plowman  to  lay  the  furrow  slices  in  one  direction. 
On  the  hillside  this  direction  is  down  the  slope,  because 
of  the  greater  ease  in  turning  the  soil  in  that  direction. 
This  plow  also  removes  the  difficulty  of  pulling  up  and 
down  the  hill.  There  is  another  type  of  moldboard  plow, 
designed  to  eliminate  "  dead  furrows  "  and  "  back  fur- 
rows." Dead  furrows  are  developed  by  the  last  furrow 
slices  of  two  lands  being  turned  in  opposite  directions, 
thereby  leaving  a  gulley  between,  which  is  often  unpro- 
ductive in  character;  the  back  furrow  consists  of  two 
furrow  slices  thrown  together,  usually  forming  a  ridge 
more  productive  than  the  average  of  the  land.  This 
plow  is  of  the  sulky  type,  the  plow  being  carried  on  wheels 
and  regulated  by  means  of  levers  and  the  traction  power. 
Two  plows  are  carried,  one  having  a  right-hand  turn  to 
the  moldboard,  and  the  other  a  left-hand  turn.  By 
using  one  plow  in  one  direction  and  the  other  in  the  oppo- 
site direction,  it  is  possible  to  begin  on  one  side  of  the  field 
and  throw  the  furrow  slice  in  one  direction  until  the 
entire  area  is  covered,  thereby  leaving  the  soil  in  a  uni- 
form condition.  Such  plows,  being  heavier  than  the 
single,  walking  plow,  are  not  adapted  to  very  uneven 
ground.  ^ 

577.  Covering  rubbish. -^- The  secondary  function  of 
(tthe  plow  is  to  cover  weeds^- manure,  and  rubbish  that 
may  be  on  the  surf acec  This  also  the  turning  plow 
does  very  effectively.~~JThe  cutting  and  turning  of  the 
sod,  rubbish,  and  weeds  is  facilitated  by -several  attach- 
ments, such  as  colters,  jointers,  and  drag  chains.     There 


672       SOILS:    PROPERTIES  AND  MANAGEMENT 

are  several  types  of  colters.  Blade  colters  are  attached 
to  the  beam  or  to  the  share  in  such  a  manner  as  to  cut 
the  furrow  slice  free  from  the  land  side.  Tiny  should 
be  adjusted  in  such  a  position  as  to  cut  the  soil  after  it 
has  been  raised  and  put  in  a  stretched  condition,  at  which 
time  the  roots  are  most  easily  severed.  This  position  is  a 
little  back  of  the  point  of  the  share.  A  knife  edge  attached 
to  the  share  is-  commonly  called  a  fin  colter.  A  jointer 
is  a  miniature  moldboard  attached  to  the  beam  for  cutting 
and  turning  under  the  upper  edge  of  the  furrow  slice, 
so  that  a  neat,  clean  turn  is  effected  without  the  exposure 
of  a  ragged  edge  of  grass  which  may  continue  growth. 
This  is  used  chiefly  on  sod  land.  A  drag  chain  is  an  ordi- 
nary heavy  log  chain,  one  end  of  which  is  attached  to 
the  central  part  of  the  beam  and  the  other  to  the  end  of 
the  double  tree  on  the  furrow  side,  and  with  enough  slack 
so  that  it  drags  down  the  vegetation  on  the  furrow  slice 
just  ahead  of  the  turning  point.  It  is  used  primarily  in 
turning  under  heavy  growths  of  weeds  or  green-manure 
crops. 

578.  Subsoil  plow.  —  There  is  a  third  type  of  plow, 
the  so-called  subsoil  plow.  The  purpose  of  this  imple- 
ment is  to  break  up  and  loosen  the  subsoil  without  mix- 
ing the  material  with  the  soil.  It  consists  essentially  of  a 
small,  molelike  point  on  a  long  shin.  This  implement  is 
drawn  through  the  bottom  of  the  furrow,  and  shatters 
and  loosens  the  subsoil  to  a  depth  of  18  inches  or  2  feet. 
j[t  is  often  useful  on  soils  having  a  dense,  hard  subsoil. 
Its  use  requires  the  exercise  of  judgment,  as  the  process 
may  prove  very  injurious  if  done  out  of  season.  As  a 
general  rule,  it  is  best  to  use  the  subsoil  plow  in  the  fall, 
when  the  subsoil  is  fairly  dry  and  may  in  a  measure  be 
recompacted  by  the  winter  rain.     Spring   subsoiling  is 


TILLAGE  673 

seldom  advisable  in  humid  regions,  owing  to  the  danger 
of  puddling  the  subsoil,  or  to  the  possibility  of  its  remain- 
ing too  loose  for  best  root  development  if  the  work  is  done 
when  the  subsoil  is  dry  enough  not  to  puddle. 

579.  Cultivators  (Fig.  70). — There  are  more  types 
of  cultivators  than  of  any  other  form  of  soil-working  im- 
plements. These  may  be  grouped  into  (1)  cultivators 
proper ;  (2)  leveler  and  harrow  types  of  cultivators ;  (3) 
seeder  cultivators.  These  implements  agree  in  their  mode 
of  action  on  the  soi vm  that  they  lift  up  and  move  it  side- 
wise  with  a  stirring  action  which  loosens  the  structure  and 
cuts  off  weeds,  and  to  a  slight  degree  covers  rubbish.  How- 
ever, the  action  is  primarily  a  stirring  one,  and,  in  general, 
it  is  much  shallower  than  that  of  the  plow.  One  impor- 
tant fact  should  be  kept  in  mind  in  cultural  operations, 
especially  those  just  following  the  plowing;  that  is,  the 
work  should  be  done  when  the  soil  is  in  the  right  moisture 
condition.  Particularly  is  this  true  in  the  pulverization 
following  the  plowing.  Plowing,  if  it  is  properly  done, 
leaves  the  soil  in  the  best  possible  condition  to  be  further 
pulverized.  It  is  properly  moistened,  and  if  the  clods 
are  not  shattered  they  are  reasonably  frail  and  may  be 
much  more  readily  broken  down  than  when  they  are 
permitted  to  dry  out.  In  drying  they  are  somewhat 
cemented  together  and  thereby  hardened.  Not  only  is 
it  desirable  in  almost  all  cases  to  take  advantage  of  this 
condition  of  the  soil,  but  the  leveling  and  pulverizing 
of  the  soil  reduces  drying  and  improves  the  character  of 
the  seed  bed. 

580.  Cultivators  proper.  —  There  is  a  great  variety 
in  types  and  patterns  of  cultivators.  They  may  be 
divided  into  large  shovel  forms  and  small  shovel  forms, 
and  the  duck-foot  form.     The  first  type  has  a  few  com- 

2x 


674       SOILS:    PROPERTIES  AND  MANAGEMENT 


paratively  large  shovels  set  rather  far  apart,  which  vigor- 
ously tear  up  the  earth  to  a  considerable  depth  and  leave 
it  in  large  ridges.  There  is  a  lack  of  uniform  action,  and 
the  bottom  of  the  cultivated  part  is  left  in  hard  ridges* 
Such  implements  are  now  much  less  used  than  they  were 
formerly,  and  may  be  considered  to  supplant  in  a  measure 
the  use  of  the  plow,  where  deep  working  without  turning 


&nJi 


Fig.  70.— Types  of  cultivators:  (1),  wheel  hoe,  or  hand  garden  culti- 
vator, with  attachments;  (2),  adjustable  small-tooth,  one-horse 
cultivator,  with  duck-foot  shovel  behind;  (3),  two- horse  spring- 
toothed  cultivator;  (4),  two-horse  sweep  or  knife  cultivator; 
(5) ,  two-horse  disk  cultivator. 


TILLAGE  675 

is  desired.  Some  of  the  wheel  hoes  used  in  orchard 
tillage  belong  to  this  type.  The  single  and  double  shovel 
plows  are  earlier  types  of  the  same  implement. 

The  small  shovel  cultivators  have  very  generally  sup- 
planted the  large  shovel  type  in  most  cultural  work. 
The  decrease  in  size  of  shovels  is  made  up  by  the  great 
increase  in  number.  Ordinarily  they  operate  to  shallow 
depths,  but  very  thoroughly  and  uniformly.  They  are 
now  much  preferred  in  all  intertillage  work  for  eradication 
of  small  weeds  and  the  formation  of  a  loose  surface  mulch. 

The  duck-foot  cultivator  —  or  sweep  as  it  is  called  in 
the  southern  states,  where  it  is  extensively  used  in  the 
cultivation  of  cotton  —  is  a  broad  blade  that  operates 
in  a  nearly  horizontal  position  an  inch  or  two  beneath  the 
surface.  The  surface  layer  of  soil  is  severed  and  raised 
slightly  from  the  under  soil,  and  is  somewhat  crumbled 
in  the  operation.  This  tool  is  very  efficient  in  establishing 
and  maintaining  a  mulch  and  in  destroying  weeds.  It 
covers  every  part  of  the  soil.  The  implement  is  increasing 
in  popularity  in  the  northern  and  eastern  states.  It  is 
not  adapted  for  use  in  very  stony  or  hard  soil. 

Another  classification,  which  has  less  relation  to  utility 
than  to  the  convenience  and  comfort  of  the  operation, 
is  based  on  the  presence  or  the  absence  of  wheels.  There 
is  a  strong  movement  toward  the  use  of  wheel  cultivators 
carrying  a  seat  for  the  operator.  These  have  a  wider 
range  of  operation  as  to  depth  and  facility  of  movement 
than  have  the  cultivators  without  wheels. 

Still  further,  there  is  the  distinction  of  shovels  from 
disks.  Disks  are  used  on  the  larger  cultivators  but  seldom 
on  the  small  ones. 

Cultivators  may  be  constructed  to  till  one  or  more  rows 
at  a  time. 


676      SOILS:    PROPERTIES  AND  MANAGEMENT 

581.  Leveler  and  harrow  types  of  cultivator  (Fig.  71). 
—  In  this  group  are  the  spike-tooth  harrow,  the  smoothing 
harrow,  the  spring-tooth  harrow,  the  disk  harrow,  the 
spading  harrow,  weeders,  and  the  Acme  and  Meeker 
harrows. 

The  spike-tooth  harrow  is  essentially  a  leveling  imple- 
ment, adapted  to  very  shallow  cultivation  of  loose  soils. 
It  is  also  something  of  a  cleaner,  in  that  it  picks  up  surface 
rubbish^  The  spring-tooth  harrow  works  more  deeply 
than  does  the  spike-tooth  harrow,  and  can  therefore  be 
used  in  many  soils  for  which  the  latter  is  not  adapted. 
In  working  down  cloddy  soil  it  brings  the  lumps  to  the 
surface,  where  they  may  be  crushed^  The  disk  harrow 
depends  for  its  primary  advantage  on  the  conversion  of 
sliding  friction  into  rolling  friction.  Its  draft  is  therefore 
less  for  the  same  amount  of  work  done.  It  has  a  vigorous 
pulverizing  action  similar  to  that  of  the  plow,  surpassing 
shovel  cultivators  in  this  respect.  The  disk  harrow  is 
not  adapted  to  stony  soil,  but  the  toothed  forms  are  as 
effective  on  such  soil  as  on  soil  free  from  stones,  as  long 
as  the  stones  are  not  large  enough  to  collect  in  the  imple- 
ment. On  the  other  hand,  on  land  full  of  coarse  manure, 
sod,  and  the  like,  the  disk  harrow  is  the  more  efficient. 
The  spading  harrow  (cutaway  disk)  is  very  little  different 
from  the  disk  harrow,  except  that  it  takes  hold  of  the  soil 
more  readily.  A  recent  attempt  to  bring  about  a  high 
degree  of  pulverization,  and  with  greater  uniformity,  is 
represented  by  the  double-disk  implements.  In  these 
implements  there  are  two  sets  of  disks,  one  set  in  front 
of  and  zigzagged  with  the  other,  and  the  two  adjusted 
so  as  to  throw  the  soil  in  opposite  directions. 

Weeders  are  a  modified  form  of  the  spring-tooth  har- 
row, adapted  to  shallow  tillage  of  friable,  easily  worked 


TILLAGE 


677 


soil,  where  the  aim  is  to  kill  weeds  and  create  a  thin  sur- 
face mulch.  They  are  wide  and  are  fitted  with  handles, 
and  therefore  have  an  intermediate  place  between  culti- 
vators proper  and  harrows.  They  are  much  used  for 
intertillage  of  young  crops. 


Fig.  71. — Types  of  harrows:  (1),  spike-tooth;  (2),  spring-tooth; 
(3),  weeder;  (4),  double  disk  (note  that  the  forward  disks  are 
solid  while  the  rear  disks  are  of  the  cut-out  type)  ;  (5),  spading 
disk ;  (6) ,  Acme.  All  these  belong  to  the  cultivator  group  of  im- 
plements. 


The  Acme  harrow  consists  of  a  series  of  twisted  blades 
which  cut  the  soil  and  work  it  over.  They  are  most 
useful  in  the  later  stages  of  pulverization  on  soil  relatively 
free  from  stones.  The  Meeker  harrow  is  a  modified  form 
of  disk,  used  primarily  for  pulverization.  It  consists  of 
a  series  of  lines  of  small  disks  arranged  on  straight  axles, 
and  is  especially  adapted  to  breaking  up  hard,  lumpy  soil. 


678       SOILS:    PROPERTIES  AND  MANAGEMENT 

In  this  particular  it  may  be  considered  as  belonging  to  the 
third  set  of  implements,  the  clod  crushers.  But  as  com- 
pared with  the  roller  on  hard  soil  it  is  more  efficient. 

582.  Seeder  cultivators.  —  Many  implements  used 
primarily  for  seeding  purposes  are  also  cultivators,  and 
their  use  is  equivalent  to  cultivation.  The  grain  drill 
is  a  good  example  of  this  group.  It  is  essentially  a  culti- 
vator —  either  shoe  or  disk  —  adapted  to  depositing  the 
grain  in  the  soil  at  the  proper  depth.  All  types  of  planters 
that  deposit  the  grain  in  the  soil  have  a  similar  action 
on  the  structure  of  the  soil.  The  ordinary  two-row  maize 
planter,  the  potato  planter,  and  the  like,  while  of  low 
efficiency  as  cultivators,  still  have  an  effect  which  is 
measurable.  This  action  is  well  seen  in  the  lister,  used 
for  planting  maize,  by  which  the  grain  is  deposited  beneath 
the  furrow,  which  is  filled  by  cultivation  after  the  grain 
is  up.  The  lister  is  generally  used  without  previously  plow- 
ing the  ground,  and  its  use  is  limited  to  regions  of  low  rain- 
fall where  the  soil  is  aerated  by  natural  processes.  Plowed 
ground  listers  have  lately  been  introduced,  which  com- 
bine the  advantages  of  deep  planting  with  proper  prep- 
aration of  the  soil. 

There  is  also  a  very  considerable  tillage  action  in  many 
harvesting  implements.  The  potato  digger,  for  example, 
very  thoroughly  breaks  up  and  cultivates  the  soil,  and 
this  process  is  one  important  reason  for  the  general 
high  yield  of  crops  following  the  potato  crop.  Bean 
harvesters  and  beet  looseners  also  have  a  similar  action 
on  the  soil. 

583.  Packers  and  crushers.  —  These  may  be  divided 
into  two  groups  —  those  implements  that  aim  to  compact 
the  soil,  and  those  the  primary  purpose  of  which  is  to 
pulverize  the  soil  by  crushing  the  lumps.     Both  kinds 


TILLAGE  679 

of  implements  have  something  of  the  same  action  on  the 
soil.  That  is  to  say,  any  implement  that  compacts  the 
soil  does  a  certain  amount  of  crushing;  and,  conversely, 
any  implement  that  crushes  the  soil  does  some  com- 
pacting. 

584.  Rollers  (Fig.  72).  —The  type  of  the  first  group 
is  the  solid,  or  barrel,  roller,  ^vhich  by  its  weight  tends  to 
force  the  particles  of  soil  nearer  together  and  to  smooth 
the  surface/)  The  smaller  the  diameter  in  proportion  to  its 
weight,  the  greater  is  the  effectiveness  of  the  roller.  Its 
draft  is  correspondingly  greater.  As  a  crusher,  the  roller 
is  relatively  inefficient  on  hard,  lumpy  soil,  because  of  its 
large  bearing  surface.  Lumps  are  pushed  into  the  soft 
earth  rather  than  crushed. 

It  should  be  mentioned  that  there  is  one  condition 
under  which  the  roller  is  effective  in  loosening  up  the  soil 
structure.  This  is  on  fine  soil  on  which  a  crust  has 
developed  as  a  result  of  light  rainfall.  Here  the  roller 
may  break  up  the  crust  and  restore  a  fairly  effective  soil 
mulch. 

Another  form  of  roller  is  the  subsurface  packer.  One 
type  of  this  implement  consists  of  a  series  of  wheels  with 
narrow,  V-shaped  rims,  which  press  into  the  soil  and  com- 
pact it  while  leaving  the  surface  loose.  The  wheels 
are  designed  primarily  to  smooth  the  land  after  plowing, 
and  tojbring  the  furrow  slices  close  together  and  in  good 
contact  with  the  subsoil,  in  order  to  conserve  moisture 
and  promote  decay  of  organic  material  that  may  be  plowed 
underp  This  packer  has  been  developed  chiefly  in  semi- 
arid^and  arid  sections  of  country  where  the  conservation 
of  moisture  is  especially  important,  but  it  might  well 
have  a  much  larger  use  for  the  same  purpose  in  sections 
of  the  country  that  are  subject  to  late  summer  and  fall 


680       SOILS:    PROPERTIES  AND  MANAGEMENT 

droughts.     While   compacting   the   soil,   this   implement 
leaves  a  mulch. 


fmuui«u««u«(((T 


Fig.  72.  — Types  of  packers  and  pulverisers  :  (1),  solid  or  barrel  roller; 
(2) ,  corrugated  roller ;  (3) ,  crusher  and  subsurface  packer ;  (4) , 
bar  roller. 

585.  Clod  crushers.  —  The  aim  of  these  clod  crushers 
is  to  break  up  lumps.  As  to  mode  of  action,  there  are 
several  forms.  The  corrugated  and  the  bar  roller  and 
the  clod  crusher  concentrate  their  weight  at  a  few  points, 
and  are  open  enough  so  that  the  fine  earth  is  forced  up 
between  the  bearing  surfaces.  They  are  very  effective 
in  reducing  lumpy  soil  to  comparatively  fine  tilth.  They 
have  very  little  leveling  effect  further  than  the  breaking- 
down  of  lumps. 

The  planker,  drag,  or  float,  variously  so-called,  con- 
sists essentially  of  a  broad,  heavy  weight  without  teeth, 
which  is  dragged  over  the  soil.  The  lumps  are  rolled 
under  its  edge  and  ground  together  in  a  manner  which 
very  effectively  reduces  their  size.  At  the  same  time 
the  soil  is  leveled,  smoothed,  and,  to  a  degree,  compacted. 
This  implement  may  well  be  used  in  the  place  of  the  roller 


TILLAGE  681 

as  a  pulverizer,  on  many  occasions.     It  is  constructed  in 
many  forms. 

586.  Efficient  tillage.  —  Efficient  tillage  requires  an 
understanding  of  the  properties  of  the  soil,  good  practi- 
cal judgment  as  to  its  condition,  facility  in  the  selection 
of  the  proper  implements  for  its  modification,  and  me- 
chanical skill  in  their  operation.  The  same  result  may 
often  be  attained  in  different  ways,  and  the  practical 
necessity  that  frequently  arises  for  the  farmer  to  get  on 
with  a  relatively  few  tillage  implements  where  a  variety 
of  soil  conditions  must  be  dealt  with  draws  heavily  on  his 
resourcefulness. 


CHAPTER   XXX 
IRRIGATION  AND   DRY-FARMING 

Irrigation  1  is  the  application  of  water  to  the  soil  tor 
the  purpose  of  growing  crops.  It  i>  supplementary  to  the 
natural  precipitation.  The  quantity  of  water  applied 
and  the  time  of  application  must  therefore  be  determined 
by  the  character  of  the  rainfall. 

587.  Relation  of  irrigation  to  rainfall. — The  limit  of 
rainfall  where  irrigation  becomes  necessary  is  not  a  fixed 

1  Widtsoe,  J.  A.  Principles  of  Irrigation  Practice.  New 
York.     1914. 

Olin,  W.  H.     American  Irrigation  Farming.     Chicago.    1913. 

Bowie,  A.     Practical  Irrigation.     New  York.     1908. 

King,  F.  H.     Irrigation  and  Drainage.     New  York.     1899. 

Paddock,  W.,  and  Whipple,  O.  B.  Fruit  Growing  in  Arid 
Regions.     New    York.     1910. 

Newell,   F.   H.     Irrigation.     New  York.     1902. 

Mead,  E.     Irrigation  Institutions.     New  York.     1903. 

Mead,  E.  Preparing  Land  for  Irrigation  and  Methods  of 
Applying  Water.  U.  S.  D.  A.,  Office  Exp.  Sta.,  Bui.  No.  145. 
1904. 

Wickson,  J.  A.  Irrigation  among  Fruit  Growers  on  the 
Pacific  Coast.  U.  S.  D.  A.,  Office  Exp.  Sta.,  Bui.  No.  108. 
1902. 

Widtsoe,  J.  A.,  and  Merrill,  L.  A.  Methods  for  Increasing 
the  Crop  Producing  Power  of  Irrigation  Water.  Utah  Agr. 
Exp.  Sta.,  Bui.  No.  116.     1912. 

Fortier,  S.  The  Use  of  Small  Water  Supplies  for  Irrigation. 
U.  S.  D.  A.,  Yearbook,  p.  409.     1907. 

Fortier,  S.  Irrigation  of  Orchards.  U.  S.  D.  A.,  Farmers' 
Bui.  404.     1910. 


IRRIGATION  AND  DRY-FARMING 


683 


amount.  Irrigation  is  practiced  in  all  parts  of  the  world 
—  in  those  regions  where  the  rainfall  is  50  and  60  inches 
a  year,  as  well  as  in  those  regions  where  it  is  only  20  inches 
or  less.  (See  Fig.  73.)  The  need  of  irrigation  is  de- 
termined by  (1)  the  time  when  the  rainfall  occurs, 
(2)    the   way  in  which    it  occurs,   whether    in  small    or 


5 

♦ 

2 

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JflN  |  Tt B  |  MAR  |  APR  |  MAY  | JIM  |  JULY  |  AU6  |Sf  PT  |  OCT  |  NOV  |  DEC 

YUMA 

BUFFALO 

■ 

■ 

1 

■ 

1 

1 

1 

1 

1 

| 

1 

1 

I 

1 

I 

1 

1 

1 

1 

1 

1 

1 

1 

I 

1 

1 

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i 

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Fig.  73.  —  Diagram  showing  the  extent  and  distribution  of  rainfall  in  an 
arid  region  (Yuma,  Arizona),  and  a  humid  region  (Buffalo,  New 
York). 


large  quantities,  (3)  the  nature  of  the  soil,  (4)  the 
air  temperature  and  wind  movement,  and  (5)  the  nature 
and  value  of  the  crops  grown.  Other  factors,  such 
as  the  cost  of  applying  water,  methods  of  tillage, 
and  market  facilities,  have  some  influence  in  deter- 
mining the  practicability  of  irrigation.  Irrigation  is 
usually  associated  with  a  low  rainfall  of  20  or  25  inches 
a  year.  Using  these  figures  as  a  measure  of  the  need 
of  irrigation  throughout  the  world,  it  appears  that  about 
60  per  cent  of  the  earth's  surface  has  so  low  a  rainfall  that 
irrigation  is  necessary  in  order  to  secure  paying  yields  of 
crops.  About  25  per  cent  of  the  earth's  surface  receives 
10  inches  or  less  of  rainfall  annually.  About  30  per  cent 
receives  between  10  and  20  inches,  and  about  10  per  cent 


684      SOILS:    PROPERTIES  AND  MANAGEMENT 

receives  between  20  and  30  Inches.     Every  continental 

area  has  its  arid  portion  where  the  rainfall  drops  below 
10  inches.  (See  Fig.  75.)  These  sections  are  usually  in 
the  interior,  but  their  position  depends  on  the  topography 
of  the  land  and  the  direction  of  the  moisture-laden  winds. 
Sometimes,  as  in  the  western  United  States,  the  coastal 
mountains  cause  an  arid  climate  in  the  adjacent  interior 
valleys,  some  of  which  extend  quite  out  to  the  ocean 
in  southern  California. 


\iPM\m\iwi\m\m  \m\xi\m  |am|ocr|w  lore  j^|rtBl»*w|v«|Nnr|i«  |jv|ac  |scn|acr  |mr|at 

SAN    DIEGO                                      BOISE 

25- 

.c  =.  --E-_                     ■--■.- 

II      -•■■Li'i       i 

i    ll              llill*      *ll 

!  1 II  i ....  1 1  1 1 1 1 1 1 . .  1 1 1 1 

TUCSON                                  DENVER 

25  - 

"i       l           i        : 

i  ___  I          it  .     _ 

=  .l.       I  l-.i      .lllll.i 

hi  ii  ii  innni  11 11 » 11 11  mm  11 11 1       im 

Fig.  74. 


■Four  types  of  rainfall.     The  diagrams  show  the  distribution 
by  months. 


It  has  been  estimated  that  the  total  available  water 
supply  is  sufficient  to  irrigate  only  one-tenth  to  one-fifth 
of  the  proportion  of  the  earth's  surface  in  need  of  such 
treatment. 


IRRIGATION  AND  DRY-FARMING  685 

588.  Extent  of  irrigated  land.  —  In  1905,  Mead1 
estimated  the  total  area  of  land  irrigated  at  100,000,000 
acres.  Since  that  date  the  practice  of  irrigation  has 
been  extended  rapidly  in  all  parts  of  the  world,  and  it  is 
probable  that  at  the  present  time  the  total  area  of  land 
irrigated  is  at  least  200,000,000  acres.  In  Egypt,  in 
Australia,  and  in  India,  as  well  as  in  the  United  States, 
large  projects  for  irrigation  developments  have  recently 
been  undertaken.  In  the  United  States,  according  to 
the  Thirteenth  Census,  the  area  of  land  irrigated  increased 
7,500,000  acres  between  1899  and  1909.  At  the  latter 
date  enterprises  for  the  provision  of  water  were  under 
way  to  cover  a  total  of  31,000,000  acres. 

589.  History  of  irrigation.  —  The  practice  of  irriga- 
tion is  very  ancient.  The  very  earliest  records  of  the 
peoples  in  the  valleys  of  the  Nile  and  Euphrates  rivers, 
in  Africa  and  Asia,  mention  large  irrigation  works. 
In  China  and  India  also  the  practice  is  very  old.  The 
remains  of  ancient  works  for  irrigation  often  amaze 
the  modern  engineer  by  their  size  and  excellence  of  con- 
struction, considering  the  facilities  that  were  available. 
As  early  as  2084  B.C.  an  artificial  lake  fifty  miles  in  cir- 
cumference was  constructed  in  Egypt,  communicating 
with  the  Nile  through  a  canal.  The  Great  Imperial 
Canal  in  China,  connecting  the  Hoangho  River  with  the 
Yangtze,  was  650  miles  long  and  had  several  lakes  in  its 
course.  In  Peru,  Mexico,  and  the  southwestern  United 
States,  there  exist  remains  of  very  extensive  irrigation 
works  of  great  antiquity.  In  Argentina  large  irrigation 
canals  may  still  be  traced  for  from  four  to  ^.ve  hundred 


1  Mead,  E.     Irrigation  Engineering  and  Practice.     American 
Cyclopedia  of  Agriculture,  p.  420,     New  York.     1907. 


686      SOILS:    PROPERTIES  AND  MANAGEMENT 

miles.  In  the  Verde  River  valley  in  Arizona,  remains 
of  the  cliff  dwellings,  which  were  scattered  long  before  the 
advent  of  the  Spanish  explorers,  are  associated  with  ex- 
tensive irrigation  canals  showing  much  skill.  The  ditches 
and  the  reservoirs  were  finished  with  hard  linings  of  tamped 
or  burned  clay,  and  in  one  instance  a  main  canal  was  cut 
for  a  considerable  distance  in  solid  rock.  Sometime 
smaller  ditch  was  sunk  in  the  bottom  of  a  large  canal,  to 
facilitate  the  movement  of  small  runs  of  water.  The 
ancient  canals  in  the  Salt  River  valley  l  had  a  length 
of  at  least  150  miles  and  were  sufficient  to  irrigate  250,000 
acres  of  land. 

In  modern  times  the  great  Assouan  dam  has  been  built 
on  the  Nile  River,  and  with  the  associated  reservoirs  it 
is  designed  to  control  the  flow  of  the  river  and  provide 
water  for  irrigation.  It  stands  as  an  example  of  present- 
day  irrigation  development  and  control. 

590.  Development  of  irrigation  practice  in  the  United 
States.  —  In  the  United  States  the  earliest  modern  people 
to  practice  irrigation  were  the  Catholic  missionaries  in 
southern  California.  The  immediate  predecessors  of 
the  present  irrigation  systems  in  the  United  States  were 
built  by  a  colony  of  one  hundred  and  forty-seven  Mor- 
mons who  went  into  the  Salt  Lake  valley  in  Utah  in  July, 
1847.  The  crops  of  these  people  were  grown  with  water 
diverted  from  City  Creek,  and  their  community  life,  to- 
gether with  their  peculiar  situation,  led  them  to  work  out 
in  the  succeeding  decades  the  fundamental  principles 
of  economic  and  social  life  as  adapted  to  irrigation  farm- 
ing.    In  the  last  thirty  years  the  practice  of  irrigation  has 


1  Forbes,  R:  H.     Irrigation  in  Arizona.     U.  S.  D.  A.,  Office 
Exp.  Sta.,  Bui.  No.  235,  p.  9.     1911. 


IRRIGATION  AND  DRY-FARMING 


687 


688     soils:  properties  and  management 

extended  rapidly  in  the  western  United  States.  It  has 
approximately  doubled  each  ten  yean  since  1879. 

Irrigation  is  employed  somewhat  generally  throughout 
the  region  west  of  the  100th  meridian,  which  rims  through 
central  Nebraska.  With  the  exception  of  limited  areas 
the  annual  rainfall  is  less  than  25  inches,  and  over  large 
areas  it  is  less  than  15  inches. 

The  methods  of  securing  water  and  applying  it  to  the 
land  have  grown  up  gradually  out  of  the  experience  of 
the  people  in  many  communities  and  under  many  condi- 
tions. Cooperative  effort  of  some  sort  is  essentia]  to 
provide  water  for  irrigation,  and  this  has  led  to  the  use 
of  several  types  of  organizations  for  the  purpose.  Nat- 
urally, the  states  concerned  have  taken  a  part  in  the 
matter  by  passing  laws  and  providing  funds  to  promote 
irrigation  practices.  Finally,  the  aid  of  the  Federal 
Government  was  enlisted.  The  enterprises  for  the  pro- 
vision of  water  for  irrigation  may  be  divided  into  seven 
groups,1  chiefly  according  to  their  legal  status :  (1)  com- 
mercial enterprises  selling  water  for  profit;  (2)  partner- 
ships among  individual  farmers  without  formal  organiza- 
tion; (3)  cooperative  enterprises,  made  up  of  water 
users;  (4)  irrigation  districts  which  are  public  corpora- 
tions; (5)  Carey  Act2  enterprises,  by  Federal  enact- 
ment authorized  August  18,  1894,  and  made  up  of 
grants  to  the  arid  and  semiarid  states,  these  states 
being  held  responsible  for  the  irrigation  of  these  grants ; 
(6)  United  States  Indian  Service  enterprises,  to  provide 
for  the  construction  of  irrigation  works  in  Indian  reser- 
vations;   and  (7)  the  United  States  Reclamation  Serv- 

1  Thirteenth  U.  S.  Census,  Chapter  14,  p.  421.     1910. 

2  Stover,  A.  P.  Irrigation  under  the  Carey  Act.  U.  S.  D.  A., 
Office  Exp.  Sta.,  Ann.  Rept.  pp.  451^88.     1910. 


IRRIGATION  AND  DRY-FARMING  689 

ice,  established  by  Federal  law  June  17,  1902,  providing 
for  the  construction  of  irrigation  works  with  the  re- 
ceipts from  the  sale  of  public  lands  in  the  arid  and  semi- 
arid  states. 

These  several  provisions  and  their  successive  growth 
in  size  suggest  the  necessity  of  large  enterprises  and  care- 
ful coordination  in  providing  water  for  irrigation.  The 
many  attractive  features  of  farming  in  arid  regions  under 
irrigation,  together  with  the  publicity  that  the  enterprises 
have  had,  have  has+ened  the  growth  of  irrigation  farming 
so  that  it  now  plays  a  very  substantial  part  in  the  agri- 
cultural business  of  the  country. 

591.  Irrigation  in  humid  regions.  —  In  the  humid 
states  —  that  is,  those  in  which  there  is  a  large  normal 
rainfall  and  in  which  crops  can  usually  be  produced  with- 
out artificial  addition  of  water  —  irrigation  has  been 
practiced  to  some  extent.  Irrigation  is  useful  (1)  where 
the  crop  has  a  high  value,  as  for  vegetables  and  small 
fruits  near  large  cities ;  (2)  where  the  quality  of  the  crop 
is  much  affected  by  unfavorable  conditions,  as  the  pro- 
duction of  wrapper  tobacco  in  northern  Florida  and  of 
rice  in  Louisiana ;  (3)  where  the  soil  is  especially  sandy ; 
and  (4)  where  the  supply  of  water  may  be  very  cheaply 
applied  to  the  land,  as  in  the  diversion  of  streams  to  adja- 
cent fields,  usually  meadows.  In  Great  Britain  and^  in 
central  and  southern  Europe,  the  diversions  of  streams 
to  near-by  grass  meadows  is  relatively  common.  Under 
all  these  conditions,  small  irrigation  enterprises  have 
been  developed  in  different  parts  of  the  eastern  United 
States.  The  rainfall  under  which  irrigation  is  practiced 
in  these  regions  ranges  from  30  to  more  than  60  inches 
annually.  The  practice  of  irrigation  in  humid  regions 
is  in  the  nature  of  an  insurance  against  dry  years.  The 
2y 


690       SOILS  :    PROPER  TIES  A  N  I)   MA  NA  0  /  M ! .  \  / 

probability  of  the  occurrence  of  these  in  the  eastern 
United  States  is  shown  in  the  following  table  '  of  rainfall 
records  for  the  ten  years  from  1900  to  1909,  inclusive:  — 


Number  or 

FlKTKl 

Number  or 

AVERAOB 

Periods  or 

Days  «  UM 

Station 

Annual 

OVER    U  l  III 

Irrigation 

Rainfall 

LESS  THAN 

was  Re- 

1 INCH  or 

quired  (a) 

Rain 

Ames,  Iowa 

30.39 

23 

190 

Oshkosh,  Wisconsin       .     .     . 

29.78 

27 

292 

Vineland,  New  Jersey   .     .     . 

47.47 

46 

352 

Columbia,  South  Carolina 

47..-).-. 

62 

568 

Selma,  Alabama 

50.75 

60 

724 

(a)  No  days  counted  until  after  a  fifteen-day  period  with  less 
than  1  inch  of  rain. 

The  aggregate  area  of  the  projects  is  small  and  amounts 
to  only  a  few  thousand  acres. 

592.  The  Reclamation  Service.  —  The  financing  of 
irrigation  enterprises  by  the  Federal  Government  through 
the  Reclamation  Service  has  been  a  wonderful  stimulus. 
The  total  number  of  acres  on  which  ditches  have  been 
constructed  or  are  in  process  of  construction  in  this  way 
aggregates  3,101,450,  in  thirty  projects  distributed  through 
seventeen  states  and  involving  a  total  expenditure  of 
hundreds  of  thousands  of  dollars.  These  projects  contem- 
plate  the  impounding  of  13,272,490  acre-feet  of  water. 


1  Williams,  M.  B.  Possibilities  and  Need  of  Supplemental 
Irrigation  in  the  Humid  Regions.  U.  S.  D.  A.,  Yearbook  1911, 
pp.  309-320. 

Also,  Teele,  R.  P.  Irrigation  in  Humid  Regions.  American 
Cyclopedia  of  Agriculture,  p.  437.     New  York,  1905. 


IRRIGATION  AND  DRY-FARMING  691 

About  one-third  of  this  area  was  irrigated  in  1915.  Many 
of  the  dams  and  eanals  involved  are  of  stupendous  size 
and  necessitate  feats  of  bold  engineering.  Often  hydro- 
electric power  is  developed  in  large  amount  in  the  passage 
of  the  water  from  the  reservoirs  to  the  fields  where  it  is 
to  be  used  to  grow  crops. 

593.  Legal,  economic,  and  social  effects  of  irrigation.  — 
The  practice  of  irrigation  on  an  extensive  scale  has  caused 
important  changes  in  the  construction  of  law  x  relative  to 
water  and  property  rights  and  in  commercial  and  social 
organization. 

Riparian  rights  in  streams  and  lakes  under  humid 
conditions,  for  purposes  of  domestic  use,  power,  and  trans- 
portation, must  be  modified  in  an  arid  country.  Jj^re 
values  of  all  real  property  depend  largely  on  the  supply 
of  water  for  purposes  of  irrigation.  The  control  and  use 
of  water  becomes  of  the  utmost  public  concern.  Conse- 
quently the  use  of  water  for  the  purpose  of  growing  crops 
takes  precedence  over  use  for  all  other  purposes  except 
domestic  use.  In  nearly  every  country  in  the  world 
where  irrigation  is  extensively  practiced,  the  state  or 
the  government  has  assumed  ownership  or  a  large  measure 
of  control  over  the  water  in  all  lakes  and  streams.  The 
necessity  of  the  use  of  water  for  irrigation  has  conferred 


1  Mead,  E.     Irrigation  Institutions.     New  York,  1903. 

Mead,  E.  Irrigation  Institutions  in  Different  Countries. 
American  Cyclopedia  of  Agriculture,  Vol.  IV,  p.  154.  New 
York,  1909. 

Hess,  R.  H.  Further  Discussion  of  American  Irrigation 
Policies.  American  Cyclopedia  of  Agriculture,  Vol.  IV,  p. 
160.     New  York,   1909. 

Hess,  R.  H.  Socio-Economic  Aspects  of  Irrigation.  Ameri- 
can Cyclopedia  of  Agriculture,  Vol.  IV,  p.  167.  New  York, 
1909. 


692      SOILS:    PROPERTIES  and  manageMi:\  I 

certain  privileges,  such  as  the  principle  of  eminent  domain, 
in  conserving  and  utilizing  water.  The  provisions  differ 
somewhat  in  detail,  but  in  general  agree  in  conferring  the 
right  to  use  water  upon  those  persons  who  can  first  make 
the  best  use  of  it  for  the  purpose  of  growing  crops.  Other 
rights  in  the  use  of  water  are  largely  subject  to  its  use  for 
irrigation.  Further,  the  tendency  is  to  attach  the  right 
to  the  use  of  water  to  the  title  to  land,  since  each  has 
value  only  as  it  is  associated  with  the  other.  However, 
in  the  attachment  of  water  from  a  particular  source  to 
any  given  area  of  land,  many  difficult  questions  may  be 
raised  which  must  be  decided  by  the  larger  principle  of 
beneficial  use. 

k  d«se  economic  dependence  among  the  people  and  a 
high  degree  of  social  coordination  grows  out  of  the  practice 
of  irrigation  farming  on  a  large  scale.  The  fertile  nature 
of  the  soil,  the  favorable  climate,  and  the  cooperation 
necessary  to  supply  water  for  irrigation,  leads  to  intensive 
methods  of  farming,  to  specialization  in  production,  and 
to  many  cooperative  enterprises,  not  only  in  agriculture, 
but  also  in  associated  industries  in  the  same  region.  These 
intensive  practices  and  the  close  personal  association 
involved  promote  a  high  intellectual  and  social  standard 
in  the  community.  Irrigation  has  been  an  efficient  school- 
master in  the  practice  and  value  of  cooperation  in  all  sorts 
of  enterprises. 

594.  Divisions  of  irrigation.  —  Two  main  parts  make 
up  the  practice  of  irrigation :  the  first  is  the  provision 
of  water,  which  is  essentially  an  engineering  problem ; l 
the  second  is  the  use  of  water  on  the  land,  which  is  es- 


1  Wilson,  H.  M.     Irrigation  Engineering,  p.  625.     New  York, 
1909. 


IRRIGATION  AND  DRY-FARMING  693 


■  l 


sentially  anfl  ■ltural  problem.  It  is  important  to 
maintain  this^Ptf  distinction  in  dealing  with  the  prac- 
tice of  irrigation,  especially  in  its  larger  aspects,  The 
two  functions  are  largely  exercised  by  different  groups 
of  men,  and  they  involve  widely  different  types  of  knowl- 
edge and  skill.  The  supreme  test  of  an  irrigation  system 
is  efficient  use  of  the  water  on  the  land  in  the  production 
of  crops. 

595.  Sources  of  water  for  irrigation.  —  The  practice 
of  irrigation  is  dependent  on  some  adjacent  supply  of 
water  that  may  be  diverted  on  to  the  land.  It  may  be 
derived  by  (1)  the  diversion  of  streams  flowing  from 
well-watered  regions;  (2)  the  melting  of  snow  on  moun- 
tain areas;  (3)  the  regulation  of  the  flow  of  streams  by 
storage  reservoirs ;  and  (4)  the  utilization  of  underground 
water  by  means  of  wells.  All  these  sources  may  require 
the  construction  of  large  and  costly  works,  which  are  well 
exemplified  in  the  structures  built  by  the  United  States 
Reclamation  Service  and  by  the  Egyptian  government 
in  the  Nile  valley.  Dams  hundreds  of  feet  high  and 
thousands  of  feet  long,  containing  millions  of  cubic  yards 
of  masonry  and  concrete,  have  been  constructed  for  these 
purposes. 

596.  Canals.  —  The  conveyance  of  the  water  from  the 
point  of  supply  to  'the  place  where  it  is  to  be  used  necessi- 
tates further  difficult  engineering  problems,  which  in  some 
cases  have  entailed  the  construction  of  large  tunnels 
under  mountains  and  the  development  of  large  pumping 
and  power  plants  as  well  as  the  construction  of  thousands 
of  miles  of  main  and  lateral  canals.  In  1909  the  length 
of  main  irrigation  ditches  in  the  United  States  was  875,911 
miles,  and  of  laterals  38,062  miles.  As  a  rule  the  water 
is  conveyed  by  gravity  flow  without  pressure.     Important 


694       SOILS:    PROPERTIES  AND  MANAGEMENT 


problems  presented  relate  to  the  prevention  of  seepage, 
erosion,  and  evaporation.  The  loss  l  or^feiter  in  transit 
from  ijts  source  to  the  field  has  been  found  to  average  60 
per  cent,  and  to  range  from  0.25  per  cent  to  as  much  as 
64  per  cent  a  mile  with  an  average  of  about  6  per  cent. 
The  seepage  water  from  canals  may  result  in  further  loss 
by  accumulating  in  low  lands,  where  the  evaporation, 
coupled  with  the  solution  of  the  soluble  salts  in  the  soil, 
causes  injurious  accumulation  of  alkali  in  the  surface  soil, 
and  in  extreme  cases  a  swampy  condition  which  destroys 
the  value  of  the  soil  for  agricultural  purposes.  In  order- 
to  prevent  seepage  many  kinds  of  lining  and  treatment 
of  the  walls  of  canals  have  been  employed.  Cement 
lining  in  different  forms,  wooden  flumes,  clay  puddling, 
oiling,  applications  of  tar,  and  silting  have  been  used. 
The  need  of  a  lining  depends  much  on  the  nature  of  the 
formation  through  which  the  ditch  passes.  Silt  is  an 
excellent  means  of  checking  seepage.  Where  clear  water 
is  carried,  the  ditch  must  usually  be  lined,  and  the  prac- 
tice of  lining  canals  in  order  to  reduce  seepage  is  increas- 
ing rapidly.  Sand  and  gravel  permit  much  seepage  and 
are  easily  eroded.  Clay  permits  little  seepage  and  is  not 
easily  eroded.  The  velocity  of  flow  of  water  in  canals 
should  not  exceed  three  feet  a  second.  In  large  canals 
this  will  not  permit  a  grade  of  more  than  six  inches  in  a 
mile;  in  very  small  ditches  a  grade  of  from  forty  to  fifty 
feet  in  a  mile  may  be  necessary  to  cause  the  same  velocity 
of  flow.     A  lining  that  is  not  subject  to  erosion,  together 

1  Teel,  R.  P.  Irrigation  and  Drainage  Investigations. 
U.  S.  D.  A.,  Office  of  Exp.  Sta.,  Ann.  Rept.  1904,  p.  36.  Also, 
Mead,  E.,  and  Etcheverry,  B.  A.  Lining  of  Ditches  and  Reser- 
voirs to  Prevent  Seepage  Losses.  Calif.  Agr.  Exp.  Sta.,  Bui. 
No.  188.     1907. 


IRRIGATION  AND  DRY-FARMING  695 

with  a  channel  that  is  deep  in  relation  to  its  width,  not 
only  reduces  seepage,  but  also,  by  permitting  the  rapid 
flow  of  water,  reduces  loss  by  evaporation. 

At  the  farm  on  which  the  water  is  to  be  used,  it  is  dis- 
tributed in  small  field  laterals  which  are  carried  on  the 
higher  ground.  Precautions  against  seepage  and  evapo- 
ration should  here  be  taken.  The  tendency  now  is  toward 
the  distribution  of  the  water  to  the  fields  by  means 
of  underground  oipes,  with  standpipes  and  valves  at 
the  points  of  discharge.  The  arrangement  of  the  farm 
laterals  must  of  course  be  determined  by  the  topography 
of  the  land,  since  the  water  flows  by  gravity. 

597.  Preparation  of  land  for  irrigation.  —  The  prep- 
aration of  the  land  for  irrigation  depends  on  the  method 
used  to  apply  the  water.  Usually  marked  irregularities 
should  be  removed  by  smoothing  the  surface.  Where 
any  sort  of  basin  method  of  irrigation  is  used,  it  may  also 
be  necessary  to  level  the  surface.  Various  types  of  scrap- 
ers and  levelers  have  been  found  useful  for  this  operation. 
Much  of  the  arid  and  semiarid  land  carries  a  growth  of 
sage  brush  or  other  bushy  vegetation,  and  of  course 
this  must  be  removed  before  smoothing  operations  can 
become  effective. 

598.  Methods  of  applying  water.  —  There  are  four 
general  methods  *  of  applying  water  to  the  soil.  These 
are  (1)  overhead  sprays,  (2)  sub-irrigation,  (3)  flooding, 
and  (4)  furrows. 

599.  Overhead  sprays.  —  By  the  overhead  spray  system 
(Fig.  76)  the  water  is  distributed  in  pipes  under  a  pres- 
sure of  forty  to  sixty  pounds  and  discharged  from  a  series 


1  Fortier,  S.     Methods  of  Applying  Water  to  Crops.     U.  S. 
D.  A,,  Yearbook  1909,  pp.  293-308. 


SOILS:  properties  and  management 

of  nozzles.  Several  types  of  nozzles  are  employed.  The 
amount  of  water  that  can  be  applied  is  relatively  small, 
and  consequently  the  method  is  used  chiefly  in  humid 
regions  to  supplement  a  rather  high  rainfall,  in  the  growth 
of  crops  of  large  value.  It  is  used  in  the  growth  of  truck 
and  small  fruit  crops  near  the  large  eastern  cities. 

The  advantages  of  the  system  are  :  — 

1.  The  water  is  conveniently  applied  at  the  d< 
point.  2.  The  system  may  be  used  on  uneven  land  and 
without  preparation  of  the  surface.  3.  There  is  no 
waste  of  land  by  ditches.  4.  The  application  of  the 
water  is  easily  controlled  by  valves  and  by  the  movement 
of  the  pipes. 

The  disadvantages  of  the  system  are  :  — 

1.  The  capacity  is  limited.  2.  The  cost  is  high  (of 
equipping  and  maintaining  the  plant,  and  for  developing 
the  pressure  requisite  to  suitably  distribute  the  water  from 
the  nozzles.  3.  There  is  possibility  of  injury  to  crops 
where  water  is  applied  on  warm,  bright  days,  since  the 
water  comes  into  contact  with  the  foliage. 

600.  Sub-irrigation.  —  Sub-irrigation  is  the  distribution 
of  water  from  underground  pipes.  These  are  buried  in 
the  soil  and  perforated  in  such  a  way  that  the  water  finds 
an  outlet  and  is  distributed  by  the  capillarity  of  the  soil 
and  by  natural  gravity  flow.  In  greenhouses  and  where 
shallow-rooted  annuals  are  grown,  lines  of  drain  tile  are 
employed,  the  water  flowing  out  at  the  joints.  Con- 
tinuous pipes  having  an  open  seam  or  perforations  have 
been  used.  Another  method  employs  a  porous  cement 
plug  which  rises  a  little  above  the  supply  pipe.  The 
object  of  the  last-named  method  is  to  avoid  the  common 
difficulty  from  the  entrance  of  roots  into  the  pipes.  The 
pipes  must  have  a  very  slight  grade  in  order  to  insure  a 


IRRIGATION  AND  DRY-FARMING  697 


Fig.  76. — 'Essential  features  of  construction  in  one  method  of  overhead 
spray  irrigation.  Water  is  supplied  under  pressure  from  under- 
ground pipes  and  is  distributed  from  small  nozzles  (N)  along  the 
axis  of  the  pipe.  Different  forms  of  nozzles  are  used  for  different 
purposes  (see  detail).  Gauge  (G)  shows  pressure,  (S)  is  pipe  sup- 
port, and  (L)  is  lever  for  turning  the  discharge  pipe,  which  is  fitted 
with  a  freely  moving  sleeve  joint. 


698       SOILS:    PROPERTIES  AND   MANAGEMENT 

uniform  distribution  of  water.  They  operate  under 
little  or  no  pressure.  The  system  has  a  number  of  ad- 
vantages, but  in  practice  these  are  usually  more  than 
offset  by  its  disadvantages.     The  advantages  are :  — 

1.  The  system  is  permanent.  2.  It  is  economical  of 
water.  3.  There  is  no  injury  to  the  physical  properties 
of  the  soil.  4.  There  are  no  obstructions  at  the  surface. 
5.  The  deep  rooting  of  crops  is  encouraged.  6.  There  is 
very  little  expense  for  supervision  of  the  distribution  of 
water.  7.  The  accumulation  of  soluble  salts  on  the  surface 
of  the  soil  by  evaporation  is  reduced.  8.  The  system  may 
sometimes  be  used  as  a  means  of  drainage  also. 

The  disadvantages  are  :  — 

1 .  There  is  a  strong  tendency  for  the  pipes  to  be  clogged 
by  the  entrance  of  roots,  especially  where  perennial  crops 
are  grown.  The  porous-plug  method  of  discharging  water 
is  designed  to  reduce  this  difficulty.  2.  The  slow  lateral 
capillary  diffusion  of  water  in  dry  soil  makes  it  necessary 
to  install  the  lines  of  pipe  near  together,  which  entails 
heavy  expense. 

The  method  is  best  adapted  to  shallow-rooted  annual 
crops,  and  least  adapted  to  orchards.  The  seepage  of 
water  from  the  pipes  attracts  the  growing  roots,  which 
are  likely  to  enter  the  pipes,  break  up  into  many  small 
fibers,  and  clog  the  system. 

There  are  soil  conditions  under  which  this  method  is 
especially  useful.  Where  the  soil  is  a  porous  sand  or 
gravel  underlaid  at  a  depth  of  four  feet  or  less  by  a  rather 
impervious  stratum,  the  water  may  be  distributed  rapidly 
from  the  pipes  so  that  it  accumulates  on  the  hard  sub- 
stratum and  saturates  the  soil,  the  pipes  being  quickly 
emptied.  There  is  then  no  tendency  for  the  roots  to 
enter  the  pipes,  and  the  porous  nature  of  the  soil  permits 


IRRIGATION  AND  DRY-FARMING  699 

the  pipes  to  be  placed  several  rods  apart,  thus  reducing 
the  expense  of  installation. 

Sub-irrigation  sometimes  occurs  naturally  under  condi- 
tions similar  to  those  just  described,  where  water  is  sup- 
plied from  springs  or  by  seepage.  Where  it  can  be  em- 
ployed, sub-irrigation  is  the  ideal  method  of  applying 
water  to  the  soil. 

601.  Methods  most  used  in  arid  regions.  —  The  two 
methods  preeminently  used  to  apply  water  to  the  soil 
under  arid  conditions  are  by  furrows  and  by  flooding. 
The  land  must  generally  be  prepared  to  some  extent  for 
either  of  these  methods,  by  smoothing  or  leveling  the 
surface,  throwing  up  levees,  or  constructing  distribution 
furrows.  It  is  a  fortunate  fact  that  the  subsoil  in  arid 
regions  is  about  as  fertile  as  the  soil,  and  therefore  grad- 
ing can  be  practiced  with  impunity.  Both  methods  have 
a  large  number  of  variations  in  detail  to  adapt  them  to 
particular  soils,  topography,  or  crops. 

The  chief  factors  determining  the  choice  between  flood- 
ing and  furrowing  are  (1)  the  nature  of  the  crop,  (2)  the 
character  of  the  soil,  (3)  the  contour  of  the  land,  and 
(4)  the  quantity  of  water  available. 

602.  Flooding.  —  Flooding  is  especially  employed 
(1)  where  the  crop  occupies  the  entire  area,  such  as  in 
grainfields  and  meadows ;  (2)  where  the  soil  is  of  medium 
porosity  and  does  not  bake  seriously  on  drying ;  (3)  where 
the  surface  is  relatively  flat ;  and  (4)  where  the  supply  of 
water  is  relatively  large. 

The  advantages  of  this  method  are  :  — 

1.  The  handling  of  water  is  easy. 

2.  There  is  economy  in  ditches. 

3.  The  necessity  of  tearing  up  the  crop  is  avoided. 

4.  The  method   is   especially   suited   to   certain  crops 


700      SOILS:    PROPERTIES  AND  MANAGEMENT 

that  grow  in  standing  water,  such  as    rice    and    cran- 
berries. 
Its  disadvantages  are :  — 

1.  A  large  quantity  of  water  is  required. 

2.  Over  irrigation,  with  consequent  seepage  and  diffi- 
culties from  alkali,  is  likely  to  occur. 

3.  On  heavy  soil,  puddling  and  checking  of  the  surface 
soil  result  from  lack  of  tillage. 

4.  Some  crops  are  injured  by  direct  contact  with  water. 

5.  The  cost  of  leveling  and  of  construction  of  levees  is 
large. 

There  are  two  main  types  of  flooding.  In  the  first 
the  water  is  turned  into  level  checks  or  blocks,  where 
it  stands  until  it  is  absorbed  by  the  soil  —  called  commonly 
closed-field  flooding.  In  the  second  type  the  water  is 
distributed  in  a  moving  sheet  or  a  series  of  small  rills, 
from  field  supply  ditches  —  called  open-field  flooding. 
This  method  is  used  only  where  there  is  a  moderate  slope 
to  carry  the  water. 

In  closed-field,  or  check,  flooding,  the  land  is  divided 
into  blocks,  each  having  a  level  surface  and  surrounded 
by  a  levee.  The  size  of  the  checks,  their  shape,  and  the 
height  of  the  levees  is  determined  by  the  contour  of  the 
land.  On  a  slope  they  may  be  very  irregular.  Small 
checks  of  one  to  three  acres  are  most  successfully  irri- 
gated, but  areas  of  twenty  or  more  acres  have  been  flooded 
in  one  block.  A  flow  of  five  to  seven  second-feet  of 
water  is  necessary  in  order  to  make  the  method  thoroughly 
successful.  One  man  can  irrigate  from  five  to  twenty 
acres  a  day,  depending  on  the  size  and  form  of  the  checks. 
The  levees  may  be  permanent,  as  is  usually  the  case 
especially  in  meadows,  or  they  may  be  thrown  up  for 
each  application  of  water.     The  permanent  levees  may 


IRRIGATION  AND   DRY-FARMING 


701 


be  broad  and  low  so  that  they  will  not  interfere  with 
harvesting.  This  method  of  flooding  is  falling  into  disuse. 
A  phase  of  check  flooding  is  the  basin  method  of  irri- 
gating orchards,  in  which  small,  shallow  basins  are  formed 
around  each  tree  and  separated  from  the  trunk  by  a  block 


Fig.  77. — Implements  used  in  irrigation  practice.  (A),  scraper  for 
making  small  levees  in  irrigation  furrows;  (B),  (C) ,  and  (D),  wood 
and  metal  topoons  used  to  close  irrigation  furrows  and  by  means 
of  the  small  openings  divide  and  regulate  the  flow ;  (E) ,  canvas 
dam  used  in  flooding  from  field  ditches.  The  edge  of  the  canvas  is 
held  down  by  a  shovelful  of  earth. 


T02       SOILS:    PROPERTIES  AND  MANAGEMENT 

of  earth  to  prevent  injury  to  the  growing  wood.  This 
method  is  used  rather  extensively  throughout  the  arid 
regions. 

In  the  open-field,  or  blind,  flooding,  the  water  is  ^up- 
plied  in  ditches  which  are  carried  across  the  contour-  ;it 
a  moderate  grade,  and  at  intervals  the  flow  is  intercepted 
by  a  canvas  dam  or  other  obstruction  and  forced  to  flow 
over  the  lower  bank,  from  which  point  it  is  distributed 
down  the  slope  and  over  the  field  in  numerous  small 
trenches.  Any  surplus  water  is  collected  in  a  ditch  at  the 
lower  side  of  the  field.  In  this  method  of  applying  water, 
constant  attention  is  required  to  guide  the  flow  and  prevent 
erosion.  One  man  can  irrigate  from  five  to  ten  acres  in  a 
day.  This  method  is  used  in  irrigating  grainfields  and 
sloping  meadowland  and  in  saturating  the  soil  in  prepa- 
ration for  a  crop. 

603.  Furrows.  —  In  the  furrow  system  of  irrigation 
the  water  is  led  out  from  the  supply  ditch  on  the  upper 
side  of  the  field  into  small,  parallel  furrows  extending 
down  or  across  the  slope  at  a  considerable  grade.  This 
system  is  used  for  cultivated  field  and  garden  crops,  and 
to  a  large  extent  in  orchards.  The  rate  of  flow  of  water 
in  the  furrows  should  not  exceed  one  to  two  feet  per 
second,  depending  on  the  nature  of  the  soil.  This  permits 
a  wide  range  of  grade,  from  2  to  10  per  cent,  where  the 
head  of  water  is  only  a  fraction  of  a  second-foot  in  each 
furrow.  The  flow  on  a  given  slope  may  be  regulated  by 
the  head  of  water  and  is  determined  by  the  porosity  of  the 
soil.  On  heavy  soil  a  small  head  and  a  steep  grade  may  be 
employed ;  on  sandy  soil,  which  washes  easily,  a  low  grade 
and  a  large  head  of  water  is  used.  The  length  of  furrows 
that  may  be  employed  depends  on  the  nature  of  the  soil 
and  the  head  of  water  available.     The  water  is  distrib- 


IRRIGATION  AND  DRY-FARMING  70S 


ROAD 


CONTOUR  CHECK  METHOD  ^od.ng) 


ROAD 


RECTANGULAR    CHECK    METHOD 


1 


ROAD 


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HAY 


ROAD 
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CORN 


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Fig.  78. — Plan  of  irrigated  farm  showing  the  methods  of  irrigating  dif- 
ferent crops  and  the  arrangement  of  the  irrigation  works  for  apply- 
ing the  water  under  the  different  conditions. 


704      SOILS:    PROPERTIES  AND  MANAGEMENT 

uted  from  the  furrows  by  percolation  and  by  capillary 
movement.  Percolation  causes  the  accumulation  of 
water  under  the  upper  end  of  the  furrows ;  capillary  move- 
ment distributes  the  water  laterally  as  well  as  downward, 


Fig.  79.  —  Diagrams  showing  the  relative  rate  of  movement  of  water 
from  irrigation  furrows  into  clay  loam  (left),  and  sandy  loam 
(right),  after  different  periods  of  time. 

and  its  rate  determines  the  distance  between  the  furrows. 
The  downward  1  movement  is  much  more  rapid  than  the 
lateral  movement,  and  both  are  very  irregular,  depending 
on  the  nature  and  structure  of  the  soil.  Ordinarily  the 
furrows  are  relatively  close  together,  to  give  greater 
uniformity  in  distribution.  In  corn,  potatoes,  berries, 
garden  vegetables,  and  crops  of  similar  character,  a  furrow 
is  placed  in  each  row,  or  at  least  in  every  other  row  as  is 


1Widtsoe,  J.  A.,  and  McLaughlin,  W.  W.  The  Movement 
of  Water  in  Irrigated  Soils.  Utah  Agr.  Exp.  Sta.,  Bui.  No. 
115.  1912.  Also,  Loughridge,  R.  H.  Distribution  of  Water 
in  the  Soil  in  Furrow  Irrigation.  U.  S.  D.  A.,  Office  Exp.  Sta., 
Bid.  No.  203.     1908. 


IRRIGATION  AND  DRY-FAB  MING 


705 


sometimes  the  case  in  strawberries,  in  order  to  permit 
harvesting. 

In  orchard  culture  two  or  more  furrows  are  placed 
between  each  two  rows.  Often  for  young  trees  a  furrow 
is  placed  on  either  side  at  a  distance  of  about  two  feet, 
this  distance  being  increased  as  the  trees  increase  in  size. 
The  furrows  are  temporary  and  are  usually  renewed 
after  each  application  of  water,  as  the  establishment  of  a 
soil  mulch  is  necessary  in  order  to  prevent  excessive  loss 
of  water  by  evaporation. 

604.  Size  and  form  of  furrows.  —  In  shape  the  fur- 
rows should  be  relatively  narrow  and  deep.  Water  is 
conserved  by  this  form  in  three  ways :  (1)  it  flows  more 
freely,  both  in  the  furrow  and  into  the  soil ;  (2)  less  surface 
is  exposed  to  evaporation ;  and  (3)  the  surface  mulch  is 
more  easily  maintained.  (See  Fig.  80.)  Under  arid  con- 
ditions a  deep  mulch l  of   six   to   eight  inches  is  most 


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Fig.  80.  —  Diagram  showing  the  relative  advance  of  water  into  the  soil 
from  a  deep  (left)  and  a  shallow  (right)  irrigation  furrow.  Note 
the  relative  extent  of  surface  soil  wet  in  the  two  cases.  A  deep 
mulch  and  deep  irrigation  furrows  aid  in  the  conservation  of 
moisture. 


1  For  tier,    S.     Evaporation   Losses  in   Irrigation  and  Water 
Requirements  of  Crops.     U.  S.  D.  A.,  Office  Exp.  Sta.,  Bui.  No. 
177.     1907. 
2z 


706       SOILS:    PROPERTIES  AND   MANAGEMENT 

efficient,  and  the  bottom  of  the  furrow  should  extend 
well  below  its  base.  This  will  allow  the  water  to  diffuse 
laterally  rapidly,  and  the  deep  dry  mulch  reduces  the 
extent  to  which  the  surface  becomes  moist,  thereby  con- 
serving moisture  and  reducing  the  accumulation  of  alkali 
at  the  surface. 

The  application  of  water  to  the  soil  in  irrigation  must 
be  guided  by  the  principles  elucidated  in  the  discussion  of 
the  physical  properties  of  the  soil,  and  its  relation  to 
moisture  and  its  control. 

605.  Units  of  measurement. — The  measurement  of 
water  in  irrigation  practice  involves  the  use  of  units  of 
volume  and  pressure.  By  the  head  is  understood  the 
volume  of  water  supplied  in  the  unit  of  time.  The  flow 
of  water  in  canals  is  usually  stated  in  units  of  flow  per 
unit  of  time,  that  is,  the  number  of  cubic  feet  per  second, 
called  the  second-foot.  Frequently  the  term  second-foot 
is  applied  to  the  volume  of  water  that  would  result  from 
a  flow  of  that  rate  throughout  the  season.  A  smaller 
unit  is  the  miner's  inch,  a  term  derived  from  mining 
practice,  which  refers  to  the  quantity  of  wrater  that  will 
flow  out  of  an  orifice  one  inch  square  under  a  constant 
pressure  which  varies  in  different  states  from  a  four  to 
an  eight  inch  head  above  the  top  of  the  orifice.  Like  the 
second-foot,  the  flow  is  frequently  rated  by  the  season. 
The  pressure  is  proportional  to  the  depth,  or  head.  It  is 
commonly  stated  in  pounds  per  square  inch.  A  column 
of  water  ten  feet  in  height  exerts  a  pressure  of  approxi- 
mately 4.34  pounds  to  a  square  inch. 

In  the  field,  water  is  commonly  measured  in  terms  of 
depth  over  an  acre.  An  acre-foot  is  the  quantity  of  water 
that  will  cover  an  acre  one  foot  in  depth.  An  acre-inch  is 
one-twelfth  of  an  acre-foot.     These  are  very  convenient 


IRRIGATION  AND  DRY-FARMING 


707 


terms  because  of  their  definiteness  and  relation  to  the 
common  method  of  stating  rainfall.  Usually  an  inch  or  a 
foot  of  water  refers  to  that  depth  over  an  acre. 

Various  mechanisms  are  employed  for  measuring l 
water  in  irrigation  practice.  The  commonest  of  these 
are  the  weir  and  the  flume.     (See  Fig.  81.)      The  weir  is 


Fig.  81.— Plank  measuring  box  and  a  Cippoletti  weir  used  in  determin- 
ing the  flow  of  irrigation  water. 


a  simple  device  to  give  the  stream  a  definite  cross  section 
and  to  aid  in  the  measurement  of  the  depth,  and  there- 
fore the  volume  of  flow.  It  is  usually  a  knife-edged  notch, 
of  a  standard  shape  calibrated  to  a  grade  stake  a  short 
distance  up  stream  from  which  the  depth  of  water  and 
its  velocity  are  rated.  The  measuring  box,  frequently 
termed  a  module,  is  a  box  for  measuring  the  flow  of  water 
from  an  orifice  under  fixed  conditions.  The  Staldate 
module,  developed  in  Italy,  is  most  generally  adopted  for 
the  purpose.     Small  streams  are  divided  by  a  knife-edge 


1  Carpenter,  L.  C.     The  Measurement  and  Division  of  Water. 
Colorado  Agr.  Exp.  Sta.,  Bui.  No.  150,  4th  ed.     1911. 


708      SOILS:    FBOPERTIES  AND  MANAGEMENT 


diverter  inserted  into  the  current,  which  diverts  a  definite 
portion  of  the  stream.     This  is  called  a  divider. 

606.  Amount  of  water  to  apply.  —  The  amount  of 
water  to  apply  to  the  soil  at  any  one  time  depends  on 
(1)  the  nature  and  condition  of  the  soil,  (2)  the  supply  of 
water,  (3)  the  crop,  and  (4)  the  season.  In  the  main, 
enough  water  should  be  applied  to  capillarily  saturate 
the  soil  to  a  depth  of  one  foot  and  to  increase  the  soil 
moisture  to  a  depth  of  three  feet.  A  fairly  dry,  fine- 
textured  soil  will  effectively  take  the  largest  irrigation. 
Some  crops  are  more  sensitive  to  water  at  one  period  of 
growth  than  at  another.  Potatoes  should  mature  in  a 
rather  dry  soil.  The  application  of  water  at  a  single 
irrigation  should  ordinarily  be  from  four  to  eight  inches. 
In  very  hot  weather  it  may  be  reduced  to  two  or  three 
inches.  In  late  fall  or  early  spring,  when  the  soil  is 
unoccupied,  the  application  may  be  relatively  larger 
provided  the  soil  is  dry. 

Excessive  irrigation  is  to  be  avoided.  While  the  total 
yield  increases  with  increase  in  the  application  of  water 
up  to  the  maximum  point,  the  unit  production  decreases.1 
The  following  brief  table,  calculated  by  Widtsoe  from 
actual  yields  of  wheat,  illustrates  this  point :  — 


Thirty  Acre-inches  op  Water  spread  over 

1  acre 

2  acres 

3  acres 

4  acres 

6  acres 

Grain  (bushels) 
Straw  (pounds) 

47.51 
4532 

91.42 
2908 

130.59 
10256 

166.16 
13204 

226.16 
17916 

1  Widtsoe,  J.  A.  The  Production  of  Dry  Matter  with  Dif- 
ferent Quantities  of  Irrigation  Water.  Utah  Agr.  Exp.  Sta., 
Bui.  No.  116.     1912. 


IRRIGATION  AND  DRY-FARMING  709 

Small  applications  of  water  are  relatively  most  efficient. 
Up  to  the  limit  where  injury  results,  the  more  concentrated 
the  soil  solution,  the  larger  is  the  yield  of  crop. 

607.  Time  to  apply  water.  —  The  best  time  to  apply 
water  depends  to  a  large  extent  on  the  nature  and  habits 
of  the  crop.  Ordinarily  the  soil  should  be  thoroughly 
moistened  at  the  time  of  planting,  in  which  case  the 
application  will  have  been  made  before  fitting  the  ground. 
For  sugar  beets  and  other  crops  planted  in  rows,  it  is 
permissible  to  irrigate  immediately  after  seeding.  The 
formation  of  a  crust  is  to  be  avoided.  After  planting, 
water  may  be  applied  at  intervals  of  two  or  four  weeks, 
or  when  the  soil  has  reached  the  stage  of  dryness  at  which 
sluggish  capillary  movement  occurs.  The  experienced 
irrigator  becomes  very  proficient  in  recognizing  this  con- 
dition. For  grain  and  forage  crops,  the  soil  should  be  well 
moistened  when  the  crop  approaches  maturity.  For  al- 
falfa, irrigation  may  be  either  shortly  before  or  just  after 
harvest  with  good  results.  For  root  crops  a  relatively 
dry  condition  of  the  soil  at  maturity  is  preferred.  The 
same  is  true  for  trees,  and  the  large  application  of  water 
late  in  the  growing  season  is  especially  to  be  avoided 
because  the  new  wood  growth  is  likely  to  be  winter- 
killed. Irrigation  in  spring,  especially  at  blossoming  time, 
is  to  be  avoided  because  it  interferes  with  the  setting  of 
fruit.  One  or  two  thorough  irrigations  in  a  season  are 
usually  sufficient  for  the  growth  of  trees.  Small  fruits 
should  have  plenty  of  water  at  the  maturity  of  the  crop. 

Where  water  is  available  in  late  fall  and  in  winter,  it 
may  be  applied  to  the  soil  and  stored  there  for  use  during 
the  following  season.     Investigations  at  the  Utah  x  station 

1  Widtsoe,  J.  A.  The  Storage  of  Winter  Precipitation  in 
Soils.     Utah  Agr.  Exp.  Sta.,  Bui.  No.  104.     1908. 


710       SOILS:    PROPERTIES  AND  MANAGEMENT 

have  shown  that  moisture  may  be  effectively  stored  in  the 
soil  to  a  depth  of  more  than  eight  feet  and  be  readily  used 
by  crops  the  next  season.  The  total  amount  of  water  to 
be  applied  depends  on  many  things.  The  following  factors 
affect  the  duty  of  water:  (1)  character  of  the  crop; 
(2)  climate ;  (3)  texture  and  structure  of  the  soil ;  (4)  depth 
of  the  soil;  (5)  fertility  of  the  soil,  including  the  total 
amount  of  soluble  material ;  (())  kind  of  tillage  practiced  ; 
(7)  thickness  of  planting ;  (8)  season  when  the  crop  grows  ; 
(9)  frequency  and  method  of  applying  water ;  (10)  amount 
and  time  of  applying  water.  A  fertile  soil  and  a  large 
and  rapid  growth  of  the  crop  go  with  economy  of  water. 
Many  of  the  above  factors,  such  as  thickness  of  planting, 
tillage  practice,  and  manner  of  using  water,  determine  the 
loss  from  the  soil  that  has  no  direct  relation  to  the  crop. 

The  total  amount  of  water  to  be  applied  l  in  irrigation 
should  range  from  five  to  twenty  inches,  with  the  tendency 
toward  the  lower  figure.  This  means  a  duty  of  280  to 
75  acres  a  second-foot  for  a  season  of  sixty  days.  From 
one  to  four  applications  of  water  are  usually  made.  The 
larger  the  plant  and  the  deeper  the  root  system,  the  larger 
the  individual  application  of  water  may  be,  and  the  fewer 
the  number  of  applications. 

608.  Conservation  of  moisture  after  irrigation.  —  The 
conservation  of  moisture  applied  by  irrigation  should  be 
provided  for  whenever  practicable.  Crops  planted  in 
rows  should  be  cultivated  as  soon  as  the  soil  is  dry  enough 
not  to  puddle.  As  suggested  above,  when  the  furrow 
method  is  employed  the  furrows  should  be  deep,  so  that 
only  a  small  part  of  the  surface  soil  will  be  wet.     Coupled 

1Widtsoe,  J.  A.  Principles  of  Irrigation  Practice,  Chapter 
XVII.     New   York.     1914. 


IRRIGATION  AND   DEY-FARMING  711 

with  this,  a  mulch  of  dry  soil  from  four  to  eight  inches 
(deep  should  be  maintained.  This  is  a  protection  against 
too  high  a  temperature  in  moist  soil  unprotected  by  shade, 
as  well  as  against  loss  of  moisture.  The  surface  of  the 
soil  should  be  kept  as  nearly  level  as  possible. 

Crops  that  are  not  planted  in  rows,  such  as  grain,  may 
be  cultivated  with  a  fine-tooth  harrow  until  they  reach  a 
height  of  from  several  inches  to  a  foot,  at  which  stage 
evaporation  from  the  soil  is  largely  prevented  by  the 
shading  of  vegetation.  If  it  is  to  be  successful  this  culti- 
vation must  begin  as  soon  as  the  seedlings  appear  above 
the  surface,  in  order  that  the  roots  may  be  forced  deep 
into  the  soil.  Then  the  top  may  be  much  twisted  with 
but  little  injury  to  the  plant,  and  that  injury  appears  to 
be  more  than  counterbalanced  by  the  tillering  of  the  plant. 
By  prompt  and  thorough  tillage  following  irrigation,  very 
much  may  be  done  not  only  to  conserve  soil  moisture  but 
also  to  prevent  the  accumulation  of  alkali  at  the  surface 
by  evaporation. 

609.  Sewage  irrigation.  —  A  phase  of  the  general  prac- 
tice of  irrigation  is  the  application  of  sewage  l  to  the  land 
for  purposes  of  crop  production.  This  supplies  plant-food 
as  well  as  water.  The  food  content,  however,  is  relatively 
small,  being  about  two  parts  in  one  thousand,  of  which 
one-half  is  organic  and  one-half  is  inorganic  material. 
In  European  countries  sewage  irrigation  is  extensively 
employed  near  cities,  but  in  the  United  States  the  practice 
has  not  been  largely  followed.  The  city  of  Boston  has 
carried  out  extensive  experiments,  and  the  city  of  Los 

1  Rafter,  G.  W.,  and  Baker,  M.  N.  Sewage  Disposal  in  the 
United  States.  New  York.  1904.  Also,  Rafter,  G.  W.  Sew- 
age Irrigation.  U.  S.  Geol.  Survey,  Water-Supply  and  Irrigation 
Papers,  Nos.  3  and  22.     1897  and  1899. 


712      SOILS:    PROPERTIES  AND  MANAGEMl  \  1 

Angeles  has  a  large  farm  irrigated  with  sewage  water. 
The  same  general  principles  prevail  in  the  use  of  water 
as  in  normal  irrigation  practice,  except  that  the  soil  may 
become  clogged  and  foul  from  the  accumulation  of  solid 
material,  especially  where  the  idea  of  disposal  oxer- 
shadows  that  of  efficient  use.  This  practice  is  used 
chiefly  for  the  production  of  hay  and  forage. 

DRY-FARMING 

The  water  supply  for  irrigation  is  sufficient  for  only  a 
small  part  of  the  earth's  surface  which  needs  such  treat- 
ment. The  remainder  of  this  vast  area  of  laud  having  a 
deficient  rainfall  must  be  utilized,  if  at  all,  by  the  most 
scrupulous  and  careful  conservation  and  use  of  the  natural 
rainfall.  The  growth  of  crops  without  irrigation  under 
such  conditions  is  termed  dry-farming.1  It  is  merely  an 
intensified  form  of  the  methods  which  are  recognized 
as  good  practice  to  conserve  moisture  in  more  humid 
regions. 

Dry-farming  is  based  on  the  principle  that  the  production 
of  dry  matter  in  crops  requires  only  a  small  part  of  the 
water  which  may  be  used  in  one  way  or  another  in  its 
growth,  and  that  a  large  part  of  that  water  is  lost  by  sur- 

1  Widtsoe,  J.  A.  Dry  Farming.  New  York.  1910.  (Appendix 
includes  a  large  list  of  references  on  dry  land  farming.) 

MacDonald,  Wm.     Dry  Farming.     New  York.     1910. 

Campbell,  H.  W.  Soil  Culture  Manual.  Lincoln,  Nebraska. 
1907. 

Chilcott,  E.  C.  Dry  Farming  in  the  Great  Plains  Area. 
U.  S.  D.  A.,  Yearbook,  pp.  451^68.     1907. 

Eriggs,  L.  J.,  and  Shantz,  H.  L.  The  Water  Requirement 
of  Plants.  Investigations  in  the  Great  Plains  in  1910  and  1911. 
U.  S.  D.  A.,  Bur.  Plant  Ind.,  Bui.  284.  1913.  Also,  the  Water 
Requirements  of  Plants.  A  review  of  literature.  U.  S.  D.  A., 
Bur.  Plant  Ind.,  Bui.  285.     1913. 


IRRIGATION  AND  DRY-FARMING  713 

face   flow,   by   seepage,   and   especially   by   evaporation, 
without  performing  any  useful  service  to  the  plant. 

610.  Practices  in  dry-farming.  —  The  practice  of  dry- 
fanning  may  be  divided  into  three  groups :  (1)  the  main- 
tenance of  such  a  condition  of  the  soil  at  all  seasons  of 
the  year  as  will  insure  the  complete  absorption  of  the 
rain-  and  snow-fall ;  (2)  the  conservation  of  the  stored 
moisture  by  appropriate  methods  of  tillage ;  (3)  the  selec- 
tion of  drought-resistant  crops  and  of  rotations  adapted 
to  the  small  use  of  water. 

611.  Storage  of  water  in  the  soil.  —  In  different  regions 
the  rainfall  occurs  at  different  seasons.  A  loose,  open 
condition  of  the  surface  soil  should  be  maintained  during 
that  period.  This  may  require  deep  plowing,  and  if  the 
subsoil  is  compact  it  may  include  subsoiling.  Where 
the  precipitation  comes  as  snow,  the  surface  should  be 
rough  so  as  to  prevent  drifting,  in  order  that  the  resulting 
water  may  be  uniformly  absorbed  by  the  soil.  Fall 
plowing  is  an  important  factor  where  much  of  the  pre> 
cipitation  comes  in  winter  and  the  soil  is  compact. 
Another  reason  for  the  maintenance  of  a  ridged  surface 
is  to  reduce  erosion  by  the  high  winds  which  frequently 
occur  in  winter  in  dry-farming  regions  and  which  cause 
the  serious  removal  of  the  soil.  The  roughened  surface 
impedes  the  wind  movement,  and  the  moist  soil  at  the 
crest  of  the  ridges  resists  erosion. 

612.  Conservation  of  moisture.  —  The  conservation  of 
the  moisture  in  the  soil  involves  two  things  —  an  increase 
in  the  capillary  capacity  of  the  soil,  and  the  prevention  of 
evaporation.  Where  the  rainfall  is  low,  the  deep  subsoil 
is  usually  very  dry.  The  rainfall  penetrates  to  a  limited 
distance  from  the  surface.  Saving  loosened  the  subsoil 
so  that  the  rainfall  is  absorbed,  the  next  step  is  to  compact 


714       -SOILS;    PROPERTIES  AND  MANAGEMENT 

the  substratum  as  much  as  possible  by  tillage  in  order  feci 
increase  its  capillary  capacity.  The  need  of  this  treat- 
ment, of  course,  depends  on  the  nature  of  the  soil,  and 
is  not  always  the  most  favorable.  It  is  undesirable  that 
this  packing  should  extend  to  the  surface.  Following  the 
plow,  the  land  is  frequently  worked  down  with  8  subsur- 
face packer,  an  implement  of  considerable  weight,  made 
up  of  openwork  rims  that  press  the  soil  together  and  at 
the  same  time  leave  a  mulch  on  the  surface.  By  acting 
on  the  lower  part  of  the  furrow  instead  of  on  the  surface, 
the  packer  brings  it  into  closer  contact  with  the  subsoil 
and  thereby  establishes  better  capillary  connection. 

After  thorough  packing  of  the  main  part  of  the  furrow, 
a  dust  mulch  is  maintained  on  the  surface.  This  should 
be  of  medium  depth  in  the  season  when  rains  are  likely 
to  occur,  and  of  somewhat  greater  depth  during  the 
dry  period.  Two  or  three  inches  is  usually  a  sufficient 
depth. 

Various  applications  of  the  principle  of  mulching  may  be 
employed.  Land  may  be  disked  before  plowing  in  fall  or 
spring,  to  hold  moisture  until  the  plowing  can  be  done. 
As  soon  as  a  crop  is  removed,  the  land  should  be  plowed 
or  fitted  and  worked  dowrn  to  a  good  mulched  surface. 
Land  should  not  be  allowed  to  stand  unworked  for  any 
considerable  time  after  harvest.  All  rowed  crops  should 
be  kept  thoroughly  mulched.  Much  may  be  done  to 
conserve  water  in  grain  and  hayfields  by  tillage.  The 
same  principles  apply  to  the  practices  that  are  used  on 
irrigated  land.  Special  revolving  toothed  implements 
have  been  devised  to  loosen  up  the  surface  soil  under 
such  conditions. 

613.  Alternate  cropping.  —  Where  the  rainfall  is  too 
light  in  a  single  season  to  permit  the  production  of  a  profit- 


IRRIGATION  AND  DRY-FARMING  715 

able  crop,  it  is  sometimes  the  practice  to  collect  and  store 
the  rainfall  of  two  seasons  in  the  soil.  This  is  the  system 
of  alternate-year  cropping.  In  the  intervening  year  the 
soil  is  carefully  fallowed  and  mulched,  to  hold  the  stored 
moisture.  That  such  long-time  storage  of  available 
moisture  is  possible  has  been  clearly  demonstrated  *  under 
dry-farming  conditions,  and  also  in  the  study  of  irrigation 
problems.  An  arid  or  a  semiarid  climate  is  especially 
favorable  for  the  formation  and  maintenance  of  an  effi- 
cient dust  mulch,  and  the  occurrence  of  dry  earth  in  the 
lower  subsoil  permits  moisture  to  be  stored  and  retained  in 
large  quantities  within  reach  of  the  roots  of  crops.  It  is 
believed  by  some  persons  that  the  practice  of  fallowing 
in  alternate  years  is  very  destructive  of  organic  matter 
in  the  soil,  and  that  it  may  be  better  to  grow  a  green- 
manure  crop  in  that  period  to  be  turned  under.  It  is 
questionable  whether  the  loss  of  water  may  not  be  a 
serious  objection  to  this. 

614.  Drought-resistant  crops.  —  For  growth  under  dry- 
farming  conditions,  crops  are  preferred  which  have  a  low 
moisture  requirement,  which  are  not  seriously  affected 
by  severe  drying,  and  which  have  a  fairly  deep  root  system. 
The  sorghums  come  in  the  first  class  and  also  fulfill  the 
second  requirement.  Corn  is  fairly  satisfactory.  Wheat, 
barley,  and  alfalfa  are  favorite  dry-farm  crops.  Drought- 
resistant  varieties  of  these  crops  are  being  sought.  A 
rotation  is  desirable  which  exposes  the  soil  as  little  as 
possible  to  evaporation,  and  permits  continuous  mulch- 
ing with  the  minimum  of  plowing. 

1  Atkinson,  A.,  Buckman,  H.  0.,  and  Gieseker,  L.  F.  Dry 
Farm  Moisture  Studies.  Montana  Agr.  Exp.  Sta.,  Bui.  87. 
1911.  Also  Burr,  W.  W.  Storing  Moisture  in  the  Soil.  Ne- 
braska Agr.  Exp.  Sta.,  Bui.  No.  114.     1910. 


716       SOILS:    PROPERTIES  AND  MANAGEMENT 

615.  Soils  associated  with  dry-farming.  -  Dry-farming 
is  often  closely  associated  with  irrigation,  being  practiced 
on  the  heavier  soils  where  the  water-storage  capacity  is 
large  and  where  the  practice  of  irrigation  is  most  difficult 
Successful  dry-farming  requires  an  annual  rainfall  of  at 
least  fifteen  inches,  and  twenty  inches  is  much  safer  as  a 
basis  for  the  practice.  A  general  principle  to  be  observed 
in  dry-farming  is  that  the  shorter  the  soil  moisture  supply, 
the  lighter  should  be  the  rate  of  seeding.  Wheat,  for 
example,  may  be  seeded  at  the  rate  of  only  twenty  pounds 


l>.) 

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Fig.  82.— Areas  of  western  United  States  where  dry- land  farming  is  or 
may  be  practiced. 


IRRIGATION  AND  DRY-FARMING  111 

to  the  acre.  The  crop  will  stool  out  strongly  and  adjust 
itself  to  the  moisture  supply.  Under  dry-land  and  irri- 
gation farming,  crops  as  a  rule  root  much  deeper  than  in 
humid  soils. 

616.  Extent  of  dry-farming.  —  In  the  United  States 
many  thousands  of  acres  in  the  Great  Plains  region,  in 
the  semiarid  northwestern  valleys,  and  in  the  Pacific 
Coast  States,  are  now  being  cropped  under  systems  of 
dry-farming  (see  Fig.  82).  Further,  the  practice  is  be- 
ginning to  be  followed  somewhat  more  definitely  in  all 
parts  of  the  world  where  similar  conditions  prevail.  The 
large  open  areas  of  land  and  the  dry  climate  in  such 
regions  have  encouraged  the  employment  of  larger  power 
equipment  in  planting  and  harvesting  the  crops,  especially 
wheat.  In  parts  of  California  machines  are  used  which 
cut,  thresh,  and  sack  the  grain  in  one  operation. 

The  study  of  the  principles  on  which  dry-farming  is 
based,  together  with  the  extension  of  their  practice, 
may  be  expected  to  bring  large  areas  of  land,  now  sub- 
stantially worthless,  to  a  measurable  degree  of  productive- 
ness. The  tendency  in  the  practice  of  both  dry-farming 
and  irrigation  is  toward  the  more  efficient  use  of  water 
for  purposes  of  crop  production,  and  to  approach  the 
actual  requirements  of  the  plant  in  the  utilization  of  water. 
In  both  cases  the  fundamental  principles  in  the  storage, 
conservation,  and  use  of  water  by  plants  must  be  observed, 
as  well  as  care  regarding  the  application  of  these  prin- 
ciples according  to  the  soil,  the  crop,  and  the  nature  of  the 
water  supply. 


CHAPTER   XXXI 
THE  SOIL  SURVEY 

The  function  of  the  soil  survey  is  to  investigate  th< 
nature  and  occurrence  of  soils  in  the  field.  The  soils  are 
classified  into  areas  having  approximately  the  same  crop 
relations  and  tillage  properties.  The  location  of  the  areas 
of  each  kind  of  soil  is  represented  on  charts  or  maps,  and 
their  character  and  chief  economic  and  agricultural  rela- 
tions are  described  in  printed  reports. 

617.  The  classification ]  of  soils  by  survey.  —  The 
occurrence  of  differences  in  the  tillage  and   manurial  re- 

1  Klassification,  Nomenclature,  und  Kartierung  der  Boden- 
arten. 

Verhandlungen  der  zweiten  internationalen  Agrogeologenkon- 
ferenz.  (Proceedings  of  the  Second  International  Agro-geological 
Conference,  Stockholm,  Chapter  V,  pp.  223-298.  (Seven  papers.) 
1911. 

Report  on  Soil  Classification.  Proc.  Amer.  Soc.  Agron.,  Vol. 
6,  No.  6,  pp.  284-288.     1914. 

Fippin,  E.  O.  The  Practical  Classification  of  Soils.  Proc. 
Amer.  Soc.  Agron.,  Vol.  3,  pp.  76-88.     1911. 

Marbut,  C.  F.  Soils  of  the  United  States.  U.  S.  D.  A., 
Bur.  Soils,  Bui.  96,  pp.  7-16.     1913. 

Coffey,  G.  N.  A  Study  of  the  Soils  of  the  United  States. 
U.  S.  D.  A.,  Bur.  Soils,  Bui.  85,  p.  114.     1912. 

Hall,  A.  D.,  and  Russell,  E.  J.  Soil  Surveys  and  Soil  Analy- 
sis.    Jour.  Agr.  Sci.,  Vol.  4,  Part  2.     1911. 

Tularkov,  N.  The  Genetic  Classification  of  Soils.  Jour. 
Agr.  Sci.,  Vol.  3,  pp.  80-85.     1909. 

Stevenson,  W.  H.,  Christie,  G.  I.,  and  WiUcox,  O.  W.  The 
718 


THE  SOIL   SURVEY  719 

quirements  of  soils,  their  crop  relations,  and  their  agri- 
cultural value  make  necessary  the  determination  of  the 
properties  of  the  soil,  that  are  chiefly  responsible  for  those 
differences,  and  their  arrangement  into  an  orderly  scheme 
of  classification.  The  aim  is  to  divide  the  land  into 
areas  of  approximately  the  same  general  character.  This 
volume  is  largely  an  exposition  of  those  properties  of  soils 
that  make  differences  in  their  crop  relations  and  manage- 
ment. It  is  evident  that  differences  are  numerous  and 
varied,  and  that  some  have  greater  significance  than 
others. 

Soils  may  be  classified  from  many  different  points  of 
view.  The  basis  may  be  purely  geological,  purely  physical, 
or  almost  entirely  chemical.  Any  one  of  these  alone  is 
likely  to  be  inadequate  for  the  purposes  of  the  agriculturist. 
The  viewpoint  of  the  agricultural  soil  survey  should  be 
such  as  to  secure  unity  in  the  crop  relations  of  each  distinct 
area  of  soil  recognized. 

The  system  of  classification  in  use  must  employ  as  a 
basis  some  combination  of  the  groups  of  properties  enu- 
merated above.  The  combination  selected  has  differed 
in  different  parts  of  the  world,  depending  on  the  training 

Principal  Soil  Areas  of  Iowa.  Iowa  Agr.,  Exp.  Sta.,  Bui.  82. 
1905. 

Mooers,  G.  A.  The  Soils  of  Tennessee.  Tennessee  Agr. 
Exp.  Sta.,  Bui.  78.     1906. 

Sherman,  C.  W.  The  Indiana*  Soil  Survey.  Dept.  Geol. 
and  Natural  Resources,  32d  Ann.  Rept.,  pp.  17-47.     1907. 

Hopkins,  C.  G.,  and  Pettit,  J.  H.  The  Fertility *of  Illinois 
Soils.     Illinois  Agr.  Exp.  Sta.,  Bui.  123.     1908. 

Hall,  A.  D.,  and  Russell,  E.  J.  Report  on  the  Agriculture 
and  Soils  of  Kent,  Surrey,  and  Essex.  Dept.  Bd.  Agr.  and  Fish- 
eries.    London.     1911. 

Kummel,  A.  B.  Soil  Surveys  as  Related  to  Geology.  N.  J. 
Bd.  Agr.,  36th  Ann.  Rept.,  pp.  162-169.     1908. 


720       SOILS:    PROPERTIES  AND  MANAGEMENT 

of  the  person  by  whom  the  survey  was  proposed,  and  the 
kinds  of  soils  and  crops  with  which  he  dealt.  Some 
persons  have  used  the  vegetation,1  especially  the  native 

vegetation,  as  a  means  of  classifying  soils.  Where  this 
is  present  it  is  an  excellent  means  of  identifying  differences, 
and  pioneers  as  well  as  others  have  always  made  use  of 
the  vegetation  growing  on  a  soil  to  detect  variation  in 
its  cropping  capacities.  Unfortunately  the  vegetation, 
whether  native  or  introduced,  being  a  result  of  natural 
causes,  affords  information  regarding  the  properties  of  a 
soil  only  when  the  correlation  has  been  worked  out. 
Further,  the  native  vegetation  is  now  seldom  present  in 
well-settled  areas,  so  that  it  is  inadequate  as  a  general 
means  of  classification,  though  very  useful  for  some 
purposes  of  comparison. 

618.  Factors  employed  in  classification.  —  In  classify* 
ing  soils,  four  primary  and  two  secondary  factors  are 
employed.  The  former  group  deals  entirely  with  the 
soil  itself ;  the  latter  group  deals  with  the  climate  or  the 
situation  in  which  the  soil  is  placed.  The  situation  exerts 
an  influence  on  the  crop  value  and  on  the  properties  of  the 
soil.  The  factors,  beginning  with  those  of  the  smallest 
range  of  occurrence,  are  as  follows  :  (1)  texture,  (2)  special 
properties,  chiefly  chemical,  (3)  kind  of  material  from 
which  the  soil  was  formed,  (4)  agency  of  formation, 
(5)  humidity  and  precipitation,  and  (6)  normal  and 
mean  temperature.  • 

The  soil  type  is  the  unit  of  classification,  and  may  be 
defined  as  an  area  of  soil  that  is  essentially  alike  in  all  the 
above  characters. 


1  Hilgard,  E.  W.     Soils,  Chapters  XXIV,  XXV,  and  XXVI. 

New  York.     1906. 


THE  SOIL   SURVEY  721 


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722       SOILS:    PROPERTIES  AND  MANAGEMENT 

619.  Texture  —  the  soil  class.  —  Of  all  the  properties 

of  the  soil,  the  one  which  is  most  apparent  and  which 
exerts  the  most  direct  influence  on  the  plant  is  the  tex- 
ture, or  fineness  of  division,  of  the  soil  particles.  Is  it  a 
clay,  a  silt,  a  sand,  a  gravel,  or  some  combination  of  these  ? 
Is  it  stony,  or  is  it  free  from  stone?  The  texture  is  the 
first  property  made  use  of  in  classifying  soil.  This  divi- 
sion based  on  texture  is  called  the  soil  class.  It  is  a  purely 
physical  division,  and  does  not  recognize  any  chemical 
or  other  differences  in  the  soil  except  as  such  differences 
may  occur  between  coarse  and  fine  materials. 

620.  Special  properties  —  the  soil  series.  —  Soils  of 
different  texture  may  be  alike  in  other  properties.  They 
may  be  all  red,  all  black,  or  all  yellow.  They  may  be 
well  drained  or  poorly  drained.  Such  a  group  of  soils  of 
different  texture  but  alike  in  all  other  properties  consti- 
tutes a  soil  series.  The  properties  by  which  the  soil 
series  is  recognized  are  (1)  color,  which  is  predominant 
in  the  separation,  (2)  content  of  organic  matter,  (3)  natural 
drainage,  (4)  content  of  lime  carbonate,  (5)  ultimate 
chemical  composition,  and  (6)  arrangement  of  the  soil  in 
the  section.  Any  one  or  a  combination  of  these  properties 
may  identify  an  area  of  soils.  Such  an  area  would  con- 
stitute a  soil  series.  These  properties  permit  the  recog- 
nition of  chemical  differences  quite  as  much  as  physical 
differences  of  the  soil  in  mass. 

If  it  were  possible  clearly  to  identify  all  the  properties 
that  may  be  recognized  in  the  series  and  class  divisions, 
there  would  be  no  need  of  employing  other  factors  in  the 
classification.  Such  a  clear  identification,  however,  is 
only  partially  possible,  and  is  further  limited  by  the 
conditions  under  which  the  soil  survey  must  be  carried 
out  in  the  field.     Many  of  these  properties  are  of  such 


THE  SOIL   SURVEY  723 

an  intimate  nature  that  they  cannot  be  recognized  by 
inspection.  However,  they  are  correlated  with  the  origin 
and  mode  of  formation  of  the  soil,  and  therefore  the  use 
of  those  factors  in  the  classification  is  justified  as  an  aid 
to  rapid  and  accurate  field  identification. 

621.  Source  of  material  —  the  soil  group.  —  The  soils 
of  a  region  may  be  similar  in  many  properties  because 
they  have  been  derived  from  the  same  kind  of  rock.  They 
may  be  similar  also  because  they  have  been  derived  from 
the  same  mixture  of  different  rock  materials.  As  a  result 
of  the  many  kinds  of  rock  and  the  different  proportions  in 
which  they  may  be  mingled,  many  groups  of  soil  series 
may  be  recognized.  Some  of  the  commoner  groups  of 
rocks  identified  with  these  differences  are  acid  and  basic 
crystalline  rocks,  shales,  sandstone,  and  limestone. 

622.  Agency  of  formation  —  the  soil  province.  —  The 
way  in  which  a  rock  formation  has  been  broken  down  and 
the  residue  brought  to  its  new  resting  place  affects  both 
the  chemical  and  the  physical  nature  of  the  resultant  soil. 
The  six  groups  of  forces  that  have  been  predominant 
in  the  formation  of  soils  are :  (1)  weathering,  or  the 
decay  and  disintegration  of  rocks  in  place,  forming  a 
residual  soil ;  (2)  biological  processes,  which  form  organic 
matter  and  give  rise  to  cumulose  soils ;  (3)  water  in 
streams,  lakes,  and  oceans,  which  reduces,  transports, 
and  sorts  soil-forming  materials,  and  which  imparts  to  its 
deposits  a  distinctly  stratified  arrangement;  (4)  atmos- 
phere, especially  as  regards  wind,  which  exerts  an  abra- 
sive and  sorting  action  similar  to  that  of  water  but  with 
a  very  much  smaller  range  in  the  texture  of  the  strata 
formed,  and  with  a  type  of  stratification  also  distinct 
from  that  formed  by  water ;  (5)  glaciation,  or  the  action 
of  continental  masses  of  ice,  the  deposits  from  which  are 


724     soils:  properties  and  management 

exceedingly  heterogeneous  in  nature  and  are  without 
sorting  or  stratification  except  as  the  action  of  wind  and 
water  may  have  combined  with  the  action  of  the  ice; 
(6)  gravity,  or  the  slow  creep  of  material  on  slopes,  which 
is  a  minor  agency  of  soil  formation  (see  Chapter  II). 

623.  Climate.  —  Soils  owe  their  origin  to  the  operation 
of  one  or  more  of  the  forces  named  above.  Usually  some 
one  of  these  agencies  is  predominant  and  gives  specific 
character  to  the  soil.  The  elements  of  climate  have  been 
used  in  the  practical  classification  of  soils  to  only  a  small 
degree,  since  the  inherent  properties  of  the  material  in 
these  divisions  are  usually  distinct  enough  to  make  sepa- 
ration easy.  The  excessive  accumulation  of  the  soluble 
salts  known  as  alkali  is  associated  with  a  low  rainfall, 
and  other  chemical  and  physical  properties  are  correlated 
with  aridity.  Three  main  divisions  in  humidity  and  pre- 
cipitation may  readily  be  made,  namely,  (1)  humid, 
(2)  semiarid,  (3)  arid.  The  exact  precipitation  limits  of 
these  divisions  depend  on  the  temperature  relations  and 
the  time  and  manner  of  occurrence  of  the  precipitation. 

In  a  world  system  of  soil  classification  the  temperature 
relations  of  the  soil  would  be  recognized,  but  this  division 
is  seldom  important  in  any  single  country. 

624.  The  practical  classification  of  soils  in  the  United 
States.  —  As  practiced  in  the  United  States,  the  classi- 
fication of  soils  l  has  disregarded  the  climatic  factor  and 
has  usually  combined  the  kind  of  rock  and  the  agencies 
of  formation  as  a  single  basis  of  separation  of  soils,  desig- 
nating the  division  resulting  therefrom  as  a  soil  province. 
In  some  areas  one  element  of  formation  is  dominant  and 

1Marbut,  C.  F.,  Bennett,  H.  H.,  Lapham,  J.  E.,  and  Lap- 
ham,  M.  H.  Soils  of  the  United  States.  U.  S.  D.  A.,  Bur. 
Soils,  Bui.  96,  p.  891.     1913. 


THE  SOIL   SURVEY  725 

in  other  areas  another  element  is  dominant.  To  this 
extent  the  classification  deviates  from  the  ideal  system 
outlined  above. 

625.  The  soil  type  and  series.  How  characterized  and 
named.  —  The  two  predominant  divisions  of  soil  are  the 
soil  type  and  the  soil  series.  The  soil  type  is  the  unit 
of  field  study  and  classification,  and  corresponds  to  a 
species  of  plant  or  animal  in  biological  classification.  It 
includes  all  those  areas  of  soil  that  are  essentially  alike 
in  all  properties  —  texture,  color,  chemical  nature,  struc- 
tural properties,  source  of  material,  and  mode  of  forma- 
tion. In  other  words,  soils  of  the  same  type  are  as  nearly 
alike  as  field  identification  will  admit.  The  soil  series  is 
a  group  of  types  differing  only  in  the  texture  of  the  differ- 
ent members.  This  may  be  said  to  correspond  to  the 
genera  in  biological  classification. 

A  name  is  given  to  each  series  of  soil  for  purposes  of 
easy  identification,  and  to  this  name  the  class  designation 
is  added,  thereby  fixing  the  identity  of  the  type.  For 
example,  the  Miami  series  includes  certain  light-colored, 
timbered,  glacial  soils  of  the  East  Central  States.  The 
Hagarstown  series  includes  certain  light  brown  to  reddish 
residual  limestone  soils,  found  in  the  blue-grass  region  of 
Kentucky  and  adjacent  states.  The  Norfolk  series  in- 
cludes lemon  yellow,  marine-deposited  soils  of  the  coastal 
plain  of  the  Atlantic  and  Gulf  regions.  Clay  loam  would 
refer  to  a  particular  texture  of  any  of  these  series,  as  the 
Miami  clay  loam,  for  example,  thus  completing  the  type 
name  of  a  soil,  which  is  made  up  of  the  series  name  and 
the  class  designation. 

The  common  practice  is  to  select  for  the  series  desig- 
nation some  geographical  name  in  the  region  where  the 
soil  is  first  identified  or  is  best  developed.     The  word 


726     SOILS:   properties  and  management 

Miami  is  taken  from  the  Miami  River  in  southwestern 
Ohio,  where  the  Miami  series  was  first  recognized. 
*~  This  system  of  a  proper  generic  name  and  a  descriptive 
class  name  is  most  widely  used  in  the  United  States  to 
identify  the  soil  type.  It  gives  a  specific  identity  of  the 
soil  in  its  situation  and  in  all  its  properties. 

Hopkins1  has  proposed  and  used  the  Dewey  Library 
System  of  numerical  naming  of  soils,  by  which  each  prop- 
erty is  given  a  fixed  series  of  numbers  and  the  identifica- 
tion number  is  obtained  by  combining  the  numbers  that 
represent  its  properties.  Whole  numbers  are  assigned  to 
important  and  definite  soil  types,  and  decimals  are  used 
for  related  types  possessing  some  distinct  variations. 
For  example,  451.2  represents  a  glacial  soil  made  up  of 
brown  loam  on  silt.  While  the  numbering  system  of 
designation  is  admirable  in  many  ways,  it  does  not  lend 
itself  to  the  same  practical  use  that  is  possible  with  a 
proper  descriptive  name. 

626.  The  equipment  for  survey  work.  —  The  most 
important  part  of  the  equipment  for  soil  survey  work  is 
the  field  man.  He  should  be  a  keen  and  careful  observer, 
and  he  should  have  had  broad  training  for  his  work.  He 
should  be  acquainted  with  the  technic  of  soils  in  the  labora- 
tory and  in  the  field.  He  should  be  familiar  with  the 
chief  physical  and  chemical  processes  and  material  in- 
volved in  soil  formation.  He  should  have  an  under- 
standing of  that  phase  of  geology  known  as  physiography. 
On  the  agricultural  side,  he  should  be  acquainted  with 
plants  and  the  methods  of  growing  the  more  important 
crops.     He  should  know  tillage  practice,  and  should  be 

1  Hopkins,  C.  G.,  and  Pettit,  J.  H.  The  Fertility  of  Illinois 
Soils.     Illinois  Agr.  Exp.  Sta.,  Bui.  123,  p.  252.     1908. 


THE  SOIL   SURVEY  727 

able  to  distinguish  between  the  properties  of  the  soil  that 
are  native  and  permanent  and  those  that  may  be  induced 
by  the  method  of  handling.  There  is  very  little  knowl- 
edge of  natural  phenomena  that  will  not  be  found  useful 
to  the  field  man  in  classifying  soils,  because  he  uses  all 
sorts  of  observations  in  making  and  checking  his  divisions 
in  soils.  In  brief,  he  should  have  a  good  training  in  the 
fundamental  technic  of  geology,  chemistry,  and  agricul- 
ture. 

In  the  way  of  physical  equipment  the  field  man  should 
have  a  good  map  of  the  region,  on  a  scale  of  one  inch  to 
a  mile  or  larger.  The  field  work  should  be  done  on  at 
least  as  large  a  scale  as  the  finished  map,  as  this  increases 
the  degree  of  accuracy.  The  map  should  show  the  roads, 
streams,  and  towns  of  the  region,  and  in  addition  the 
topography,  location  of  houses,  and  other  natural  and 
cultural  features  which  are  useful  in  placing  boundaries 
of  soil.  Where  a  satisfactory  map  is  not  available  the 
field  man  must  make  such  a  map  x  during  the  progress  of 
the  soil  survey.  For  this  purpose  a  Gannett  plane  table, 
a  sight  alidade,  and  some  method  of  measuring  distance  — - 
preferably  an  odometer,  such  as  is  used  for  counting  the 
revolutions  of  a  buggy  wheel  —  are  necessary.  Cloth- 
back  drawing  paper  is  generally  used. 

Where  a  suitable  base  map  is  already  available,  a  set 
of  pencils  of  different  colors  for  representing  each  type  of 
soil  on  the  map  as  it  is  recognized  is  essential.  A  horse 
and  buggy  is  the  usual  method  of  conveyance.  For  ex- 
amining the  soil  a  soil  auger  is  used  (see  Fig.  83).  This 
consists  of  a  one-and-one-half-inch  wood  auger  attached  to 
a  half-inch  pipe  rod  with  a  T  handle,  making  a  total  length 

1  Instruction  to  Field  Parties.     U.  S.  D.  A.,  Bur.  Soils.     1914. 


728       SOILS:    PROPERTIES  AND  MANAGEMENT 


of  about  thirty-eight  inches.  By  the  use  of  additional 
sections  the  length  may  be  increased.  The  end  of  the 
auger  may  be  modified  by  cutting  off 
the  screw  and  the  cutting  jaws,  to 
better  adapt  it  to  the  work  in  ><>il. 
Generally  a  bottle  of  muriatic  acid 
for  detecting  carbonates,  and  strips 
of  sensitive  litmus  paper  of  red  and 
blue  for  testing  for  soil  acidity,  are 
useful  adjuncts  to  the  equipment.  In 
arid  regions  where  important  quanti- 
ties of  alkali  are  met  with,  the  field 
man  should  be  supplied  with  a  modi- 
fied Wheatstone  bridge  and  chemical 
equipment  necessary  for  the  detection 
and  measurement  of  the  important 
salt  constituents.1  A  geologist's  ham- 
mer for  examining  soil  and  rock  should 
be  added,  together  with  such  other 
minor  equipment  as  may  increase  the 
convenience  and  efficiency  of  the  work. 
A  substantial  field  book  should  be 
provided,  for  notes  on  the  character 
of  soil  types  and  other  observations 
and  data,  and  for  records  of  borings 
and  samples.  The  notes  should  be 
carefully  classified.  Muslin  bags  of 
about  one  quart  capacity  should  be  used  for  collecting 
and  shipping  the  samples  to  the  laboratory  for  mechani- 
cal   analysis.      Where    the   natural  field    structure    and 

x  Davis,  R.  0.  E.,  and  Bryan,  H.  The  Electrical  Bridge 
for  the  Determination  of  Soluble  Salts  in  Soils.  U.  S.  D.  A., 
Bur.  Soils,  Bui.  61.     1910. 


Fig.  83— Auger  used 
in  the  examination 
of  soils.  (A), 
handle;  (B),  joint; 
(C) ,  worm  with 
modified  cutting 
edge. 


THE  SOIL  SURVEY  729 

moisture  conditions  of  the  sample  are  to  be  preserved, 
wide-mouth,  sealed-top,  metal  or  glass  containers  should 
be  used.  Aluminum  cans  are  usually  most  suitable,  as 
they  are  not  corroded  by  the  sample. 

627.  Procedure  in  the  field.  —  The  area  for  survey 
having  been  selected,  the  field  party  —  which  usually 
consists  of  two  men,  a  chief  and  an  assistant  —  proceeds 
to  examine  the  soils  of  the  district.  Headquarters  are 
temporarily  established  in  a  convenient  village  or  coun- 
try residence,  and  excursions  are  made  into  the  adjacent 
territory.  The  routes  are  laid  out  carefully  and  system- 
atically with  the  purpose  of  examining  the  soils  of  the 
entire  area.  The  party  proceeds  along  the  highway, 
with  frequent  stops  and  side  excursions  into  the  field, 
examining  the  soil  to  a  depth  of  three  or  more  feet  with 
the  auger.  In  humid  regions  the  basis  of  the  soil  classi- 
fication is  a  section  of  soil  three  feet  deep.  In  arid 
regions,  where  alkali  is  prevalent,  a  six-foot  section  is  usu- 
ally the  basis  of  classification,  and  occasionally  much 
deeper  examinations  are  made  for  studying  the  position  of 
the  water  table.  The  soil  is  examined  especially  with  refer- 
ence to  its  texture,  structure,  color,  drainage,  content  of 
organic  matter,  depth  of  different  strata,  and  special  chemi- 
cal properties  such  as  lime  and  alkali.  The  natural  vege- 
tation is  observed,  and  note  is  taken  of  the  type  and 
growth  of  crops  as  well  as  the  extent  and  species  of  forest 
trees. 

Borings  and  other  observations  are  made  from  point 
to  point  as  the  appearance  of  the  soil,  the  topography, 
the  conformation  of  the  country,  and  the  character  of  the 
vegetation  may  suggest.  The  frequency  and  position  of 
observations  are  determined  entirely  by  the  judgment  of 
the  field  man.     They  may  be  made  every  few  rods  or  at 


730       SOILS:    PROPERTIES  AND  MANAGEMENT 

much  wider  intervals.  In  getting  acquainted  with  new 
types,  more  borings  and  detailed  observations  arc  n<  < 
sary  than  after  the  soil  properties  have  become  familiar 
and  can  be  more  readily  identified.  Where  the  soil  is 
highly  variable,  much  more  frequent  observations  are 
necessary  than  where  it  is  more  uniform.  As  the  survey 
proceeds  the  field  man  progresses  from  point  to  point, 
along  the  highway  and  in  the  field,  on  foot  or  by  convey- 
ance as  may  be  more  convenient,  extending  his  observa- 
tions about  half  the  distance  to  the  next  highway  in  order 
that  all  the  territory  may  be  covered  most  conveniently. 
Usually  the  trip  is  arranged  in  a  circuit.  All  areas  of  soil 
essentially  alike  in  their  properties  and  plant  relations  are 
recognized  as  of  the  same  soil  type,  and  their  position  on 
the  map  is  represented  by  one  of  the  colors.  As  the 
observations  proceed,  a  change  in  the  character  of  the  soil 
may  occur.  When  this  change  becomes  of  such  char- 
acter and  importance  as  to  cause  difference  in  agricultural 
relations  and  to  be  recognizable  under  the  plan  of  classi- 
fication outlined  above,  a  new  type  is  recognized.  The 
boundary  line  between  the  two  types  must  be  carefully 
traced  out  by  observation  and  by  borings.  As  the  work 
proceeds  ^ther  types  of  soil  may  be  recognized  and  the 
boundaries  are  determined  and  represented  on  the  map, 
each  type  being  indicated  by  a  particular  color  or  symbol. 
A  large  number  of  types  of  soil  may  be  recognized  in  each 
area  surveyed.  The  character  and  relationships  of  these 
must  be  studied  carefully  in  order  to  decide  how  they 
may  be  grouped  in  series  and  larger  units. 

In  practice  it  is  usually  better  for  the  field  party  to 
first  make  general  observations  over  the  area,  in  order 
to  recognize  the  main  divisions  of  the  soil  that  may  later 
require  subdivision  into  types.     To  this  end  all  available 


THE  SOIL   SURVEY  731 

facts,  particularly  concerning  the  geology  of  the  region, 
should  be  familiar  to  the  survey  man  before  active  field 
work  is  begun.  It  is  easier  and  results  in  a  greater  degree 
of  accuracy  to  first  recognize  the  larger  divisions  of  an 
area  of  soil,  and  later  work  out  the  types,  than  to  be  con- 
cerned from  the  very  beginning  entirely  with  these  ele- 
mental subdivisions. 

During  the  progress  of  the  field  observation  the  rela- 
tionship of  each  type  of  soil  to  natural  and  cultivated 
plants  should  be  studied,  and  the  tillage  properties  of  the 
soil  noted.  The  farmers  also  may  be  interviewed  con- 
cerning their  soils,  as  to  tillage  properties,  crop  relations, 
and  response  to  methods  of  improvement.  In  short,  all 
available  data  concerning  the  character  of  the  soils  of  the 
region  should  be  sought. 

Records  are  made  in  the  field  notebook  descriptive  of 
the  average  character  of  each  type  of  soil.  The  descrip- 
tion of  typical  borings  may  be  recorded  and  their  location 
noted  on  the  map.  Preliminary  samples  may  be  taken 
and  sent  to  the  central  laboratory  for  physical  or  chemical 
examination,  to  check  the  judgment  of  the  field  man. 

628.  Collection  of  soil  samples.  —  Samples  of  soil  for 
laboratory  examination  should  be  taken  only  after  the 
field  man  is  thoroughly  familiar  with  each  type  of  soil 
and  can  select  a  location  that  accurately  represents  the 
average  material  of  the  type.  Attention  should  be  given 
to  the  slope,  drainage,  abnormal  modifications,  and 
manurial  treatment  of  the  soil  at  that  point.  Therefore, 
in  survey  work  samples  are  collected  only  in  the  latter 
part  of  the  season.  One  or  more  samples  of  each  im- 
portant type  of  soil  are  taken.  The  material,  to  the 
amount  of  a  quart,  is  preserved  in  cloth  bags.  Usually 
each  sample  is  divided  into  two  parts,  one  representing 


732       SOILS:    PROPERTIES   AND  MAN AGEME\  ! 

the  soil  and  the  other  the  subsoil.     If  there  is  a  marked 

change  in  appearance  or  texture  in  the  subsoil,  other  divi- 
sions of  the  sample  may  be  made.  Usually  a  composite 
of  a  number  of  borings  over  an  area  of  several  squ 
rods,  or  even  of  several  acres,  may  be  necessary  in  order 
to  secure  an  accurate  sample  and  to  obtain  enough  ma- 
terial. A  composite  of  several  representative  borings 
made  over  a  considerable  area  gives  a  more  nearly  accu- 
rate sample  than  is  possible  in  a  single  boring.  The 
possibility  of  local  variations  is  very  great,  and  their 
effect  is  reduced  when  composite  sampling  is  done. 

Each  bag  should  bear  a  tag  which  is  given  a  number 
and  on  which  is  placed  the  name  of  the  type,  the  location 
of  the  sample  in  the  section,  and  a  brief  description  of  the 
material.  The  same  data  are  recorded  in  the  field  note- 
book, which  is  finally  preserved  as  a  part  of  the  permanent 
office  record  of  the  survey.  The  description  in  the  note- 
book may  be  amplified  more  than  is  possible  on  the  tag. 
The  location  where  each  sample  was  taken  should  be 
accurately  marked  on  the  field  map  by  a  number  corre- 
sponding to  the  number  of  the  sample.  Usually  each 
sample  is  given  a  number,  and  the  parts  are  indicated  by 
a  letter,  proceeding  from  the  surface  downward.  Where 
the  material  is  very  wet  and  likely  to  become  lumpy 
when  dry,  it  may  be  dried  in  a  thin  layer  before  being 
finally  bagged  for  shipping  or  preservation.  Care  should 
be  bestowed  on  every  part  of  the  operation  of  collection, 
describing,  numbering,  tagging,  tying,  and  shipping,  in 
order  to  insure  accuracy  and  permanency  of  the  record. 

The  soil  auger  is  generally  used  in  taking  the  sample 
and  in  examining  the  soil  section.  The  worm  of  the 
auger  is  bored  into  the  soil  until  it  is  filled.  It  is  then 
withdrawn  and  the  soil  is  removed.     The  soil  may  be 


THE  SOIL    SURVEV  733 

collected  on  one  or  more  squares  of  oilcloth,  or  it  may  be 
placed  directly  in  the  appropriate  bags.  The  worm  of  the 
auger  having  been  cleaned,  it  is  inserted  into  the  same 
hole  and  advanced  until  it  is  again  full,  when  it  is  with- 
drawn and  cleaned  as  before.  This  operation  is  repeated 
until  the  desired  depth  is  reached.  Where  the  soil  is  a 
very  heavy  clay,  it  may  be  advisable  to  only  partially 
fill  the  worm  with  soil.  Where  the  soil  is  very  dry  and 
pulverizes  to  a  dust,  it  may  slip  off  the  worm,  in  which 
case  water  may  be  added  to  make  it  adhere.  The  upper 
part  of  the  hole  should  be  cleaned,  and  -it  may  be  slightly 
enlarged  so  as  to  prevent  contamination  with  the  material 
from  the  lower  part  of  the  section.  Where  there  is  rubbish 
on  the  surface,  this  should  be  removed  previous  to  begin- 
ning the  collection  of  the  sample. 

In  very  stony  soil  the  auger  is  not  suited  to  taking  a 
sample,  either  for  examination  or  for  record.  In  such 
soil  a  shovel  may  be  used,  or  the  sample  may  be  taken  in 
a  road  or  some  other  cut  by  means  of  a  geologist's  hammer. 

The  face  of  the  section  should  be  removed  to  a  depth 
of  several  inches,  in  order  to  eliminate  weathered  or  con- 
taminated material  which  may  not  be  typical  of  the  soil 
section.  Usually  a  difference  in  color  and  physical 
properties  of  the  soil  indicates  a  modification  of  the  typi- 
cal material. 

629.  The  accuracy  and  detail  of  the  soil  survey.  —  The 
accuracy  and  detail  of  the  soil  survey  depend  on  many 
things.  Assuming  an  adequate  preparation  on  the  part 
of  the  field  man,  there  are  limitations  in  accuracy  imposed 
by  the  scale  on  which  the  map  is  made  and  the  nature  of 
the  soil.  The  smaller  the  scale  of  the  map  used  in  the 
field,  the  less  is  the  detail  that  may  be  represented.  The 
commonest  scale  employed  is  one  inch  to  a  mile.    Some 


734       SOILS:    PROPERTIES  AND   MANAGEMENT 

states  use  a  larger  scale,  and  in  reoomioissance  survi 

a  smaller  scale  is  used.     While   a    large   scale    incres 
the  detail  that  may  be  represented,    it   also   multiplies 
the   difficulties    of   making    an     accurate     classification 
because    it   increases   the   number   of    properties   to   be 
observed. 

The  nature  and  occurrence  of  soils  in  the  field  involves 
more  variations  than  can  be  shown  on  the  map.  The 
boundaries  of  soil  types  grade  into  one  another,  and  it 
may  not  be  possible  to  mark  the  division  within  several 
rods.  Sometimes  even  a  wider  range  occurs.  The  accu- 
racy with  which  the  boundary  may  be  determined  and 
drawn  depends  very  much  on  the  way  in  which  the  two 
adjacent  soils  have  been  formed.  If  they  are  very  dif- 
ferent, the  boundary  may  be  very  distinct.  Some  types 
of  soil  are  characterized  by  local  variations  in  sections, 
or  from  point  to  point,  which  are  on  too  small  a  scale  to 
be  recognized  as  a  type.  Variations  may  be  induced  in  a 
type  due  to  differences  in  topography,  drainage,  or  culti- 
vation. Where  the  properties  do  not  bring  about  an 
important  change  in  the  crop  relations  of  the  soil,  they 
may  be  ignored.  Differences  due  to  cultivation  are 
generally  disregarded.  The  soil  survey  is  made  to  cover 
a  period  of  years,  and  only  permanent  differences  should 
be  considered. 

Variations  in  the  soil  must  be  considered  in  relation  to 
the  scale  of  the  map.  On  a  scale  of  one  inch  to  .a  mile 
the  minimum  area  that  can  be  shown  is  about  ten  acres. 
Occasionally,  where  the  difference  in  type  constitutes  a 
striking  contrast,  the  small  area  may  be  somewhat  ex- 
aggerated in  size.  An  area  of  muck  soil  having  high 
value  for  the  production  of  truck  crops  might  be  such  an 
exception. 


THE  SOIL   SURVEY  735 

630.  The  soil  survey  report.  —  The  soil  survey  report 
consists  of  two  parts,  the  printed  report  and  the  map 
showing  the  distribution  of  the  soil  types.  The  printed 
report  accompanying  the  soil  map  should  be  a  brief  but 
comprehensive  summary  of  the  observations  of  the  field 
party  in  the  areas  surveyed.  It  should  cover  six  types  of 
information:  (1)  location  and  boundaries  of  the  area; 
(2)  general  physical  features;  (3)  climate;  (4)  agricul- 
tural history  and  development;  (5)  description  of  the 
soils;  (6)  suggestions  for  improvement  in  the  manage- 
ment of  the  soil  that  may  have  been  determined  by  the 
survey. 

The  description  should  point  out  the  salient  topographic 
forms,  the  range  in  elevation,  the  nature  and  development 
of  the  drainage,  the  transportation  facilities,  and  the  dis- 
tribution of  population  and  of  farm  areas.  The  discus- 
sion of  climate  should  note  the  monthly  mean  tempera- 
ture and  amount  of  precipitation ;  the  character  of  the 
extreme  ranges  in  these ;  the  direction  of  prevailing  winds ; 
and  the  occurrence  of  any  special  features,  such  as  untimely 
frosts,  sleet  and  hail  and  windstorms,  and  the  nature  of 
local  variations  in  climate  that  may  be  due  to  the  prox- 
imity of  bodies  of  water  or  topographic  features.  The 
agricultural  history  should  note  the  source  and  character 
of  the  agricultural  population,  the  chief  products  and  any 
changes  that  have  occurred  in  their  production,  and  the 
present  status  of  the  area. 

The  description  of  the  soils  should  be  in  two  parts. 
First,  the  grouping  of  the  types  into  series  and  larger 
divisions,  with  the  geological  and  topographic  relations 
of  these  groups  and  a  clear  statement  of  the  characteristic 
properties  of  each  group.  Any  important  characteris- 
tics that  are  common  to  two  or  more  types  or  series,  such 


786      SOILS:    PROPERTIES  AND  MANAGEMENT 

as  a  deficiency  in  humus,  lime,  or  drainage,  should  be 
pointed  out.  Secondly,  a  detailed  description  of  each 
type  following  a  uniform  outline  of  properties,  including 
color,  texture,  depth,  structural  peculiarities,  and  mines* 
logical  and  chemical  features.  Following  this,  Attention 
should  be  drawn  to  the  location  and  extent  of  the  tvpc 
in  the  area,  and  to  its  mode  of  origin,  drainage  condition-. 
and  economic  relations,  including  the  crop  rotation  and 
extent  of  development. 

In  making  suggestions  for  the  treatment  of  the  soils  a 
clear  distinction  should  be  drawn  between  methods  of 
soil  management  and  improvement,  and  questions  of  farm 
organization  and  management.  The  data  collected  by 
the  soil  survey  man  will  usually  lead  him  to  confine  hk 
suggestions  to  the  former  group. 

631.  The  soil  map  (Fig.  84).  — The  soil  map  is  de- 
signed primarily  to  show  the  geographic  position  and  ex- 
tent of  each  type  of  soil.  Therefore  an  accurate  base  may), 
showing  important  natural  and  cultural  features  as  noted 
above,  is  essential.  The  scale  of  the  map  must  be  adapted 
to  the  amount  of  detail  to  be  shown.  The  comma 
scale  in  use  in  the  United  States  is  one  inch  to  a  mile.  In 
reconnoissance  surveys  a  scale  of  one  inch  to  six  miles  is 
usually  employed.  The  map  is  printed  in  colors  or  in 
symbols  representing  the  different  types  of  soil.  Symbols 
may  be  added  to  the  color  to  indicate  further  variation, 
such  as  the  presence  of  much  stone,  occurrence  of  ledge 
rock,  or  a  swampy  condition.  On  the  right-hand  border 
of  the  map  a  legend  to  the  colors  or  symbols  is  given,  and 
they  may  be  arranged  in  accordance  with  the  scheme  of 
classifying  the  soils  to  show  their  relationship.  On  the 
left-hand  border,  the  character  of  the  profile  of  each  type 
of  soil  is  indicated  by  a  series  of  legends. 


Bui.  60,  Bureau  of  Soils,  U.  S.  Dept.  of  Agriculture 


PLATE  1 


Volusia  Volusia  Dunkirk  Huntington         Dunkirk  Muck 

silt  loam  loam  gravelly  loam  loam  ciay 

Fig.  84.  —  Part  of  the  Madison  County,  N.  Y.,  soil  map  showing  the  topography 


THE  SOIL   SURVEY  737 

632.  The  extent  of  soil  surveys  in  the  United  States.  — 
The  detailed  survey  and  mapping  of  soils  by  the  Bureau 
of  Soils  of  the  United  States  Department  of  Agricul- 
ture, according  to  the  scheme  outlined  above,  has  been  in 
progress  since  1899.  On  January  1,  1915,  about  330,000 
square  miles  had  been  covered  by  detailed  surveys  and 
435,000  square  miles  had  been  covered  by  reconnoissance 
surveys.  In  addition,  several  miscellaneous  surveys  have 
been  made  in  outlying  provinces  such  as  Porto  Rico, 
Panama,  Philippine  Islands,  and  Alaska.  The  total  num- 
ber of  soil  types  and  series  recognized  is  approximately 
2000  and  600,  respectively. 

633.  Surveys  by  state  institutions.  —  Several  states 
are  engaged  in  soil  survey  work,  either  independently  or 
in  cooperation  with  the  United  States  Bureau  of  Soils. 
The  states  that  have  undertaken  this  work  independently 
have  carried  it  out  in  the  same  general  manner  as  in  the 
Federal  survey.  Some  of  the  states  that  are  working 
independently  have  confined  their  investigations  to  recon- 
noissance surveys  on  a  large  scale.  Tennessee  has  pub- 
lished a  general  report,  with  a  map,  on  the  soil  areas  of 
the  state,  with  special  reference  to  their  geological  rela- 
tions. Account  is  also  taken  of  the  texture,  chemical 
composition,  and  other  properties.  Iowa,  Missouri,  and 
Illinois  and  Ohio l  have  published  similar  general  reports 
showing  soil  areas  based  chiefly  on  origin.  Illinois,  In- 
diana, and  New  Jersey  have  also  published  detailed  reports 
on  particular  areas.  In  their  work  the  principles  of  clas- 
sification laid  down  above  have  been  followed  in  a  general 
way,  but  with  emphasis  on  certain  selected  properties. 

1  Coffey,  C.  N.,  and  Rice,  T.  D.     Reconnoissance  Soil  Survey 
of  Ohio.     U.  S.  D.  A.,  Bur.  Soils,  Field  Op.,  p.  134.     1912. 
3b 


738       SOILS:    PROPERTIES  AND  MANAGEMENT 

Illinois  has  given  special  prominence  to  color,  and,  in  ad- 
dition to  the  general  description  of  the  soil  types,  includes 
data  derived  from  chemical  analyses  to  show  the  itore  o! 
plant-food  in  the  surface  layers.  The  Indiana  and  Mis- 
souri surveys  have  combined  a  purely  geological  scheme  of 
classification  on  the  basis  of  origin,  with  certain  properties 
of  practical  importance,  such  as  texture,  color,  and  content 
of  humus,  but  without  observing  a  systematic  order. 
The  New  Jersey  survey  includes  rather  full  data  <>n  the 
chemical  composition  of  the  soil  types,  in  addition  to  the 
usual  discussion  of  their  properties  and  relationships. 

634.  Surveys  in  other  countries.  —  Several  countries 
have  undertaken  some  type  of  soil  or  agrogeological  sur- 
vev.  These  surveys,  which  have  been  undertaken  in 
Germany,  France,  Italy,  Russia,  Great  Britain,  and  Japan, 
have  aimed  at  a  broad  practical  classification  of  soils 
based  on  their  agricultural  values  and  tillage  properties. 
Several  thousand  square  miles  have  been  covered  by  the 
surveys  in  each  of  these  countries.  Colored  charts  are 
published  to  accompany  the  descriptive  reports.  In  these 
surveys  the  classification  is  largely  genetic,  in  combina- 
tion with  a  consideration  of  the  more  evident  physical 
and  chemical  properties,  which  are  recognized  and  grouped 
in  the  field  in  much  the  same  manner  as  in  the  American 
surveys.  The  details,  of  course,  are  considerably  differ- 
ent in  the  reports  of  the  different  countries.  In  Germany 
the  maps  are  geological-agronomic  in  character;  that  is, 
prominence  is  given  to  both  the  geological  and  the  crop 
relations  of  the  soils.  Their  physical  and  chemical  proper- 
ties are  pointed  out  and  are  used  in  the  classification. 
Similar  methods  are  followed  in  France  and  Japan. 

In  England  the  areas  of  soil  are  determined,  first,  by 
means  of  their  texture ;   secondly,  by  means  of  their  con- 


THE  SOIL   SURVEY  739 

tent  of  humus  and  lime  carbonate,  with  which  color  and 
drainage  are  associated;  and  thirdly,  by  means  of  the 
geological  formation  and  mode  of  origin.  Fairly  com- 
plete mechanical  and  chemical  analyses  of  representative 
samples  of  the  important  types  are  included,  and  the  rela- 
tion of  the  soils  to  crops  and  farm  practice  is  discussed  at 
some  length.  The  grouping  of  the  types  into  series, 
groups,  provinces,  and  the  like,  is  not  so  distinct  as  in  the 
American  surveys.  That  the  fundamental  importance 
of  the  larger  factors  in  classification  are  recognized  is 
shown  in  the  discussion  of  the  relation  of  precipitation 
and  temperature  to  the  properties  and  agricultural  uses 
of  the  soil,  in  which  the  controlling  influence  of  these 
over  large  areas  is  pointed  out. 

635.  Uses  of  the  soil  survey.  —  The  soil  survey  is 
useful  in  many  ways,  but  it  is  not  a  final  investigation. 
It  is  to  be  regarded  rather  as  a  means  of  determining  the 
status  of  the  soil  and  related  conditions  in  the  field. 
These  may  throw  light  on  many  farm  practices  and  lead 
to  their  improvement.  More  frequently  the  soil  survey 
points  to  lines  of  further  investigation  that  should  be 
carried  out. 

The  uses  of  the  soil  survey  may  be  conveniently  divided 
into  two  groups  —  its  use  to  the  individual,  and  its  use 
to  the  state.  For  the  individual,  the  soil  survey  (1)  points 
out  the  character  and  location  of  the  several  types  of  soil 
on  his  farm  which  may  be  correlated  with  particular 
crops  and  farm  practices ;  (2)  shows  him  the  relationship 
of  soils  over  wide  areas,  which  may  form  a  basis  for  the 
adoption  of  new  crops  or  new  methods  of  soil  manage- 
ment; (3)  provides  a  reliable  central  source  of  informa- 
tion concerning  soil  conditions ;  (4)  standardizes  methods 
of  description  and  representation  of  soils ;    (5)  reveals  in 


740       SOILS:    PROPERTIES   AND  MANAGEMENT 

many  cases  important  problems  of  soil  improvement  that 
need  attention;    (6)  affords  a  guide  in  the  exchange  of 

real  estate  and  in  the  selection  of  land  for  particular  pur- 
poses. For  the  state  the  soil  survey  (1)  shows  its  soil 
resources;  (2)  by  the  collection  of  this  data  at  a  central 
point,  affords  the  basis  for  the  correlation  of  all  other 
types  of  information,  the  character  of  which  is  affected 
by  the  soil  relations;  (3)  shows  in  main  cases  the  occur- 
rence and  importance  of  large  questions  of  soil  improve- 
ment, and  may  point  out  the  need  for  further  investiga- 
tions; (4)  gives  a  basis  on  which  much  of  the  results  of 
experiments,  investigations,  and  observations  on  soil  im- 
provements, crop  growth,  and  in  many  cases  farm  man- 
agement, should  be  applied ;  (5)  is  a  means  of  commu- 
nication and  mutual  understanding  between  the  state 
institutions  concerned  with  agricultural  information  and 
the  individual  farmer;  ((>)  by  affording  a  basis  of  facts, 
promotes  sound  commercial,  social,  and  governmental 
development. 

The  soil  survey  is  essentially  an  inventory  of  the  re- 
sources in  land  and  closely  allied  interests.  It  helps  the 
farmer  to  understand  the  situation  of  his  farm  and  its 
relations  to  other  farms.  It  helps  the  state  to  get  ac- 
quainted with  its  domain,  and  promotes  a  better  sense  of 
mutual  understanding  and  helpfulness.  The  soil  survey 
in  some  form  is  an  essential  step  in  sound  community 
building,  for  the  success  of  most  interests  —  commercial, 
social,  and  institutional  —  rests  ultimately,  to  a  large 
extent,  on  the  character  and  value  of  the  soil 


AUTHORS'    INDEX 


Adams,  G.  E.,  Salt  as  fertilizer,  544. 
Age  ton,  C.  U.*,  Lime    and    magnesium, 

538. 
Aikman,  C.  M.,  Composition  of  manure, 
595. 
Production  of  manure,  597. 
Alway,   F.   G.f   Composition  of  humus, 

148. 
Ames,  J.  W.,  Lime  in  soil,  378. 

Soil  investigation,  69,  70. 
Ammon,  C,  Hygroscopicity,  203. 
Appiani,  G.,  Silt  cylinder,  91. 
Ashley,  H.  E.,  Colloidal  clay,  161. 
Estimation  of  colloids,  167. 
Plasticity,  172. 
Atkinson,    A.,    Storage    of    moisture    in 

soil,  715. 
Atterberg,  A.,  Classification  of  soil  par- 
ticles, 96. 
Cohesion  test,  176. 
Measurement  of  cohesion,  181. 
Mechanische  Bodenanalyse,  84. 
Plasticity,  171. 
Silt  cylinder,  91. 

Baker,  M.  N.,  Sewage  irrigation,  711. 

Bancroft,   W.   D.,   Colloidal    chemistry, 
153. 

Barakov,  F.,  Carbon  dioxide  in  soil,  409. 

Baumann,   A.,   Composition  of   humus, 
133. 

Beal,  W.  H.,    Absorptive    capacity    of 
litter,  603. 
Handling  manure,  602. 

Bennett,    H.    H.,   Classification  of   soils 
in  U.  S.,  724. 

Bertrand,  G.,.  Manganese,  531. 

Biltz,  W.,  Soil  solution,  345. 

Bizzell,  J.  A.,  Composition  of  drainage 
water,  372. 
Legume  cultures,  545. 

Blanck,  E.,  Law  of  minimum,  554. 

Bolly,  H.  L.,  Disease-producing  organ- 
ism, 426. 

Boullanger,  E.,  Sulfur,  524. 


Boussingault,  J.  B.,  Snow  and  tempera- 
ture, 306. 
Bouyoucos,  G.  J.,  Bibliography  of  soil 
heat,  289. 
Manure  and  soil  temperature,  316. 
Radiation,  302,  304. 
Soil  temperature,  301. 
Specific  heat  of  soil,  295. 
Texture  and  conductivity,  309,  312. 
Bowie,  A.,  Practical  irrigation,  682. 
Breazeale,  J.  F.,  Acid  toxicity,  379. 

Estimation  of  organic  matter,  143. 
Brenchley,    W.    E.,    Soil    solution    and 

plant  growth,  347. 
Briggs,  L.  J.,  Analysis  of  soil,  143. 
Capillary  movement,  225. 
Classification  of  particles,  96. 
Hygroscopic     and    capillary    water, 

208. 
Hygroscopic  moisture,  206. 
Mechanical  soil  analysis,  84. 
Moisture  equivalent,  220. 
Soil  solution  in  situ,  343. 
Water  requirements  of  plants,    245, 

712. 
Wilting  point,  258. 
Britton,   W.    E.,   Availability    of    ferti- 
lizers, 510. 
Bronet,    G.,    Penetration    of    fertilizers, 

354. 
Brown,  B.  E.,  Carbonized  materials,  141, 
144. 
Fertilizers  and  acidity,  381. 
Brown,  C.  F.,  Alkali  land  reclamation, 

399. 
Brown,  C.  W.,  Bacteria  in  soil,  439. 
Brown,  P.  E.,  Bacteria  in  soil,  432. 
Bryan,  H.,  Soil  analysis,  93. 
Buckingham,   E.,   Capillary  movement, 
232. 
Capillary  water,  217. 
Diffusion  of  gases  in  soil,  483. 
Movement   of   water    vapor   in   soil, 

241. 
Natural  mulching,  277. 


741 


742 


AUTHORS    INDEX 


Buckman,  H.  O.,  Formation  of  residual 
clay,  25. 
Moisture  control,  379. 
Storage  of  soil  moisture,  71"). 
Buddin,  \Y\,  Partial   sterilization  of  soil, 

471. 
Burmester,  H.,  Temperature  of  soil,  821. 
Burr,  W.  W.,  Storage  of  soil  moist  ure, 
715. 

Caldwell,  J.  S.,  Wilting  point,  258. 
Cameron,  F.  K.,  Estimation  of  organic 
matter,  141*. 

Litmus  paper  test,  386. 

Physical  condition  of  soil,  181. 

Soil  solution,  346. 

Solubility  of  phosphate.-. 

Test  for  cohesion,  176. 
Campbell,  B.  W„  Dry-farming.  71  J. 
Carpenter,     L.     C,     Measurement     of 

\v;it«T,  707. 
Cates,  J.  S.,  Mulch  and  moisture,  280. 
Cato,  M.  P.,  Land  drainage,  629. 
Chamberlin,  T.  C,  Drift  less  area,  67. 
Chamberlain,  C.  W.,  Molecular  attrae- 

tion,  206. 
Chester,  F.  D.,  Bacteria  in  soil,  432. 
Chilcott,  E.  C,  Dry-farming,  712. 
Christie,  G.  I.,  Soil  survey  of  Iowa,  718. 
Clark,  F.  W.,  Data  of  geochemistry,  5, 
24,  72. 
Composition  of  loess,  59. 
Coffey,  C.  N.,  Reconnoissance  survey  of 
Ohio,  737. 
Classification  of  soils,  718. 
Conn,  H.  J.,  Bacteria  in  soil,  431. 
Coppenrath,  E.,  Catalytic  agents,  529. 
Coville,  F.  W.,  Acid-tolerating   plants, 

384. 
Coville,  J.  V.,  Acids  in  plants,  379. 
Cox,  H.  R.,  Mulch  and  moisture,  280. 
Craig,  C.  E.,  Weeds  and  crop  growth, 

281. 
Crosby,  W.  O.,  Colors  of  soils,  76. 
Cushman,  A.  S.,  Air  elutriator,  86. 
Colloids,  161. 
Plasticity,  172. 
Czapek,  J.,  Solvent  action  of  roots,  406. 
Czermak,   W.,    Hygroscopic   coefficient, 
190. 

Darwin,  C,  Earthworms,  19,  422. 
Daubree,  A.,  Solubility  of  orthoclase,  24. 
Davis,  N.  B.,  Plasticity,  172. 
Davis,  R.  O.  E.,  Electrical  bridge,  728. 
Salts  and  capillarity,  229. 


Deherain,   P.   P.,  Lime  and  photj 
D.nuloii,  A  ,  Sulphur,  '■: 

Penetration  <>f  fertilizer 

Digby,    K   .    Miuenil   manure,  490. 
Diller,  Q.  S.,   Composition   ..f    li: 
soil,  66. 
Residual  alt 

Dobeneck,  A    I  ration,  897. 

lfygroseopicitv 
Dorsch,  F.,  Availability  of  fertilize 
I)....  ('    \\ "..  Alkali  lan<l  reelamat in  ., 
399. 

Composition  of  alkali,  898. 
Duchaoek,  !■'..  Paotoria  io  nfl, 

Fermentation  and  phosphates,  520. 
Dugardin,  M  .  Bntftti 

i  >upre.  M     A.,  Capillary  innveii,. 
Dyer,    B  ,    Citric    acid    method    soil    an- 
alysis. 886. 
Plant  food  in  Broadbalk  Bold,  .'W7 

Ebermayer,  E  ,  Temperature  of  soil,  322. 
Ehrenbers,  P.,  Effect  of  Hoi!  ■ntionptioi, 

I7(i 

Estimation  of  colloids,  168. 
l'.ichhoni,  EL,  Absorption  by  chabazite, 

356. 
Elliott,  C.  G.,  I^nd  drainage,  6:27,  839. 
659. 
Siae  of  tile.  681. 
Ernest,  A.,  Carbon  dioxide  production, 

139,  408. 
Etcheverry,   B.   A.,   Linings  for  canals, 


Failyer,  G.  H.,  Absorption  by  soils,  351. 

Composition  of  soil,  70. 

Minerals  in  soil  separates,  101. 
Faure,  L.,  Land  drainage,  627. 
Feilitzen,  H.  von,  Sulfur,  525. 
Fippin,  E.  O.,  Causes  for  granulation,  188. 

Classification  of  soils,  718. 
Fletcher,  C.  C,  Soil  analysis,  93. 
Flugel,  M.,  Law  of  minimum,  554. 
Forbes,  R.  H.,  Irrigation,  686. 
Fortier,    S.,    Application    of     irrigation 
water,  695. 

Mulches  under  irrigation,  705. 

Orchard  irrigation,  682. 

Small  water  supply,  682. 
Frank,    B.,    Effect   of   heat    on    soluble 

matter,  466. 
Fraps,  J.  S.,  Availability  of  phosphates, 
338. 

Fertilizers,  546. 


AUTHORS'    INDEX 


743 


Frear,  W.,  Losses  of  manure,  605. 
Fred,  E.   B.,   Effect  of  soil  antiseptics, 

468. 
French,  H.  F.,  Drainage,  629. 
Freundlich,  H.,  Kapillar  chemie,  153. 
Friedlander,  K.,  Soil  antiseptics,  470. 

Gaither,  E.  W.,  Lime  in  soil,  378. 

Soil  investigation,  69,  70. 
Gallagher,  F.  E.,  Absorption  of  gases, 
367. 
Cohesion  tests,  176. 
Physical  condition  of  soil,  181. 
Temperature      and      hygroscopicity, 
208. 
Gardner,  F.  D.,  Fertilizers  and  acidity, 

381. 
Gedroiz,  K.  K.,  Phosphate  fertilizer,  519. 
Geikie,  A.,  Geology,  text  of,  40. 
Georgeson,  C.  C.,  Manure  and  soil  tem- 
perature, 316. 
Gerlach,    U.,    Composition    of   drainage 
water,  350. 
Drainage  water,  370. 
__Gieseker,  L.  F.,  Storage  of  soil  moisture, 

715. 
-—Gilbert,  J.  H.,  Composition  of  drainago 
water,  240. 
Gile,  P.  P.,  Lime  and  magnesium,  538. 

Nitrogen  in  plant  nutrition,  491. 
Girard,  A.,  Soil  antiseptics,  465. 
Goddard,  L.  H.,  Cost  of  drainage,  655. 
Goessman,  C.  A.,  Poultry  manure,  589. 
Golding,  J.,  Partial  sterilization,  474. 
Grandeau,  L.,  Estimation  of  humus,  144. 
Greig-Smith,  R.,  Effect  of  partial  ster- 
ilization, 472. 

Haberlandt,  H.,  Cohesion  test,  175. 

Heat  and  germination,  290. 
Hall,  A.  D.,  Accumulation  of  soil  nitro- 
gen, 463. 

Classification  of  soil  particles,  96. 

Classification  of  soils,  718. 

Composition  of  crops,  418. 

Composition  of  drainage  water,  369. 

Composition  of  manure  gases,  594. 

Crop  adaptation  and  texture,  105. 

Denitrification,  457. 

Fermentation  of  manure,  591. 

Losses  of  manures,  600. 

Losses  of  nitrates,  454. 

Losses  of  nitrogen,  500. 

Lysimeter  records,  266. 

Manures,  579. 

Mechanical  analysis,  103. 


Hall,  A.  D.,  Nitrification,  453. 
Nitrogen  fertilizers,  547. 
Residual  effect  of  manures,  614. 
Soil  separates,  102. 
Soil  solution  and  plant  growth,  347. 
The  Soil,  11. 
Halligan,  J.  E.,  Composition  of  rotted 
manure,  596. 
Fertilizers,  546. 
Hart,   R.   A.,   Alkali  land  reclamation, 
399. 
Handling  manure,  602. 
Sulfur  as  a  fertilizer,  526. 
Hartwell,  B.  L.,  Acidity  test,  387. 

Fermentation  and  phosphates,  520. 
Hasenbaumer,  J.,  Catalytic  agents,  529. 

Colloidal  chemistry,  153. 
Hassler,  C,  Colloid  chemistry,  153. 
Headden,  W.  P.,  Nitrates  in  alkali   soil, 

292. 
Heinrich,    R.,    Hygroscopic    coefficient, 

257. 
Heinze,  B.,  Soil  antiseptics,  470. 
Hellriegel,    H.,    "Water    requirement    of 

plants,  246,  249,  250. 
Henneberg,  W.,  Absorbed  bases,  354. 

Absorption,  353. 
Henry,  W.  A.,  Production  of  manure,  588. 
Hess,  E.  H.,  Lime,  541. 
Hess,  R.  H.,  Social  aspect  of  irrigation, 

691. 
Hilgard,  E.  W.,  79,  82. 

Absorption  and  temperature,  367. 
Alkali  land  reclamation,  399. 
Churn  elutriator,  88. 
Composition  of  alkali,  393. 
Dilution  of  plant  food  in  soil,  332. 
Estimation  of  humus,  144,  147. 
Retentive  power  of  soil  for  water,  221. 
Roots  and  humus,  127. 
Soil  analysis,  96. 
Soil  and  climate,  62,  72. 
Soils  of  arid  and  humid  regions,  71. 
Temperature      and      hygroscopicity, 

208. 
Vegetation    and    soil    classification, 
720. 
Hills,  J.  L.,  Commercial  fertilizers,  562. 
Hiltner,  L.,  Effect  of  soil  antiseptics,  469. 
Hoffman,   C,   Fermentation   and    phos- 
phates, 520. 
Hopkins,  C.  G.,  Library  system   of  soil 
naming,  726. 
Manure  and  the  rotation,  616. 
Nitrogen  storage  by  legumes,  621. 
Soil  survey  of  Illinois,  719. 


u 


AUTHORS     INDBX 


Hoiiston,  H.  A.,  Estimation  of  huniun, 

142. 
Hunt,  T.  F.,  Manure  and  the  rotation, 
616. 
Nitrogen  fertilizer 
.  Hutchinson,   H.   B.,   Nitrogen  assimila- 
tion, 495. 
Organic  matter  in  soil,  188* 
Partial  sterilization  of  soil,  470. 

Jenkins,  E.  H.,  Availability  of  fertilizers, 
510. 
Commercial  fertilizers,  564. 
Jodidi,  EL  I-.,  Nitrogen  compounds.  138. 
Johnson,  B.  W.,  Availability  of  fert  P 

610.  y 

Composition  of  soil  air,  477. 
Johnston,  J.,  Early  drainage  in  America, 

630. 
Jones,  C.  H.,  Commercial  fertilizer,  .">f._\ 

Kellerman,  K.  F.,  lime,  887. 

Nitro-cultures,  462. 
Litmus  test,  386. 
Kellev,  W.  P.,  Ammonia  as  plant  food, 
488. 

Magnesium  and  nitrate*,  888, 
Kcllner,  ( )..  Ammonia  as  plant-food,  495. 
Kelly,  M.  P.,  Manganese,  530. 
Kerr,  W.  C,  Composition  of  mar! 
Kiesselbach,  T.  A.,   Water  requirement 
of  corn,  248. 
~~f]&\n%,   F.   H.,   Absorption   and   produc- 
tivity, 368. 

Aspirator,  124. 

Capillary  and  ground  water,  21  I. 

Capillary  movement,  225. 

Drainage  and  soil  temperature,  314. 

Drainage  and  free  water,  I'M. 

Effect  of  barometric  pressure,  234. 

Effective  diameter  of  soil  particles, 
123. 

Effective    surface    of   soil   particles, 
125. 

Irrigation,  682. 

Land  drainage,  627. 

Movement  of  ground  water,  235. 

Pore  space  in  soil,  116,  117. 

Slope  and  soil  temperature,  319. 

Spring  plowing,  284. 

Surface  tension  and  temperature,  227. 

Temperature      and      hygroscopicity, 
208. 

Water  requirement  of  plants,  247. 

Wind  breaks  and  moisture,  285. 
Kinnison,  C.  S.,  Plasticity,  171. 


Kinsl<  \ 

Klippart,  .1.  EL,  I. arid  dr 

Knisety,   \    I 

Koch,  A.,  Theory  as  to  - 

Konig,  .) 

Colloid  rliemi-r ,  .  .   1  ">  i. 

!..-fT,   If.   P.,   Roots  and  humus, 
127. 
Krdfw  I  ria  in  soil,    1 

Fermentation  and  phosphates,  520. 
KQmmel.     \      K  .    BoU     and    n< 
surveys,  719. 

Lang,  C,  Radiation 

l.apham,  J.  E.,  Classification  of  soils  of 

1'.. 
l.apham.    M     If  .   01— Ifteathffl  of  soils 

of  United  State-. 
Capillary  moven 
Lawes,  J.    B  ,   Composition   of   drainage 

Nitrogen  in  plant  nutrition,  491. 

Leather,  •'    W.,  Waier  reciuirements  of 

plant- 
LeClerc,  J.  A.,  Acid  toxicity,  379. 

Liebenberg,    R-   von,   Specific   heat  of 

soils,  294. 
l.iebig.  J.  J.  von.  Composition  of  plants, 
491. 
Law  of  minimum,  858* 
Solvent  action  of  root  - 
Lint,  H.  C,  Sulfur  and  soil  aotd 
I.ipman,  C.  B.,  Bacteria  in  arid  soils,  73. 
Lipman,  J.  G.,  Availability  of  fertilizers, 
510. 
Fermentation  of  manure,  591. 
Green  manures  818. 
Loew,  ().,  Lime  and  magnesia,  538. 
I.ohnis,  F.,  Bacteria  in  soil,  438. 
Loughridge,   R.   H.,   Crop   tolerance  to 
alkali,  395. 
Distribution   of   irrigation   water    in 

soil,  704. 
Hygroscopic  water,  204. 
Soil  separates,  102. 
Lynde,  C.  J.,  Capillary  movement,  231. 
Lyon,  T.   L.,   Composition  of  drainage 
water,  372. 
Legume  cultures,  545. 
Lysimeter  tanks,  241. 

McBride,  F.  W.,  Estimation  of  humus, 

142,  144. 
McCall,  A.  G.,  Solution  of  soil  in  situ, 

344. 


authors'  index 


745 


McCaughey,  W.  J.,  Color  of  soil,  77. 

Soil-forming  minerals,  100. 
MacDonald,  W.,  Dry-farming,  712. 
McLane,    J.    W.,   Moisture   equivalent, 
220. 

Soil  solution  in  situ,  343. 
McLaughlin,    W.    W.,    Capillary   move- 
ment, 224. 

Movement  of  irrigation  water,  704. 
Marbut,  C.  F.,  Classification  of  soils  of 
United  States,  718,  724. 

Soils  of  United  States,  34. 
Marchal,  E.,  Ammonification,  447. 
Mares,  M.  N.,  Sulfur,  525. 
-Mayer,  A.,  Optimum  moisture,  263. 
Mayo,  U.  S.,  Bacteria  in  soil,  432. 
Maze,  P.,  Law  of  minimum,  554. 
Mead,  E.,  Extent  of  irrigation,  685. 

Irrigation  institutions,  682. 

Legal  status  of  irrigation,  691. 

Lining  for  canals,  694. 

Preparation  for  irrigation,  682. 
Mellen,    C.    R.,    Drainage   of   Johnston 

farm,  630. 
Merrill,  G.  P.,  Color  of  soil,  77. 

Residual  clay,  granite,  27. 

Pock  weathering,  13,  26,  62. 

Weathering  of  gneiss,  66. 

Zeolitis,  357. 
Merrill,  L.  A.,  Irrigation  of  crops,  682. 
Merzbacher,  G.,  Origin  of  loess,  60. 
Miles,  M.,  Drainage  in  Europe,  629. 
Miller,  N.  H.  J.,  Nitrogen  assimilation, 
495. 

Organic  matter  in  soil,  136. 
Miner,  H.  L.,  Commercial  fertilizers,  562. 
Mitscherlich,     A.     E.,     Estimation     of 
colloids,  168. 

ir.vgroscopicity,  208. 

Law  of  Minimum,  553. 

Water  and  plant  growth,  253. 
Molisch,  H.,  Enzymes  of  roots,  407. 
Montemartini,  L.,  Catalysis,  530. 
Montgomery,    E.    G.,    Water    require- 
ments, 245,  248,  249. 

Water  requirement  of  corn,  251. 
Mooers,  G.  A.,  Soil  survey  of  Tennessee, 

719. 
Mulder,  T.  J.,  Organic  matter  of  soil,  131. 
Miiller,  R.,  Solubility  of  minerals,  24. 

Newell,  F.  H.,  Irrigation,  682. 
Niklas,  H.,  Colloid  chemistry,  153. 
Norton,  J.   H.,  Composition  of  surface 

water,  373. 
Noyes,  A.  A.,  Properties  of  colloids,  153. 


Oberlin,  C,  Soil  antiseptics,  465. 
Olin,  W.  H.,  American  irrigation,  682. 
Osborne,    T.     B.,    Beaker     method    of 
mechanical     analysis,     96. 
Classification  of  soil  particles,  90. 

Paddock,  W.,  Irrigation  of  fruit,  682. 
Pagnoul,    M.,   Effects   of   carbon   bisul- 
fide, 466. 
Parks,   J.,   Drainage  and  soil   tempera- 
ture, 314. 
Patten,  A.  J.,  Bacteria  in  soil,  439. 
Patten,  H.  E.,  Absorption  of  gases,  367. 
Absorption  by  soils,  351. 
Heat  of  condensation,  209. 
Heat  transfer,  309,  313. 
Specific  heat  of  soil,  295. 
Temperature      and      hygroscopicity, 
208. 
Patterson,  H.  J.,  Lime,  541. 
Peake,    W.    A.,    Estimation   of   organic 

matter,  143. 
Pember,  F.-  R.,  Fermentation  and  phos- 
phates, 520. 
Penny,  C.  L.,  Clover  as  green  manure, 
621. 
Green  manures,  619. 
Penrose,  R.  A.  F.,  Composition  of  resid- 
ual soil,  33. 
Peters,  E.,  Absorbed  bases,  354. 

Absorption  of  potassium,  350. 
Peterson,  W.   H.,  Sulphur  as  fertilizer, 

526. 
Petit,  A.,  Temperature  of  soil,  300. 
Pettit,    J.    H.,    Soil   survey   of    Illinois, 

719. 
Pfaundler,  L.,  Specific  heat  of  soil,  294. 
Pfeiffer,  T.,  Effect  of  soil  antiseptics,  470. 
Law  of  minimum,  554. 
Carbon  dioxide  as  soil  solvent,  409. 
Pick,  H.,  Estimation  of  colloids,  168. 
Pickel,   G.   M.,    Composition   of   muck. 

37. 
Piper,  C.  V.,  Green  manures,  619. 
Pitra,  J.,  Bacteria  in  soil,  437. 

Fermentation  and  phosphates,  520. 
Plummer,  J.  K.,  Acid  materials,  376. 
Potts,  E.,  Texture  and  conductivity,  309. 
Pranke,  E.  J.,  Cyanamid,  503. 
Prianischnikov,  D.,  Availability  of  phos- 
phorus, 518,  521,  535. 
Puchner,  H.,  Cohesion  test,  176. 

Composition  of  soil  separates,  102. 
Measurement  of  cohesion,  178. 
Pugh,  E.,   Nitrogen  in  plant  nutrition. 
491. 


746 


AUTHORS'   INDEX 


Rafter,  G.  W.,  Sewage  irrigation,  711. 
Ramann,  E.,  Carbon  dioxide  in  soil  air, 
480. 

Colloid  chemistry,  153. 
Reed,  H.  S.,  Oxidation  by  roots,  407. 

Toxic  material  in  soils,  136. 
Reid,  F.  R.,  Catalysis,  529. 
Rice,  T.  D.,  Reconnoissance,  737. 
Richthofen,  F.,  Character  of  loess,  59. 
Roberts,  I.  P.,  Losses  of  manun 

Production  of  manure,  587. 
Robinson,  F.  R.,  Lime,  537. 
Robinson,    F.    W.,    Root    nodules    and 

nitrogen  storage,  621. 
Robinson,  T.  R.,  Litmus  test,  386. 
Robinson,  W.  O.,  Color  of  soils,  77. 

Manganese,  531. 
Rodewald,    H.,   Estimation   of   colloids, 

168. 
Rostworowski,    S.,  Absorption   by    per- 

mutite,  867. 
Russell,  E.  J.,  Plant  food  elements,  3. 

Classification  of  particles,  96. 

Classification  of  soils,  718. 

Crop  adaptation  and  texture,  105. 

Mechanical  soil  analysis,  103. 

Nitrogen  fertilizers,  547. 

Partial  sterilization  of  soils,  470. 

Sewage  sick  soils,  474. 
Russell,  I.  C,  Subaenal  deposits,  62. 

Sachs,  J.,  Solvent  action  of  roots,  406. 
Sackett,  W.  CJ.,  Ainmonification,   146 

Availability  of  fertilizers,  511. 
Salisbury,  R.  D.,  Driftless  area,  07. 

Glacial  geology,  47. 
Sanborn,  J.  W.,  Roots  of  crops,  81. 
Sargent,  C.  L.,  Acidity  tests,  387. 
Saussure,  T.  de,  Composition  of  plants, 

490. 
Schantz,    H.    L.,    Water    requirements, 
245,  712. 

Wilting  point,  258. 
Schlicter,   C.   S.,   Effective  diameter  of 

particles,  123. 
Schone,  E.,  Elutriator,  87. 
Schreiner,  O.,  Absorption  by  soil,  351. 

Carbonized  material  in  soil,  141,  144. 

Composition  of  humus,  134,  136. 

Organic  substances  in  soil,  132. 

Oxidation  by  roots,  407. 
Schubler,  G.,  Cohesion  test,  175. 
Schucht,   F.,   Mechanische  Bodenunter- 

suchung,  84. 
Schulze,  B.,  Temperature  of  soil,  321. 
Schulze,  F.,  Water  extract  of  soil,  341. 


Schutt,  M    A.,  Losses  of  manure,  583 

599,600. 
Seelhorst,  C.  von,  Water  requir. 

261,  . 
Sheppard,  J.  H.,  Roots  of  crops,  82. 
Sherman,  ('.  W  ,  Boil  survey  of  Indiana, 

719. 

mine,  497. 
Organic  sol  D  soil,  132. 

Shutt.   F.  T.,   Nitrogen  storage  by   !<•-:- 

nines,  621. 
Simniermacher,     W  ,      Lime     :in<l     phos- 
phates, 536. 
Skinner,  J.  J.,  Creatinine 

Organic  matter  in  soil,  136. 
Smith,  C.  I).,  Root  nodules  and  nitrogen, 

storage,  621. 
Smith,  H.  E.,  Bacteria  in  frozen  soil,  433. 
Snyder,  H.,  Ash  of  humus.  1  l :» 
Complete  solution  of  soil,  330. 
Composition  of  humu.s,  149. 
Production  of  humus,   I  HI 
Sj)illinau.  \V    J.,  «ree:>  manures,  619. 
Stevenson.  W.   II..  Soil  survey  of  Iowa, 

718. 
Stewart,  -I.  B..  Moisture  control,  285. 

Capillary  movement,  225. 
Btohman,  P.,  Absorption,  353,  354. 
Stoklasa,  J.,  Bacteria  in  soil,  437,  439. 
Carbon  dioxide  production,  139. 
Carbon    dioxide    production   in   soil, 

408. 
Carbon  dioxide  in  soil  air,  479,  482. 
Fermentation  and  phosphates,  520. 
Stonr,  F.  H.,  Farm  manure,  578. 
Green  manure,  619. 
Humus  and  capillarity,  219. 
Poultry  manure,  588. 
Stormer,   K.,   Effect  of  soil  antiseptics, 

MM. 
Stover,  A.  P.,  Carey  Act,  688. 
St  rem  me.     II  .    Estimation    of    colloids, 

167. 
Sullivan,  M.  X.,  Catalysis,  529. 

Manganese,  531. 
Swezey,  G.  D.,  Temperature  of  soil,  321, 
485. 

Teel,  R.  P.,  Losses  of  irrigation  water. 

694. 
Ten  Eyck,  A.  M.,  Roots  of  plants,  81. 
Thaer,  W.,  Properties  of  colloids,  153. 
Thome,  C.  E.,  Application  of   manure 
607. 
Cement  versus  dirt  floors,  604. 
Effect  of  food  on  manure,  582. 


AUTHORS    INDEX 


747 


Thome,  C.  E.,  Farm  manure,  581 

Losses  of  manure,  599. 

Manure  and  the  rotation,  616. 

Production  of  manure,  587. 

Reinforcing  manure,  value,  610. 

Value  of  manure,  590,  601. 
Trowbridge,  A.  C,  Classification  of  sedi- 
ments, 31. 
Truog,  E.  A..  Sulfide  test  for  soil  acidity, 

387. 
Tularkov,  N.,  Classification  of  soils,  718. 
Tull,  Jethro,  Effects  of  tillage.  490. 

Ulrich,  R.,  Specific  heat  of  soil,  295. 
Underwood,   T.    M.,    Soil   solution   and 
plant  growth,  347. 

Vail,  C.  E.,  Composition  of  humus,  148. 
Van  Bemmelen,  J.  M  ,  Adsorption,  359. 

Colloids,  161. 

Colloidal  humus,  365. 

Colloidal  material,  346. 

Color  of  soil,  77 

Composition  of  humus,  133. 

Estimation  of  colloids,  167. 

Soil  solution,  345. 
Van  Slyke,   L.   L.,  A  general  fertilizer, 
571. 

Composition  of  manure,  584. 

Fertilizers,  546. 

Fertilizer  mixtures,  565. 

Production  of  manure,  587. 
Van  Suchtelen,  F.  H.  H.,  Soil  solution  in 

situ,  344. 
Veitch,  F.  P.,  Complete  solution  of  soil, 
330. 

Composition  of  soil,  66. 

Test  for  acidity,  390. 
Voelcker,  J.  A.,  Composition  of  rotted 
manure,  596. 

Manure,  579. 

Soil  acidity,  383. 
Von  Engeln,  O.  D.,  Glaciation  and  agri- 
culture, 70,  71. 
Voorhees,    E.    B.,    Availability    of    fer- 
tilizers, 510. 

Poultry  manure,  588. 

Waggaman,  W.  H.,  Absorption  by  soil, 

351. 
Wagner,  F.,  Conductivity  in  soil,  310. 

Manure  and  soil  temperature,  316. 

Texture  and  conductivity,  309. 
Wagner,  H.,  Law  of  minimum,  553. 
Wagner,   P.,   Availability  of  fertilizers, 

510. 


Wahnschaffe,  F.,  Silt  cylinder,  92. 
Warington,  R.,  Causes  of  granulation, 
•186. 
Colloids,  161. 
Composition  of  crops,  418. 
Composition  of  drainage  water,  240. 
Denitrification,  456. 
Estimation  of  organic  matter,  143. 
Evaporation  losses,  271. 
Lime  and  granulation,  194. 
Nitrification,  453. 
Snow  and  soil  temperature,  306. 
Warren,   G.   M.,   Marsh  land  drainage, 

627 
Waters,  H.  J.,  Lime,  541. 
Watson,  G.  C,  Production  of  manure, 

587. 
Way,  J.  T.,  Absorption  by  soil,  355. 

Colloids,  161. 
Weir,  W.  W.,  Soil  acidity,  383. 

Test  for  carbonates,  388. 
Welitschkowsky,   D.   von,   Temperature 

and  movement  of  water,  234. 
Wheeler,  H.  J.,  Acid-tolerating  plants, 
38. 
Acidity  tests,  387. 
Forms  of  lime,  542. 
Plants  injured  by  acidity,  385. 
Salt  as  a  fertilizer,  544. 
Wheeler,  W.  P.,  Poultry  manure,  582. 
Whipper,  O.  B.,  Irrigation  of  fruit,  682. 
Whitbeck,  R.  H.,  Glaciated  and  residual 

soils,  70. 
Whitney,  M.,  Apparent  specific  gravity, 
114. 
Soil  classes,  104. 
Soil  solution,  346. 
Specific  gravity  of  soil,  113. 
Whitson,  A.  R.,  Soil  acidity,  383. 

Test  for  carbonates,  388. 
Wickson,  J.  A.,  Irrigation  of  fruit,  682. 
Widtsoe,    J.    A.,    Amount   of   water   to 
apply,  710. 
Capillary  movement,  224. 
Dry-farming,  712. 
Dry  matter  and  water,  254. 
Irrigation  water  and  yield,  708. 
Movements  of  irrigation  water,  704. 
Principles  of  irrigation,  682. 
Storage  of  water  in  soil,  709. 
Water   requirement   of   plants,   248, 
251. 
Wiegner,  G.,  Absorption  by  permutite; 

357. 
Wiley,  H.  W.,  Complete  solution  of  soil, 
328. 


748 


AUTHORS'  index 


Wiley,    H.   W.,    Estimation  of  organic 

matter,  143,  144. 
Mechanical  soil  analysis,  84,  92. 
Soil  analysis,  338. 
Willcox,  O.  W.,  Soil  survey  of  Iowa,  718. 
Williams,  H.  F.,  Soil-forming  minerals, 

100. 
Williams,    M.    B.,    Irrigation   in   humid 

regions,  690. 
Wilson,  H.  M.,   Irrigation    engineering, 

692. 
Wing,  H.  H.,  Losses  of  manure,  599. 

Production  of  manure,  587. 
Winton,  A.   L.,  Commercial  fertilizers, 

564; 
Wolff,  E.,  Composition  of  manure,  595. 
Wollny,  E.,  Capillary  movement,   226, 

228,  230. 


Wollny,  E.,  Capillarity  and  temperature, 
214 
Color  and  temperature,  302. 
Composition  of  soil  air,  139. 
Effect  of  earthworms,  422. 
Gravitational    movement   of   water, 

234. 
Optimum  moisture,  262. 
Roots  and  humus,  127. 
Slope  and  temperature,  319. 
Water  requirements  of  plants,  246. 
Water  and  soil  temperature,  315'. 
Woodward,   S.    M.,    Land   drainage  by 
pumping,  627. 

Yoder,  P.  A.,  Centrifugal  elutriator,  89. 

Zsigmondy,  R.,  Colloid  chemistry,  153. 


INDEX 


Absolute    specific    gravity   of    minerals, 
112. 

of  soil,  113. 

of  soil  particles,  113. 
Absorption  by  the  soil,  349. 

causes,  355. 

formation    of    insoluble    substances, 
358. 

influence  of  chabazite,  356. 

influence  of  colloids,  165,  359,  360. 

influence  of  organic  matter,  150,  365. 

influence  of  silicates,  363. 

influence  of  zeolites,  355. 

influence  on  soil  analysis,  340. 

insolubility  of  absorbed  substances, 
354,  358. 

of  ammonia,  366. 

of  carbon  dioxide,  366. 

of  gases,  366. 

of  heat,  300. 

of  nitrogen  and  oxygen,  367. 

of  phosphoric  acid,  357. 

relation  to  temperature,  367. 

relation  to  drainage,  368. 

relation  to  productiveness,  368. 

selective,  362. 

time  necessary,  353. 
Absorptive    power    of    different    plants, 

414. 
Absorbed  bases,  solubility  of,  354. 
Abundance  of  common  minerals,  11. 

of  plant-food  elements,  5. 
Acetic  acid  secreted  by  roots,  408. 
Acid,  a  flocculating  agent,  159. 

in  plant  juices,  336. 

rocks,  7. 

soils,  375. 

soils  caused  by  ammonium  sulfate, 
499. 

test  for  carbonates,  388. 
Acidity  and  climate,  382. 

and  colloids,  165. 

and  forests,  380. 
and  sulfur,  382. 
and  plants  indicating,  382. 


Acidity,    effect  on  phosphate  fertilizer, 
519. 

relation  to  bacteria,  436. 

relation  to  fertilizers,  381. 

quantitative  determinations,  389. 

tests  for,  386. 
Acid  phosphate,  514. 

for  reinforcing  manure,  610. 
Acids  formed  from  fermenting  manure, 
593. 

in  plant  juices,  379. 

secreted  by  plant  roots,  408. 
Acme  harrow,  677. 
Adobe,  awlian  soil,  47,  59. 

described,  61. 

wind  origin,  16. 
^Eolian  soils,  deposition  of,  58. 

adobe,  61. 

composition  of,  60,  62. 

distribution,  61. 

loess,  59. 

sand  dunes,  63. 
Aeration  and  denitrification,  456. 

and  toxic  materials,  133. 

effect  of  drainage  on,  631. 

effect  on  nitrification,  452. 

influence  on  decay,  129. 

promoted  by  soil  organisms,  422. . 
Aerobic  bacteria,  433. 

bacteria  and  decay,  444. 

fermentation  of  manure,  592. 
Agencies  of  rock  decay,  14. 
Agricere  in  soil,  275,  472. 
Air  of  the  soil,  475. 

analyses,  478,  139. 

carbon  dioxide,  139,  478. 

composition  of,  477. 

control  of,  486. 

effect  of  organic  matter  on,  139,  476. 

effect  of  soil  moisture  on,  476. 

effect  of  texture,  475. 

effect  of  tillage,  487. 

function  of,  480. 

movement  of,  483. 

volume,  475. 


749 


750 


INDEX 


Air-slaked  lime,  539. 
Alga?,  aid  to  nitrogen  fixation,  464. 
Alinit,  nitro-culture,  463. 
Alkali  in  soils,  392. 

accumulation  of,  397. 

and  irrigation,  397. 

and  the  mulch,  282. 

block  alkali,  392. 

composition,  393. 

correction,  399. 

control,  402. 

effect  on  plants,  394. 

effect  of  method  of  irrigation,  706. 

formation  of,  724. 

outfit  for  testing,  728. 

salts  in  soil,  391. 
Alkali  land,  drainage  of,  659. 
Alkali  lands,  management  of,  399. 
Alkali  lands  of  foreign  countries,  398. 
Alkali  spots,  402. 
Alluvial  soils,  described,  39. 

distribution  of,  41. 

humus  and  nitrogen  in,  148. 
Alternaria,  disease  organism,  426. 
Alternate  cropping,  714. 
Aluminum,  6. 
Aluminum  phosphate,  337. 
Aluminum  in  soil  separates,  102. 
Amendments  of  the  soil,  534. 

calcium  sulfate,  542. 

calcium  carbonate,  540. 

caustic  lime,  539. 

common  salt,  543. 

effect  on  nitrification,  536. 

effect  on  tilth  and  bacteria,  534. 

effect  on  toxic  materials,  537. 

liberation  of  plant-food,  535. 

lime  and  granulation,  193. 

muck,  545. 
Amid  nitrogen,  plant-food,  497. 
Ammonia,  absorption  of,  353,  366. 

as  plant-food,  494. 

from  plant  decay,  139. 

salts  and  acidity,  381. 

test  for  acidity,  387. 
Ammonification  in  soil,  446. 
Ammonification,   effect  of  partial   ster- 
ilization, 470. 
Ammonium  sulfate,  as  fertilizer,  499. 
Amount  of  water  to  use  in  irrigation,  710. 
Anaerobic  bacteria,  433. 
Anaerobic    bacteria    and    putrefaction, 

444. 
Anaerobic  fermentation  of  manure,  593. 
Analysis,   mechanical,   of  soil,   97,    104, 
107. 


Analysis,  mineralogical,  100. 

of  adobe.  I 

of  air,  139.  478. 

of  alkali,  393. 

of  arid  and  humid  soils,  72,  147. 

of  coastal  plain  soils,  70,  66. 

of  crops,  419. 

of  cumulose  soil,  37. 

of  cyanamid,  503. 

of  drainage  water,  351,  371,  374,  500 

of  gases  from  manure,  594. 

of  glacial  soils,  68,  70. 

of  granite  and  residual  soil,  27. 

of  humus,  149. 

of  humus  ash,  145. 

<>f  limestone  and  residual  clay,  27,  33, 
68. 

of  litter,  580. 

of  loess,  60. 

of  manure,  581,  582,  583,  584,  588. 
595. 

of  marl,  38. 

of  residual  soils,  66,  68,  70. 

of  soils,  humus,  147,  148. 

of  soil,  organic  matter,  146. 

of  soil  separates,  101,  102. 

of  water  extract,  348. 
Animals,   capacity   to  produce  manure, 
587. 

effect  on  granulation,  192. 

effect    on    composition    of    manure, 
581,  582,  584. 

soil-forming  agent,  18. 
Antiseptics,  treatment  of  soil  with,  465. 
Apatite,  mineral,  9. 

as  a  fertilizer,  512. 
Apophyllite,  solubility,  339. 
Apparent  specific  gravity,  113. 
Application  of  water,  time,  709. 
Arginine,  plant-food,  497. 
Arid  and  humid  soils,  71. 

composition  of,  72. 

properties  of,  72,  82. 

soil   particles,    composition   of,    100, 
101. 
Arrangement  of  soil  particles,  108. 
Artificial  mulch,  273. 
Ash  composition  of  humus,  145. 
Ash  constituents  of  plants,  416. 
Aspergillus  niger  in  soil,  427. 
Atmospheric    pressure    affects    soil    air, 

484. 
Attraction  of  soil  particles,  206. 
Auger,  for  soil  examination,  727,  732. 
Augite,  9. 
Availability  of  organic  fertilizer,  510. 


INDEX 


751 


Availability  of  plant-food  and  bacteria, 

428. 
Availability  of  soil  water,  diagram,  262. 
Azotobacter  bacteria,  464. 

Bacillus,    denitrificans,   alpha  and  beta, 
456. 

mesentericus,  reducing  organism,  455. 

mycoides,  reducing  organism,  455. 

pestifer,  reducing  organism,  455. 

radicicola,  459,  545. 

rndiobacter,  464. 

ramosus,  reducing  organism,  455. 

subtilis,  reducing  organism,  455. 

vulgatus,  reducing  organism,  455. 
Back  furrow,  671. 
Bacteria  and  ammonification,  446. 

and  plant  decay,  129. 

and  root  nodules,  458. 

carbon  dioxide  production,  408. 

conditions  for  growth,  433. 

distribution,  429. 

functions,  436. 

in  frozen  soil,  431. 

influence  of  sulfur  on,  525. 

influence  on  organic  matter,  435. 

inoculation  with,  460. 

nitrate  reduction,  455. 

number  in  soil,  430. 

nitrogen  supply,  428. 

non-symbiotic,  460. 

solvent  action  of,  439. 
Bacteria  in  soil,  427,  428. 
Bacterial   action,   effect  on  phosphates, 

520. 
Bacteroids,  460. 
Bacterium  ellenbachensis,  463. 
Balanced  fertilizer,  551. 
Barnyards,  covered  for  manure,  605. 
Barometric  pressure  and  drainage,  234. 
Basalt,  8. 

Bases  absorbed,  solubility  of,  354. 
Bases,  absorbed  by  colloids,  165. 
Bases  and  toxicity,  379. 
Bases  in  plant  ash,  378. 
Bases,  removed  in  drainage,  378. 
Basic  rocks,  7.  - 
Basic  slag  phosphate,  513. 
Basic  soil,  effect  on  phosphates,  519. 
Bacillus  amylobacter,  440. 
Bedding,  absorptive  capacity,  603. 
Biotite,  9. 

Biotite,  solubility,  330. 
Black  alkali,  392. 
Blood,  dried,  as  fertilizer,  507. 
Bones  as  fertilizer,  511. 


Bone  phosphate,  511. 

Bone  tankage,  512. 

Brands  of  fertilizer,  555. 

Broadcasting  fertilizer,  570. 

Bromberg,      composition     of      drainage 

water,  370. 
Bureau  of  soils,  classification  of  alkali, 

396. 

Calcium,  4,  6. 

Calcium  carbonate,  7,  10. 

Calcium  carbonate  in  soil,  333. 

Calcium  combinations,  539. 

Calcium  compounds,  root  action  on,  406. 

Calcium  cyanamid,  502. 

Calcium  hydrate  in  fertilizer,  566. 

Calcium  in  soil  separates,  101. 

Calcium  loss  in  drainage,  372. 

Calcium  nitrate,  505. 

Calcium  phosphate,  effect  of  bacteria  on, 

439. 
Calcium  salts,   as  amendments,   effects, 

534. 
Calcium    sulfate    to    reinforce    manure, 

609. 
Calculation  of  air  space  of  soils,  238,  477. 

of  apparent  specific  gravity,  113. 

of  free  water,  238. 

of  number  of  soil  particles,  118. 

of  pore  space,  116. 

of  surface  of  soil  particles,  120. 

of  wilting  point,  260. 
Canals  for  drainage,  construction,  635. 
Canals  for  irrigation,  693. 
Capillarity  and  texture,  214. 
Capillarity  carries  nitrates    to    surface, 

454. 
Capillary  movement,  221. 

and  alkali  formation,  298. 

in  wet  and  dry  soil,  225. 

influenced  by  thickness  of  film,  223. 

influenced  by  surface  tension,  227. 

influenced  by  texture,  229. 

influenced  by  structure,  232. 

intercepted  by  green  manure,  624. 
Capillary  water,  201,  210. 
Capillary  water,  amount,  213. 

and  organic  matter,  218. 

and  structure,  217. 

and  surface  tension,  213. 

and  texture,  214. 

estimation  of,  219. 

factors  affecting  amount,  213. 

form  of  surface,  212. 

relation  to  plant,  261. 
Capillarity  and  ground  water,  214. 


752 


INDEX 


Carbohydrates    and    nitrogen    fixation, 

464. 
Carbohydrates  as  source  of  energy,  466. 
Carbohydrates  in  soil,  127. 
Carbon,  4,  6. 
Carbon  dioxide,  absorption  of,  366. 

as  an  influence  on  climate,  4'.». 

and  root  action,  406,  408. 

end  product  of  decay,  138. 

functions  in  soil,  481. 

in  soil  air,  21,  139,  410,  478. 

in  atmosphere,  139. 

product  of  decay,  130. 

production  by  bacteria,  408. 

use  in  soil  analysis,  339. 
Carbon  disulfide  as  soil  antiseptic,  465. 
Carbon  disulfide  from  plant  decay,  140. 
Carbonate  of  lime,  640. 
Carbonates  of   lime,  effect  on   nitrates, 

536. 
Carbonates,  acid  test  for,  388. 
Carbonates  in  earth's  crust,  11. 
Carbonates,  effect  on  soil,  482. 
Carbonation  as  weathering  agent,  19. 
Carbonized  material  in  soil,  140,  144. 
Carriers,  in  fertilisers,  555. 
Catalytic  action  of  soil,  528. 
Catalytic  fertilizer,  528. 
Catch  crops,  and  nitrate  conservation, 

455. 
Caustic  lime,  539. 

Cement  pit  storage  of  manure,  604. 
Centrifugal  soil  analysis,  93. 
Cephalothecium,    disease    organism    in 

soil,  426. 
Cereals,  absorptive  power,  414. 
Chabazite,  356. 
Checking  of  losses  by  evaporation,  272. 

of  losses  by  leaching,  267. 
Chemical  agents  of  rock  decay,  14. 
Chemical  analysis  of  soil,  327. 

complete,  328. 

extraction  with  acids,  329. 

extraction  with  water,  340. 
Chemical  changes  due  to  heat,  291. 
Chemical    composition    of    soils.       See 

Analysis. 
Chili  saltpeter,  498. 
Chloride  of  potash,  523. 
Chlorine,  6. 

Chlorine  as  fertilizer,  544. 
Chlorite,  9. 

Chloroform  as  soil  antiseptic,  473. 
Cistern  storage  of  manure,  605. 
Citric  acid  method,  336. 
Class,  the  soil,  defined,  103. 


Class,  the  soil,  in  classification,  722. 

Classification  of  rocks,  6. 

Classification  of  soil  material,  geological, 

31. 
Classification  of  soil  particles,  95. 
Classification  of  soil,  factors  used,  720. 
Classification  of  soil,  outline  of,  721. 
Clostridium  paatorianum,  463. 
Clay,  character  of  separate,  98. 
Clay,  colloidal,  161. 
Climate  and  acidity,  382. 
Climate  and  efficiency  of  fertilizer,  568. 
Climate,  influence  on  soil,  65. 
Climate  as  factor  in  soil  classification, 

724. 
Clouds,  effect  on  radiation,  306. 
Clod  crushers,  678. 
Coefficient  of  cohesion,  175. 
Coefficients  of  drainage,  651,  652. 
Coefficients  of  expansion  of  rocks,  17. 
Coefficient  of  friction,  229. 
Coefficient  of  hygroscopicity,  208. 
Coefficient  of  plasticity,  171. 
Coefficient  of  wilting,  257,  258. 
Cohesion  defined,  173. 
Cohesion,  determination,  174. 
Cohesion,  control,  183,  197. 
Cohesion,  factors  affecting,  178. 
Cold  and  heat  as  weathering  agents,  16. 
Collectotrichum,    disease    organism    in 

soil,  426. 
Colloids  in  the  soil,  153. 
Colloids,  absorption  by,  359. 

and  acids,  165. 

chemistry,  153. 

effect  of  roots  on,  411. 

estimation  of,  166. 

factors  affecting,  165. 

organic,  140. 

preparation  of,  163. 
Colloidal  humus,  133. 

materials,  346. 

matter,  relation  to  phosphates,  517. 

phases,  158. 

state,  154. 
Colluvial  soils  described,  38. 
Color  of  soil,  73. 
Color  of  soils,  due  to  weathering,  20. 

due  to  humus,  75. 

due  to  iron,  75. 

agricultural  significance,  78. 

and  temperature,  301. 
Color  of  arid  and  humid  soils,  72. 

of  glacial  soil,  53. 

of  marine  soil,  44. 

of  residual  soil,  33. 


INDEX 


753 


Colters,  types,  672. 
Commercial  value  of  fertilizer,  560. 
Complete  solution  of  soil,  328. 
Composition,  of  arid   and   humid   soils 
72. 

of  animal  manures,  584. 

of  alkali,  391. 

of  cumulose  soil,  37. 

of  crops,  419. 

of  drainage  water,  369. 

of  gases  from  manure,  594. 

of  glacial  soil,  52. 

of  glacial  and  residual  clay,  68,  69. 

of  humus,  131,  140. 

of  horse  and  cow  manure,  581. 

of  loess,  60. 

of  marl,  38. 

of  manure  litter,  580. 

of  plants,  127,  418. 

of  residual  and  marine  soil,  66. 

of  rotted  manure,  596. 

of  residual  soil,  33. 

of  soil  in  general,  2. 

of  soil-forming  minerals,  9. 

of  soil  due  to  weathering,  29. 

of  soil  separates,  101. 

of  subsoil,  332. 

of  soil  air,  139,  477. 

of  surface  water,  373. 

of  yard  manure,  578. 
Composting  manure,  613. 
Concentration  and  growth,  417. 
Concrete  tile,  641. 
Condensation,  heat  of,  209. 
Conduction  of  heat  in  soil,  307. 
Conservation  of  moisture  in  irrigation, 

710. 
Convection  in  soil,  307. 
Coprolites  as  fertilizer,  512. 
Cornell  tanks,  composition  of  drainage 

water,  372. 
Cornell  University,  bacteria  in  soil,  429. 

effect  of  magnesium,  539. 

nitrates  in  soil,  451. 
production  of  manure,  587. 

soil  inoculation,  462. 
Cottonseed  meal,  as  fertilizer,  507. 
Cover  crops  and  green  manure,  620. 
Covered  barnyards  for  manure,  605. 
Cow  manure,  composition,  584. 
Creatinine,  plant-food,  497. 
Cropping,  alternate  in  dry-farming,  714. 
Cropping,  effect  in  soil  ventilation,  488. 
Crops,  composition  of,  419. 
drought-resistant,  715. 
effect  of  alkali  on,  394. 

So 


Crops,  efficiency  of  fertilizer,  568. 

feeding  power,  415. 

for  green  manure,  622. 

suited  to  basic  soils,  385. 

suited  to  acid  soil,  384. 
Crumb  structure,  109. 
Crushers  and  packers  as  soil  pulverizer, 

678. 
Cultivators,  types,  673. 
Cultures,  mixed  nitrogen-fixing,  464. 
Cultures  of  nitrogen-fixing  bacteria,  458, 

461. 
Cumulose  soils,  35. 
Cyanamid,  nitrogen  fertilizer,  502. 
Cyanamid  in  fertilizer  mixtures,  566. 
Cytase,  enzyme,  440. 

Dam,  canvas,  701. 
Damping-off  fungi  in  soil,  425. 
Dead  furrow,  objectionable,  671. 
"Dead"  furrows,  for  drainage,  635. 
Decay  of  green  manure  in  soil,  622. 

of  organic  matter  in  soil,  128. 

organic,  and  soil  temperature,  315. 

and  putrefaction,  443. 

of  rocks,  law  of,  24. 
Decomposition  of  organic  matter,  440. 
Decomposition  of  rock,  14. 
Deep-tilling  plow,  667. 
Deflocculation  of  sodium  nitrate,  499. 
Delta  defined,  41. 
Denitrification,  456. 
Denudation,  rate  of,  in  U.  S.,  14. 
Denudation  by  Mississippi  River,  15. 
Deoxidation  as  weathering  agent,  20. 
Department  of  Agriculture,  U.  S.  Nitro- 

culture,  462. 
Depth  of  moisture  storage  in  soil,  710. 

for  plowing,  669. 

of  soil  in  relation  to  humus,  148. 

of  soil  mulch,  278. 

of  tile  drains,  646. 
Detention  of  plant-food  in  soil,  332. 
Dewey  system  of  classification,  726. 
Diabase,  6. 

Diffusion  of  gases  in  soil,  483. 
Dihydroxystearic  acid,  376. 
Diorite,  6. 

Disease  of  plants  and  moisture,  253. 
Diseases  of  plants,  effect  of  lime,  536. 
Disease  organisms  in  soil  426. 
Disease  resistance,  effect  of  phosphorus, 
550. 

effect  of  nitrogen,  549. 
Disintegration  of  rock,  14. 
Disk  harrow,  single  and  double,  676. 


754 


INDEX 


Disk  plow,  666. 

Disking  to  hold  moisture,  714. 

Ditching  machines,  649. 

Dolomite,  6,  9. 

Drag  as  soil  pulverizer,  680. 

Drain  till,  quality,  640. 

Drainage  and  absorption,  368. 

Drainage  and  denitrification,  456. 

coefficient,  651. 

effects  on  soil,  630. 

effect  on  soil  ventilation,  477,  488. 
'   effect  on  soil  bacteria,  484. 

extent  of  need,  628. 

efficiency  of  fertilizer,  568. 

formation  of  humus,  152. 

history  of  development 

of  irrigated  and  alkali  land,  659. 

of  land,  indications  of  need,  627. 

methods,  634. 

muck  and  peat  soil,  658. 

promoted  by  soil  organisms,  422. 

reclamation  of  alkali  land,  400. 

relation    to   green   manure   practice, 
624. 

relation  to  colloids,  166. 

run-off  checked,  269. 

systems,  arrangement,  643. 

toxic  materials  eliminated,  137. 

use  of  explosives,  661. 

tile  drains,  639. 

vertical,  660. 
Drainage  water  at  Bromberg,  370. 
Drainage  water,  composition,  369,  351. 
Drainage  water,  relation  of  phosphates 

and  carbon  dioxide,  482. 
Drains,  protection  of  joints,  642. 
Dried  blood,  507. 
Drift  defined,  52. 
Drought-resistant  crops,  715. 
Dry-farming  practices,  713. 

principles,  712. 

drought-resistant  crops,  715. 

soils  best  suited  for,  716. 

extent,  717. 
Drying  and  wetting,  effect  on  granula- 
tion, 187. 
Dust  mulch,  272. 

Dust  mulch  under  irrigation,  705,  710. 
Duty  of  water  in  irrigation,  708. 
Dynamite,  drainage  by  means  of,  661. 

Earthworms,  action  on  soil,  19. 
Earthworms  and  productiveness,  422. 
Effective  diameter  of  soil  particles,  122. 
Effective  surface  of  soil,  125. 
Effects  of  organic  matter  on  soil,  150. 


Electrical    production    of    nitrogen    f.r 

tUtor, 
Elements  of  plant-food,  3. 
Enzymotic  action  of  bacteria,  129. 
Enzymes,  effect  on  plant-food,  438. 
Enzymes  in  roots,  407 
Epidote,  9. 
Kn.sion,  agencies  causing,  14. 

ctTcct  of  dramas 

in  ditches,  limits  of  Krade,  636. 

ice  as  agen< 

in  irrigation  canals,  694. 

rateot.  in  U.B.,  14. 

wind  action,  l~>. 
Eskers,  50. 
Evaporation  and  alkali,  282. 

and  temperature.  314. 

and  wind  movement,  285. 

at   Kotham.-ted.  871. 

from  plants,  27<>. 

prevented  by,  272. 

rainfall  lost  by,  271. 
•i  from  animals,  577. 
Exhaustion  of  soil,  410. 
Expansion  of  minerals  by  heat,  16. 
Explosives,  drainage  by  means  of,  661. 

Factors  for  plant  growth,  3. 

Factory-mixed  fertilizer,  563. 

Fall  plowing,    relation    to    dry-farming, 

713. 
Fall  and  spring  plowing,  283. 
Farm  manures,  577. 
Farm  manure,  waste,  597. 
Fats  in  soil.  127. 
Fats,  effect  on  capillarity,  211. 
Feeding  power  of  plants,  414. 
Feldspar,  as  potash  fertilizer,  524. 
Feldspar  in  soil  separates,  101. 
Fermentation,  effect  on  phosphates,  520. 
Fermentation  of  manure,  591. 
Ferrie  phosphate  availability,  337. 
Fertilizers,  489. 
Fertilizers  and  acidity,  381. 

amounts  to  use,  572. 

application  of,  570. 

brands,  555. 

catalytic,  530. 

commercial  value,  560. 

commercial,  extent  of  use,  492. 

effect  on  toxic  material,  137. 

factors  in  efficiency,  568. 

for  special  crops,  571. 

grades  of,  562. 

home  mixing,  563. 

incompatible  material,  565. 


INDEX 


755 


Fertilizers,  inspection,  556. 

mixed,  561. 

nitrogen,  493. 

penetration  into  soil,  354. 

practice,  546. 

potash,  522. 

systems  of,  573. 

sulfur,  524. 
•      trade  value,  559. 
Fertilizing  crops,  571. 
Filler  in  fertilizer,  556. 
Filler  in  fertilizer,  muck,  545. 
Film  water,  206,  210. 
Fineness  in  phosphates,  513. 
Finger  lakes,  50. 
Fire-fanging,  manure,  596. 
First  bottom  land,  42. 
Fish  as  fertilizer,  508. 
Floats  to  reinforce  manure,  610. 
Flocculation,  159. 

Flooding,  as  means  of  soil  infection,  425. 
Flooding,  irrigation  by,  699. 
Flume,  for  measurement  of  water,  707. 
Flushing,  correction  for  alkali,  401. 
Food  of  animal,  effect  on  manure,  582. 
Food  elements,  sources,  4. 
Food  material  in  crops,  418. 
Food  of  plants,  absorption  of,  404. 
Forces  of  weathering,  14. 
Formic  acid  secreted  by  roots,  408. 
Forests  and  acidity,  380. 
Forest  trees,  mycorrhize  of,  428. 
Free  water  in  soil,  236. 
Free  water,  bad  effects,  262. 
Freezing,  effect  on  granulation,  189. 

relation  to  soil  colloids,  166. 
Fresh  versus  leached  manure,  601. 
Friction  and  available  water,  256. 
Friction  coefficient,  229. 
Frost  as  soil  pulverizer,  680. 
Frost  as  weathering  agent,  17. 
Frozen  soil,  bacteria  in,  431. 
Fruiting  effect  of  nitrogen,  548. 
Fruits,  feeding  power,  416. 
Functions  of   fertilizers,    elements,    547, 
549,  551. 

of  soil  to  plant,  1. 

of  water  to  plant,  243. 
Fungi  and  fire-fanging  of  manure,  596. 
Fungi  in  soil,  423. 
Furrow,  back,  productive,  671. 

correct  position,  668. 

dead,  objectionable,  671. 

depth  and  width,  669. 

use  in  irrigation,  702. 
Fusarium  as  a  disease  organism,  426. 


Gabbro,  6. 

Gases,  from  manure,  594. 

Gel  colloids,  159. 

Geological  classification  of  soil  material, 

31. 
Germination,  effect  of  heat  on,  290. 
Glacial  drift,  47. 
Glacial  ice,  as  erosive  agent,  16. 
Glacial  lakes,  55. 
Glacial  soil,  composition,  52. 
Glacial  soil,  humus  in,  54. 
Glacial  soils,  47. 
Glauconite,  solubility,  339. 
Gneiss,  6. 

Grade  of  drains,  645. 
Grain,  effect  of  phosphorus,  550 

effect  of  potassium,  551. 

effect  of  nitrogen,  548. 
Granite,  6,  8. 

weathering  of,  27. 
Granular  soil,  111. 
Granulation,  185. 
Granulation,  cause  of,  187. 

defined,  170. 

effect  of  plow,  195. 

effect  on  cohesion,  179. 
Granulation  of  soil  and  optimum  mois- 
ture, 263. 

effect  of  drainage,  630. 

modified  by  tillage,  663. 
Granulabacter,  group  of  bacteria,  464. 
Granules  in  soil,  109. 
Grass  crops,  feeding  power,  415. 
Gravel,  99. 

Gravitational  water,  201,  233. 
Gravitational  water,  injurious  to  crops, 
261. 

calculation  of,  237. 

control  of,  drainage,  627. 

movement,     factors    affecting,     233, 
235. 

movement  of,  233. 

study  of,  238. 
Green  manure,  151. 
Green  manure,  and  acidity,  379. 

conditions  for  plowing  under,  624. 

constituents  added,  621. 

crops,  622. 

decay  in  soil,  622. 

denitrification,  547. 

effects,  619. 

lime  relations,  625. 

relation  to  the  rotation,  625. 
Ground  limestone,  540. 
Ground  water  and  capillarity,  214. 
Group,  the,  in  soil  classification,  723. 


756 


INDEX 


Growing  season,  effect  of  drainage,  631. 

Growth,  effects  of  nitrogen,  547. 

Guano,  as  fertilizer,  507. 

Guarantee,  fertilizer,  558. 

Gypsum,  9,  542. 

Gypsum,  correction  for  alkali,  400. 

Gypsum,  lime  fertilizer,  539. 

Gypsum,    solvent   action   of    roots    on, 

406. 
Gypsum  to  reinforce  manure,  609. 

Handling  manure,  602. 

Hardpan,  alkali,  effect  on  drainage,  659. 

Harrow  as  cultivator,  676. 

Heat,  absorption  of,  300. 

and  cold,  mechanical  action,  16. 

chemical  and  physical  changes  due 
to,  291. 

condensation,  209. 

convection  in  soil,  307. 

functions  of,  in  soil,  289. 

effect  on  soil,  466. 

unit  defined,  314. 

sources  in  soil,  292. 

specific  heat  of  soil,  294. 
Head  of  water,  defined,  706. 
Head  of  water  for  irrigation,  702. 
Heaving  of  soil  indicates  wetness,  628, 

632. 
Heiden's  formula  for  manure  production, 

588. 
Helminthosporium,     disease     organism, 

426. 
Hematite,  9. 

Hematite,  hydration  of,  21. 
Herring-bone  system  of  drainage,  645. 
High-grade  fertilizer,  562. 
Hillside  plow,  670. 
Histidine,  plant-food,  497. 
Home-mixing  fertilizer,  563. 
Home-mixing  fertilizer,  method,  567. 
Hoof  meal  as  fertilizer,  507. 
Hornblende,  9. 

Hornblende  in  soil  separates,  101. 
Horse  manure,  composition,  584. 
Humid  and  arid  soils,  71. 
Humid  climates,  irrigation  in,  689. 

soils,  composition  of,  72. 

soils,  organic  matter  of,  146,  147. 
Humidity,  effect  on  hygroscopicity,  207. 
Humus,  absorption  by,  365. 
Humus  ash  composition,  145. 

and  conductivity,  310. 

and  capillarity,  219. 

and  efficiency  of  fertilizer,  569. 

and  roots  of  plants,  127. 


Humus  and  run-off,  269. 

and  specific  heat,  297. 

as  plant-food,  495. 

composition  of,  131. 

content  of  soils,  147. 

effect  of  drainage  on  formation,  632. 

effect  on  hygroscopicity,  204. 

effect  on  granulation,  190. 

effect  on  cohesion,  170.  • 

effects  on  soil,  150. 

i>tim:ition  of,  142,  144. 

in  glacial  soil,  54. 

nature  of,  in  soil,  130. 

specific  gravity  of,  113. 
Humus,  defined,  130. 
Hydration  as  weathering  agent,  20. 
Hydrochloric  acid  used  in  soil  survey, 

728. 
Hydrochloric  acid  solution  of  soil,  329. 
Hydrochloric   acid   test  for   carbonates, 

388. 
Hydrogen,  4,  6. 

Hydrogen,  from  plant  decay,  140. 
Hygroscopic  coefficient,  257. 
Hygroscopic  moisture,  201,  202. 
Hygroscopic    moisture    and    plasticity, 
171. 

relation  to  colloids,  168. 

relation  to  plants,  256. 
Hygroscopicity,  determination  of,  208. 

Ice,  as  erosive  agent,  16. 

Ice-formed  soils,  52. 

Ice  sheet,  47. 

Igneous  soil-forming  rocks,  6. 

Implements,  tillage,  664. 

Infection  by  soil  fungi,  425. 

Inoculation  of  soil  for  legumes,  460. 

Inorganic  colloids,  158,  162. 

Insects  in  soil,  423. 

Inspection  of  fertilizers,  556. 

Interception  losses  in  forests,  287. 

Iodine  in  ash  of  sea  weed,  404. 

Iron,  4,  6. 

Iron  a  catalytic  fertilizer,  529. 

Iron  as  a  soil  color,  75. 

Iron  phosphate  available,  337. 

Iron  in  soil  separates,  102. 

Irrigation,  amount  of  water  applied,  708. 

amount  of  water  to  apply,  710. 

and  alkali,  397. 

application  of  water  and  yield,  208. 

canals,  693. 

canals,  linings,  694. 

conditions  that  warrant,  683. 

development  in  the  United  States,  686 


/ 


INDEX 


757 


Irrigation,  erosion  in  canals,  694. 
flooding,  699. 
furrow,  702. 
history,  685. 
in  humid  regions,  689. 
legal,    economic    and    social    effects, 

691. 
land,  drainage  of,  659. 
land,  extent  of,  685. 
methods  of  water  supply,  688. 
methods  of  applying  water,  695. 
movement  of  water,  704. 
preparation  of  land,  695. 
relation  to  rainfall,  682. 
setting  of  fruit,  709. 
sewage,  711. 
sources  of  water,  693. 
sub,  696. 

theory  and  practice,  682.  . 

units  of  water  measurement,  706. 

Jointer,  672. 

Joints,  protection  in  tile  drains,  642. 

Kainit,  potassium  salt,  522. 
Kainit  to  reinforce  manure,  609. 
Kames,  50. 

Kaolin,  specific  heat,  299. 
Kaolinite,  9. 

Lacustrine  soils,  56. 

Lagoons,  40. 

Land  drainage,  indications  of  need,  627. 

Land  plaster,  542. 

Land  plaster  to  reinforce  manure,  609. 

Leaching  of  manure,  599. 

Leather  meal  as  fertilizer,  507. 

Legumes  and  symbiosis,  458. 

Legumes,  feeding  power,  415. 

Legume    manure,    nitrogen    added    by, 

621. 
Leguminous  green  manures,  623. 
Lento-capillarity,  257. 
Lento-capillarity,  defined,  224. 
Leucite,  solubility,  339. 
Level  tillage,  286. 
Lime  as  amendment,  effects,  534. 
Lime  carbonate,  333. 
Lime,  a  catalytic  fertilizer,  528. 

effect  on  bacteria,  432,  436. 

effect  on  nitrates,  536. 

effect  on  toxic  substances,  536. 

effect  on  efficiency  of  fertilizer,  569. 

effect  on  sanitation  of  soil,  138. 

effect  on  granulation,  193. 

fertilizer  mixtures  of,  566. 


Lime,  flocculating  agent,  160. 

forms  of,  539. 

formation  of  humus,  152. 

green  manure  and,  625. 

loss  in  drainage,  372. 

manure  and,  612. 

nitrogen  fixations,  464. 

relation     to     available     phosphorus, 
517. 

relation  to  soil  colloids,  166. 

relation  to  soil  diseases,  426. 

run-off  relationships,  269. 

soil  separate  content,  101. 

soil  and  subsoil  content,  378. 
Lime-magnesia  ratio,  538. 
Lime  phosphate,  effect  of  bacteria   on, 

439. 
Limestone,  6,  8. 
Limestone  for  soil,  539. 
Limestone,  residual  soil  from,  33. 
Limestone  soil,  not  rich  in  lime,  28. 
Limestone,  weathering  of,  27. 
Limewater  test  for  acidity,  390. 
Limonite,  9. 

Linseed  meal  as  fertilizer,  507. 
Liquid  manure  compared  with  solid,  585. 
Lister,  seeder  cultivator,  678. 
Listing,  effect  on  soil  ventilation,  487. 
Lysimeter  described,  239. 
Litmus  test  for  acidity,  386. 
Litter,  absorptive  capacity,  603. 
Litter  in  manure,  578,  580. 
Loam,  defined,  104. 
Loess,  ^Eolian  soil,  58. 

description  and  composition,  59. 

wind  origin,  15. 

soil  distribution  of,  51. 
Loss  of  manure  in  handling,  583,  600. 
Low-grade  fertilizer,  562. 

Machinery,  tillage,  664. 
Macroorganisms  in  soil,  421. 
Macrosporium,  disease  organism  in  soil, 

426. 
Magnesium,  4,  6 
Magnesium  carbonate,  and  nitrates,  536. 

catalytic  fertilizer,  530. 

in  soil  separates,  102. 
Manure,  489. 

Manure,  amount  produced  by  animals, 
587. 

commercial  value,  589. 

composition,  causes  of  variation,  580. 

composition  of  gases  from,  594. 

composition  of  rotted,  596. 

composting,  613. 


758 


INDEX 


Manure,  covered  yards  and  pits,  605. 

damnification  of,  457. 

destruction    of    organic     matter     in 
feed,  597. 

effect  of  food  of  animal,  582. 

effect   of   handling   on    composition, 
583,  600. 

effects  on  the  soil,  613. 

effect  on  soil  ventilation,  488. 

farm,  597. 

farm  corrects  alkali,  403. 

fermentation,  501. 

frequent  small  applications,  607. 

fresh  versus  leached,  601. 

fire-fanging,  596. 

functions,  489. 

green,  619. 

green  and  lime,  625. 

Heidens  formula  for  production,  588. 

lime  and,  612. 

muck  and,  613. 

needs  and  plant  food  deficiency,  334. 

organic  and  nitrification,  450. 

plowing  under,  608. 

reinforcement,  609. 

residual  effects,  614. 

rotation  relation,  615. 

small  versus  large  applications,  608. 

spreader,  608. 

storage  in  open  piles,  606. 

yard  composition,  578. 
Marble,  6,  43. 

Marine  soil  compositions,  66. 
Marl,  539. 
Marl,  composition,  38. 

found  under  muck,  37. 
Marsh  mud  composition,  37. 
Maturity,  effect  of  nitrogen,  548. 
Maturity,  effect  of  phosphorus,  550. 
Maximum  water  content,  262. 
Measurement  of  water  in  irrigation,  706. 
Meat,  as  fertilizer,  507. 
Mechanical  analysis,  84. 
Mechanical  analysis  of  samples,  728. 
Meeker  harrow,  677. 
Metamorphic  soil-forming  rocks,  6. 
Methane  in  soil  air,  140. 
Mica  in  soil  separates,  101. 
Microcline,  solubility,  338. 
Microorganisms,  424. 
Microorganisms,  effect  of  sulfur,  525. 
Mineral  acid,  method  of  analysis,  338. 

colloids,  162. 

definition  of,  7. 

matter,   decomposition   by   bacteria, 
437. 


Mineral  acid,  nutrients  of  feed  in  ma- 
nure, 597. 

phosphates  as  fertilizer,  .*il2. 
Mim  r:ii>,  sbeorbsd  by  plants,  416. 

law  of  decay,  24. 

relative  abundance,  11. 

rock-forming,  8. 

soil-forming,  8. 
Mineralogical  character  of  soil  separates, 

99. 
Miner*!  inch  of  water,  defined,  7<>d. 
Minimum,  law  of,  .V.I 

Mississippi  River,  denudation  by,  15. 

Mixed  fertilizers,  ffl. 
Mollification  of  structure,  187. 
Module,  for  measuring  water,  709. 
Moisture  of  the  soil,  198. 

conductivity  relations,  :<1<>. 

capacity  of  soil,  effect  of  drainage, 
681. 

capillary  form,  210. 

conservation  in  irrigation,  710. 

content,  effect  on  soil  air,  476. 

control  of,  264. 

effect  on  bacteria,  434. 

effect  on  cohesion,  179. 

effect  of  movement  on  soil  air,  484. 

equivalents  of  soil,  220. 

forms  of,  200. 

gravitational,  233. 

hygroscopic,  202. 

maximum  content,  221. 

methods  of  stating 

relation  to  colloids,  165. 

relation  to  decay,  129. 

relation  to  plowing,  196. 

used  by  plant,  261. 

uses  to  plant,  243. 
Moldboard  plows,  667. 
Moldboard,     shapes     for     best     result, 

196. 
Molds  in  soil,  ammonify  proteins,  427. 
Mole  drainage,  638. 
Moraine,  terminal,  50. 
Movement,  of  soil  air,  483. 

heat,  307. 

heat  factors  affecting,  308,  310. 

moisture,  capillary,  221. 

moisture,  factors  affecting  capillarity, 
223. 

moisture,  gravitational,  233. 

moisture,  thermal,  241. 

water,  affected  by  friction,  223. 

water,  affected  by  texture,  229. 

water,  affected  by  structure,  232. 
Muck,  defined,  36. 


INDEX 


759 


Muck,  as  fertilizer,  545. 

and  manure,  613. 

and  moisture  control,  272. 

nitrogen  compounds  in,  133. 

specific  growth  of,  113. 
Mulch  and  the  control  of  alkali,  402. 
Mulch,  depth  of,  278. 

depth  in  irrigation  farming,  705,  715. 

dry-farming,  use  in,  714. 

effectiveness  in  arid  regions,  277. 

effect  other  than  on  moisture,  280. 

factors  of  effectiveness,  275. 

formation  of,  276. 

functions  of,  274. 

kinds  of,  273. 

management  of,  278. 

of  soil,  233. 

r6sum6  of  control,  278. 

usefulness  of,  282. 

water  saved  by,  279. 
Mulching  grain  crops,  282. 
Muriatic  acid  used  in  soil  survey,  728. 
Muscovite,  9. 
Muscovite,  solubility,  339. 
Mycorrhiza,  relation  to  fertility,  423. 
Mycotrophic  plants,  423. 

Natural  mulch,  273. 
Negative  acidity,  376. 
Nematodes  in  soil,  424. 
Nephelite,  solubility,  339. 
Nitrate  assimilation  by  bacteria,  456. 
Nitrate  of  calcium,  fertilizer,  505. 
Nitrate  of  soda,  as  plant  food,  498. 
Nitrate,  reduction  of,  455. 
Nitrates,    conserved   by   green   manure, 
620. 

constituent  of  alkali,  392. 

effect  of  lime  on,  536. 

in  soil,  effect  of  absorption  on,  341. 

loss  from  soil,  454. 

product  of  decay  processes,  130. 

returned  to  surface,  454. 
Nitric  acid  method  of  analysis,  338. 
Nitric  acid  solution  of  soil,  329. 
Nitrification  in  soil,  447. 

effect  of  aeration,  452. 

effect  of  antiseptics,  465. 

effect  of  carbon  bisulfide,  466. 

effect  of  depth,  453. 

effect  of  organic  matter,  449. 

effect  of  sod,  452. 

temperature  for,  435. 
Nitrobacter  in  soil,  448. 
Nitrogen,  4,  12. 

absorption  of,  367. 


Nitrogen,  added  to  soil  by  legumes,  621. 

available  from  atmosphere,  501. 

chemical  estimation,  329. 

cycle,  443. 

effects  on  plant  growth,  547. 

effect  on  toxic  material,  137. 

fertilizers,  493. 

fertilizer  from  the  air,  501. 

fertilizers,  organic,  507. 

fixation,  457. 

fixation  and  mycorrhiza,  428. 

fixation  by  molds,  427. 

fixation,  non-symbiotic,  463. 

fixing  organisms,  463. 

forms  in  soil,  493. 

forms  used  by  plants,  448,  494. 

found  in  animal  manures,  597. 

in  humus,  147. 

loss  in  drainage,  372. 

loss  from  Rothamsted  soil,  500. 

necessary   to   plants,    early    studies, 
491. 

supply  and  bacteria,  428. 

utilized  by  bacteria,  459. 
"Nitrogen"  culture,  461. 
Nitrosococcus  in  soil,  448. 
Nitrosomonas  in  soil,  448. 
Nodules  on  plant  roots,  458. 
Number  of  soil  particles,  118. 
Nutrient  salts,  absorption  of,  by  plants, 
404. 

absorption  by  soils,  349. 

selective  absorption,  362. 

Odometer  used  in  soil  survey,  727. 

Oils,  effect  on  capillarity,  214. 

Oils  in  soil,  128. 

Oily  material,  and  capillary  movement, 

226,  228. 
Oily  materials  in  soil,  275. 
Olivine,  9. 

Open  ditches,  construction,  635. 
Open-ditch  drainage,  objections,  634. 
Optimum  water  content,  262. 
Orchards,  manure  in,  617. 
Organic  colloids,  157,  161. 
Organic  fertilizer  availability,  510. 
Organic  constituents  of  soils,  2,  12,  128, 

131. 
Organic  decay  and  soil  temperature,  315. 
Organic  matter,  12. 

absorption  by,  365. 

and  capillary  water,  218. 

catalytic  action,  529. 

composition  in  soil,  135. 

effect  on  granulation,  190. 


760 


INDEX 


Organic  matter,  effect  on  nitrification, 
449. 

effect  on  phosphates,  520. 

effects  on  soil,  150. 

effect  on  soil  bacteria,  435. 

efficiency  of  fertilizer,  569. 

estimation  of,  141. 

losses  in  digesting  food,  597. 

maintenance  in  soil,  151. 

relation  to  air  in  soil,  476. 

soil  content,  126. 

soils  of  U.  S.  content,  146. 

source  of  carbon  dioxide,  479. 

specific  heat  relations,  297. 
Organic  nitrogen,  plant  food,  497. 
Organisms  in  soil,  421. 

effect  of  heat  on,  289. 

effect  of  drainage  on,  632. 

macro-  in  soil,  421. 

micro-  in  soil,  424. 
Orthoclase  mineral,  9. 

solubility  of,  338. 

weathering  of,  22. 
Osmotic  activity  of  plant  roots,  412. 
Outlets  to  the  drains,  656. 
Oxbows,  40. 

Oxidation  and  soil  fertility,  137. 
Oxidation  as  weathering  agent,  19. 
Oxidation  of  sulfur  in  soil,  525. 
Oxygen,  4,  6. 

Oxygen,  absorption  of,  367. 
Oxygen,  effect  on  soil  bacteria,  433. 
Oxygen  in  soil,  air  functions,  480. 
Oxygen  in  soil,  relation  to  carbon  dioxide, 
479. 

Packers  and  crushers,  678. 
Packer,  sub-surface,  679. 
Partial  solution  of  soil,  329,  331. 
Particles,  in  soil,  chemical  composition, 
101. 

in  soil,  mineral  composition,  99. 

in  soil,  physical  character,  98. 

of  soil,  classification,  95. 

of  soil,  number,  118. 

surface  exposed  in  soil  by,  120. 
Peas,  materials  used  as  food,  496. 
Peat,  defined,  36. 
Peat,  nitrogen  compounds  in,  133. 
Peat,  specific  gravity  of,  113. 
Penicillium  glaucum  in  soil,  427. 
Percolation  of  water,  233. 

effect  of  pressure  on,  233. 

effect  of  texture  and  structure  on, 
235. 

effect  on  air  movement,  483, 


Percolation  of  water,  losses  from,  265. 
mi,  control,  267. 

objection  to,  in  irrigation,  704. 
Rothamsted  figures  on. 
Peridot  | 

IVrmutite,  absorption  by,  357. 
Peruvian  guano,  508. 
Pliillipsite,  solubility,  339. 
Phosphate,  acul,  to  r<  nil orce  manure,  610. 

bone,  early  manufacture,  492. 

calcium,  root  action  on 

effect  of  bacteria  on,  439,  437. 

fertilizer.",  .">1  1. 

fertilizers,   relative  availability,   516. 

insoluble,  358. 

of  iron  and  aluminum,  337. 

row  rock,  to  reinforce  manure,  610. 

relation  to  carbon  dioxide,  482. 

reverted,  616. 
Phoaporic  acid,  absorption  of,  352,  357. 
Phosphorite  as  fertiliser,  512. 
Phosphorus  in  soil,  4,  6. 

effect  on  toxic  material,  137. 

effects  on  plant  growth,  549. 

in  soil  separates,  101. 
Physical  agencies  of  weathering,  14. 

absorption,  359. 

changes  due  to  heat,  291. 

character  of  soil  separates,  98. 

effect  of  organic  matter,  150. 
Piedmont  soils,  34. 
I'lauioclase,  '.». 

Plane  table  for  soil  survey,  727. 
Planker,  as  soil  pulveriser,  680. 
Plant  food,  elements  of,  in  soil,  3. 
Plant    food,  deficiencies    and    manurial 
needs,  334. 

distribution  in  liquid  and  solid  ma- 
nure, 586. 

elements,  abundance  of,  5. 

elements,  essential,  417. 

elements,  sources  of,  4. 

appearing  in  manure,  597. 

in  soil  minerals,  101. 

in  plants,  418. 

limiting  elements,  4. 

proportion  of  feed  in  manure,  581. 

relation  to  bacteria,  428. 

relation  to  dilution,  332. 

shown  by  analysis,  330. 

supplied  in  sewage,  711. 
Plant  growth,  effects  of  nitrogen,  547. 

factors  for,  3. 

functions  of  water,  243. 

and  the  soil  solution,  347. 

and  strength  of  soil  solution,  417. 


INDEX 


761 


Plant  nutrients,  absorption  of,  404. 
Plant  roots  in  soil,  424. 
Plant  roots,  solvent  action,  405. 
Plants,  absorptive  power,  412. 

acid  in  juices,  379. 

available  water  for,  261. 

composition  of,  128. 

effect  of  alkali  on,  394. 

effect  on  availability  of  phosphorus, 
518. 

effect  on  granulation,  192. 

indicating  an  acid  soil,  382. 

requirements  for  growth,  3. 

soil-forming  agents,  18. 

soil  shelters,  287. 

succession  on  soil,  1. 

utilize  simple  and  complex  material, 
131. 

water  requirements  of,  244. 
Plasticity,  causes  of,  172. 
Plasticity,  defined,  170. 
Plow  attachments,  671. 

effect  on  granulation,  195. 

hillside,  670. 

as  tillage  implement,  665. 

subsoil,  672. 

sale  of,  670. 
Plowing,  correct  position  of  furrow,  668. 

depth,  669. 

fall,  relation  to  dry-farming,  713. 

under  manure,  608. 
Poncelet's  formula,  652. 
Pore  space  in  spherical  particles,  109. 

in  soil,  115. 

in  soil,  calculation,  116. 
Porosity  and  diffusion  of  gases,  483. 
Positive  acidity,  375. 
Potassium  in  soils,  4,  6. 

absorption  of,  358. 

chloride,  523. 

effects  on  plant  growth,  551. 

effect  on  toxic  material,  137. 

fertilizers,  522. 

nitrate  test  for  acidity,  388,  389. 

soil  separates  content,  101. 

solubility,  339. 

sulfate,  523. 
Poultry  manure,  composition,  582,  588. 
Pressure,  atmospheric,  affects    soil    air, 

484. 
Properties  of  soil  separates,  98,  99,  101. 
Protein  decay,  447. 
Proteins  in  soil,  127. 

Protozoa,    relation    to    soil    productive- 
ness, 471. 
Province,  the,  in  soil  classification,  723. 


Puddled  soil,  110. 

Pulverizing  action  of  plow,  195,  665. 
Putrefaction  and  decay,  443. 
Putrefaction,  products  of,  445. 

Quartz,  mineral,  9. 

Quartz,  specific  heat,  299. 

Quartz,  in  soil  separates,  99. 

Quartzite,  6. 

Quicklime,  539. 

Quicksand,    management    in    drainage, 


Radiation,  effect  of  moisture,  305. 

Radiation  from  soil,  302. 

Rainfall,  distribution  in  world,  683,  686. 

effect  on  soil  ventilation,  483. 

relation  to  irrigation,  682. 
Raw  rock  phosphate,  512. 
Reclamation     service     of     the     United 

States,  688,  690. 
Reconnoissance  soil  surveys,  737. 
Reduction  of  nitrates,  455. 
Reinforcement  of  manure,  609. 
Residual  effects  of  manure,  614. 
Residual  soils,  colors,  33. 

characters  of,  32. 

composition  of,  66. 

defined,  31. 

distribution  of,  34. 

origin  of,  31. 
Resinous  material  in  soil,  275. 
Reverted  phosphate,  515. 
Ridge  tillage  versus  level,  287. 
Riparian  rights,  modified  under  irriga- 
tion, 691. 
Ripening  and  moisture,  253. 
Rodents  in  soil,  421. 
Rock  flour,  68. 
Rock  rot,  68. 
Rock-forming  minerals,  8. 
Rock,  definition  of,  7. 
Rocks,  acid,  7. 

basic,  7. 

classification  of,  6. 

coefficients  of  expansion,  17. 

decay  agencies  of,  14. 

law  of  decay,  24. 

soil-forming,  6. 

solubility  in  sodium  carbonate,  26. 

weathering  of,  13. 
Rolling  and  moisture  control,  284. 
Roller  as  soil  pulverizer,  679. 
Root  crops,  feeding  power,  415. 
Root-hairs  and  food  absorption,  404. 
Root-hairs,  relation  to  soil  particles,  405. 


762 


INDEX 


Root   development,   effect    of   drainage, 
63L 

Root  excretions,  188. 
Roots,  effect  on  colloids,  411. 

effect  of  phosphorus,  550. 

effect  on  soil  ventilation,  488. 

entrance  into  tile,  642. 

extent  of  surface,   11 J 

nodules  on,  458. 

of  plants,  distribution,  81. 

of  plants  and  humu-,  Il'7 

of  plants  under  dry-farming,  717. 

of  plants  in  soil,  424. 

oxidizing  enzymes  in,  407. 

solvent  action,  405. 
Rotation  and  green  manure,  I .. 

of  crops  and  maintenance  of  humus, 
152. 

of  crops  and  manure,  616. 

of   crops,   remedy    for   soil   diseases, 

Rothamsted,    composition    of    drainage 
water,  370. 

drain  gauges,  266. 

losses  of  nitrogen,  500. 
Run-off,  losses  from 

Salt,  as  amendment,  543. 
Sampling  soil  in  the  held,  731. 
Sand,  99. 
Sand  dunes,  63. 
Sandstone,  6,  8. 

Sanitation  of  soil,  effect  of  drainage,  633. 
Sanitation  of  soil  and  toxic  material,  137. 
Saturated  soil,  201. 
Sawdust  as  litter,  603. 
Schist,  6. 

Scraping,  correction  for  alkali,  401. 
Second-food  of  water,  defined,  7()f>. 
Sediment  in  water,  effect  on  erosion,  637. 
Sedimentary,  soil-forming  rocks,  6. 
Seeding  machines  as  cultivators,  678. 
Seepage,  extent  and  prevention  in  irri- 
gation, 694. 
Selective  absorption,  362. 
Selective  absorption  by  colloids,  165. 
Separates,  mineralogical  character,  99. 
Separates,  physical  character  of,  98. 
Separates  of  soil,  84. 
Series,  the,  in  soil  classification,  722. 
Series  of  soil,  named,  725. 
Serpentine,  9. 
Sewage  irrigation,  711. 
Sewage-sick  soil,  474. 
Shading  and  moisture  loss,  287. 
Shale,  6. 


Sheep  manure,  composition,  584. 

Shelters  and  mofctaN  control,  - 

Siderit. 

Silica  in  soil  separates,  102. 

Silicates,  absorption  by,  360. 

Silicates,  influence  on  absorption,  887. 

Silicon,  6. 

Silt,  character  of  separate 

Silt-l.:i>in>,  in  drains,  656. 

Silvinit,  potassium  salt,  IS 

Single-grain  structure,  110, 

Sise  of  colloidal  particles,  154. 

Size  of  soil  particles,  95. 

Sise  of  the  drains,  650,  654. 

Slate,  6. 

Slag  phosphate,  818. 

Slope  and  temperature,  317. 

Slime  molds  in  soil,   »_'■"> 

Snow,  effect  on  soil  temperature,  306. 

Sod,  effect  on  nitrification,  452. 

Sodium,  8 

Sodium  compounds,  as  fertiliser,  544. 

Sodium  nitrate,  as  plant  food,  498. 

Sol.  eolloi.Lil  condition,  159. 

Soldatc.  module,  707. 

Solid  manure  compared  with  liquid,  585. 

Solubility  of  rocks  in  sodium  carbonate, 

of  soil  and  texture. 

of  the  soil,  327. 
Solution  and  soil  formation,  21. 
Solution  in  soil  and  plant  growth,  417. 
Solvent  action  of  roots,  40.".. 
Solvents  of  the  soil,  328. 
Soil  amendments,  534. 

best  suited  for  dry-farming,  716. 

classification,  factors  used,  720. 

composition,  general,  2. 

defined,  1. 

organic  matter,  126. 

organisms,  421. 

sampling  in  the  field,  731. 

solution  in  situ,  342. 

solution  and  plant  growth,  347. 

survey,  accuracy  and  detail,  733. 

survey,  equipment  for,  726. 

surveys,  extent,  737. 

survey  map,  736. 

survey  report,  735. 

survey,  its  uses,  739. 

survey,    principles    of    classification, 
718. 

type,  name,  725. 
Soil  and  subsoil,  arid  and  humid  regions, 

82. 
Soil  and  subsoil,  defined,  79. 


INDEX 


763 


Soil-forming  minerals,  8. 
Soil-forming  rocks,  6. 
Soil  mulch  versus  dust  mulch,  275. 
Sour  soils,  375. 
Sources  of  plant  food,  419. 
Specific  gravity,  absolute,  112. 
apparent,  113. 
of  minerals,  112. 
of  soil  separates,  113. 
Specific  heat  of  soil,  294. 
Spray  system  of  irrigation,  695. 
Spring,  early  plowing,  283. 
Stall  manure  versus  yard  manure,  601. 
Starch,  effect  of  potassium,  551. 
Stassfurt  salts,  522. 
Storage  of  water  in  soil,  279. 
Straw,  effect  of  nitrogen,  548. 
effect  of  phosphorus,  550. 
relation  to  denitrification,  456. 
Stone  drains,  638. 
Stone  mulch,  273. 

Structure  and  capillary  movement,  232. 
and  capillary  water,  217. 
and  conductivity,  310. 
defined,  108,  170. 
effect  on  diffusion  of  gases,  483. 
effect  on  gravitational  movement,  235. 
modified  by  tillage,  663. 
relation  to  air  in  soil,  476. 
temperature  relations,  304. 
Subirrigation,  696. 
Subsoil  and  soil  defined,  79. 
Subsoil,  composition  of,  332. 
Subsoil  plow,  672. 

Subsoiling,  relation  to  dry-farming,  713. 
Substitution  of  bases,  355. 
Substitution  of  elements  by  plant,  417. 
Subsurface  packer,  679. 
Subsurface  packing,  714. 
Sulfates  as  fertilizer,  526. 
Sulfate  of  ammonia  as  fertilizer,  499. 
Sulfate  of  potash,  523. 
Sulfur,  4,  6. 

Sulfur  and  acidity,  382. 
Sulfur  as  fertilizer,  524. 
Sulfur  bacteria  in  soil,  439. 
Sulfur  dioxide,  from  plant  decay,  140. 
Sulfur,  free,  as  fertilizer,  524. 
Sulfuric  acid  solution  of  soil,  329. 
Superphosphate  fertilizer,  514. 
Surface  of  roots,  412. 
Surface  of  soil  particles,  120. 
Surface  tension,  211. 

and  capillary  movement,  227. 
and  capillary  water,  213. 
Surface  water,  composition  of,  373. 


Swine  manure,  composition,  584. 
Symbiosis,    nitrogen    fixation    through, 

457. 
Syenite,  6,  8. 
Systems  of  fertilization,  573. 

Talc,  9. 

Tankage  as  fertilizer,  507. 
Temperature  changes,  affect  soil  air,  484. 
Temperature  in  fermenting  manure,  593. 
Temperature  of  soil  and  air,  485. 

annual,  321. 
•  annual  range,  322. 

control,  325. 

daily  range,  324. 

effect  of  drainage,  631. 

effect  of  slope,  317. 

effect  of  snow,  306. 

evaporation  influence,  314. 

factors  affecting,  293. 

influence  on  absorption,  367. 

influence  on  bacterial  activity,  435.' 

influence   on   efficiency   of   fertilizer, 
568. 

influence  decay,  129. 

influence  on  hygroscopicity,  207.  

influence  on  movement  of  water,  234. 

influence  on  organic  decay,  315. 
Tenacity  of  soil,  174. 
Terrace,  defined,  41. 
Texture,  adaptation  of  crops  to,  105. 

defined,  83. 

effect  on   capillary  movement,   214, 
229. 

effect  on  cohesion,  179. 

effect  on  conductivity,  308. 

effect  on  hygroscopicity,  204. 

relation  to  air  in  soil,  475. 

relation  to  gravitational  movement, 
235. 

relation  to  soil  classification,  722. 

relation  to  solubility  of  soil,  331. 

relation  to  wilting  point,  260. 

rock,  and  weathering,  23. 

influence  on  absorption,  355. 

influence  on  diffusion  of  gases,  483. 

temperature  relations,  304. . 
Thermal  movement  of  water,  241. 
Tile,  quality,  640. 
Tile  drains,  639. 

carrying  capacity,  651. 

cost,  654. 

dangers  from  roots,  642. 

depth,  646. 

formula  for  size  of  tile,  652. 

grade,  645. 


764 


INDEX 


Tile  drains,  interval,  647. 

laying,  649. 

outlets,  656. 

silt  basins,  656. 

size,  650,  654. 

surface  intakes,  656. 

trenches,  construction,  648. 
Tile  drainage  in  alkali  land,  659. 
Tile  drainage  in  muck  and  peat  soil,  658. 
Till,  glacial  soil,  53. 
Tillage,  objects,  663. 

relation  to  colloids,  166. 

relation  on  granulation,  194. 

relation  to  means  of  soil  infection, 
425. 

relation  to  run-off,  269. 

relation  to  soil  ventilation,  487. 

implements,  664. 

practices,  663. 
Tillering  and  moisture,  253. 
Tilth,  effect  of  lime,  534. 
Tilth  of  soil,  184. 

Toxic  material  and  drainage,  262,  632. 
Toxic  material  in  soil,  136. 
Toxic  substances,  effect  of  lime,  636. 
Toxicity  and  loss  of  bases,  378. 
Trade  values  of  fertiliser,  559. 
Transpiration  ratio,  244. 
Transpiration  and  fertility,  250. 
Transported  soils,  31,  46. 
Type  of  soil,  named,  725. 
Type  of  soil,  the  unit,  720. 

Unavailable  soil  water,  256. 
Underdrains,  advantages,  635. 
Underdrains,  tile,  639. 
Units  in  water  measurement,  706. 
Urea,  derived  from  cyanamide,  504. 
Urea,  product  of  decay,  130. 
Urine  in  manures,  584,  586. 

Value  of  fertilizer,  trade,  559,  562. 

Value  of  manure,  commercial,  602. 

Vegetables,  feeding  power,  415. 

Vegetation  as  means  of  soil  classifica- 
tion, 720. 

Vegetative  growth,  favored  by  nitrogen, 
547. 

Ventilation  of  soil,  effect  of  drainage, 
487,  630. 

Vertical  drainage,  660. 

Volcanic  dust,  63. 

Water,  amount  to  apply  in  irrigation,  710. 
available  to  plants,  261. 
application,  time,  709. 


Water,  as  solvent  in  soil  analysis,  340. 

as  weathering  agent,  14. 

carrying  power,  40. 

content,  maximum,  262. 

oontral  of,  264. 

capillary  form,  210. 

effect  of  movement  on  soil  air,  483. 

equivalents  of  soil,  220. 

expansive  power  in  freezing,  17. 

forms  in  soil,  200. 

functions  of,  in  plant  growth,  243. 

gravitational  form,  233. 

hygroscopic  form,  202. 

interception  losses,  287. 

losses  from  evaporation,  270. 

losses  from  run-otT,  M& 

relation  of  application  to  yield,  708. 

requirements  of  plants,  244. 

saved  by  mulch,  279. 

sources  for  irrigation,  693. 

specific  heat,  relation  to,  298. 

storage  in  soil,  713. 

surfaces  in  soil,  212. 

unavailable  in  soil,  256. 
Water-logged  land,  659. 
Water-slaked  lime,  539. 
Weathering,  conditions  affecting,  22. 

forces  of,  14. 

in  arid  and  humid  regions,  23. 

of  rocks,  13. 

special  cases,  27. 
Weeder,  cultivator,  676. 
Weeds  and  crop  growth,  280. 
Weight  of  soil,  115. 
Weir,  for  measurement  of  water,  707. 
Wetting  and  drying,  effect  on  granula- 
tion, 187. 
White  alkali,  392. 
Wilt  of  crops,  a  soil  organism,  425. 
Wilting  coefficient,  257,  258. 
Wilting  point,  calculation  of,  260. 
Wilting  point,  relation  to  texture,  261. 
Wilting,  when  it  occurs,  257. 
Wind,  effect  of  suction  on  soil  air,  486. 
Wind,  erosion  by,  15. 
Wind,  soils  formed  by,  58. 
\Vrind-breaks  and  moisture  control,  285. 
Wood  ashes,  as  fertilizer,  523. 
Wool  waste,  as  fertilizer,  507. 
Worms  in  soil,  422. 

Yard  manure,  578. 

Zeolites,  9,  355. 

Zinc  sulfide  test  for  acidity,  387. 

Zircon,  9. 


Printed  in  the  United  States  of  America. 


14  DAY  USE 

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LD  21A-40m-2,'69 
(J6057sl01476 — A-32 


General  Library 

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